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📄 asm.html <!--{ "Title": "A Quick Guide to Go's Assembler", "Path": "/doc/asm" }--> <h2 id="introduction">A Quick Guide to Go's Assembler</h2> <p> This document is a quick outline of the unusual form of assembly language used by the <code>gc</code> Go compiler. The document is not comprehensive. </p> <p> The assembler is based on the input style of the Plan 9 assemblers, which is documented in detail <a href="https://9p.io/sys/doc/asm.html">elsewhere</a>. If you plan to write assembly language, you should read that document although much of it is Plan 9-specific. The current document provides a summary of the syntax and the differences with what is explained in that document, and describes the peculiarities that apply when writing assembly code to interact with Go. </p> <p> The most important thing to know about Go's assembler is that it is not a direct representation of the underlying machine. Some of the details map precisely to the machine, but some do not. This is because the compiler suite (see <a href="https://9p.io/sys/doc/compiler.html">this description</a>) needs no assembler pass in the usual pipeline. Instead, the compiler operates on a kind of semi-abstract instruction set, and instruction selection occurs partly after code generation. The assembler works on the semi-abstract form, so when you see an instruction like <code>MOV</code> what the toolchain actually generates for that operation might not be a move instruction at all, perhaps a clear or load. Or it might correspond exactly to the machine instruction with that name. In general, machine-specific operations tend to appear as themselves, while more general concepts like memory move and subroutine call and return are more abstract. The details vary with architecture, and we apologize for the imprecision; the situation is not well-defined. </p> <p> The assembler program is a way to parse a description of that semi-abstract instruction set and turn it into instructions to be input to the linker. If you want to see what the instructions look like in assembly for a given architecture, say amd64, there are many examples in the sources of the standard library, in packages such as <a href="/pkg/runtime/"><code>runtime</code></a> and <a href="/pkg/math/big/"><code>math/big</code></a>. You can also examine what the compiler emits as assembly code (the actual output may differ from what you see here): </p> <pre> $ cat x.go package main func main() { println(3) } $ GOOS=linux GOARCH=amd64 go tool compile -S x.go # or: go build -gcflags -S x.go "".main STEXT size=74 args=0x0 locals=0x10 0x0000 00000 (x.go:3) TEXT "".main(SB), $16-0 0x0000 00000 (x.go:3) MOVQ (TLS), CX 0x0009 00009 (x.go:3) CMPQ SP, 16(CX) 0x000d 00013 (x.go:3) JLS 67 0x000f 00015 (x.go:3) SUBQ $16, SP 0x0013 00019 (x.go:3) MOVQ BP, 8(SP) 0x0018 00024 (x.go:3) LEAQ 8(SP), BP 0x001d 00029 (x.go:3) FUNCDATA $0, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB) 0x001d 00029 (x.go:3) FUNCDATA $1, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB) 0x001d 00029 (x.go:3) FUNCDATA $2, gclocals·33cdeccccebe80329f1fdbee7f5874cb(SB) 0x001d 00029 (x.go:4) PCDATA $0, $0 0x001d 00029 (x.go:4) PCDATA $1, $0 0x001d 00029 (x.go:4) CALL runtime.printlock(SB) 0x0022 00034 (x.go:4) MOVQ $3, (SP) 0x002a 00042 (x.go:4) CALL runtime.printint(SB) 0x002f 00047 (x.go:4) CALL runtime.printnl(SB) 0x0034 00052 (x.go:4) CALL runtime.printunlock(SB) 0x0039 00057 (x.go:5) MOVQ 8(SP), BP 0x003e 00062 (x.go:5) ADDQ $16, SP 0x0042 00066 (x.go:5) RET 0x0043 00067 (x.go:5) NOP 0x0043 00067 (x.go:3) PCDATA $1, $-1 0x0043 00067 (x.go:3) PCDATA $0, $-1 0x0043 00067 (x.go:3) CALL runtime.morestack_noctxt(SB) 0x0048 00072 (x.go:3) JMP 0 ... </pre> <p> The <code>FUNCDATA</code> and <code>PCDATA</code> directives contain information for use by the garbage collector; they are introduced by the compiler. </p> <p> To see what gets put in the binary after linking, use <code>go tool objdump</code>: </p> <pre> $ go build -o x.exe x.go $ go tool objdump -s main.main x.exe TEXT main.main(SB) /tmp/x.go x.go:3 0x10501c0 65488b0c2530000000 MOVQ GS:0x30, CX x.go:3 0x10501c9 483b6110 CMPQ 0x10(CX), SP x.go:3 0x10501cd 7634 JBE 0x1050203 x.go:3 0x10501cf 4883ec10 SUBQ $0x10, SP x.go:3 0x10501d3 48896c2408 MOVQ BP, 0x8(SP) x.go:3 0x10501d8 488d6c2408 LEAQ 0x8(SP), BP x.go:4 0x10501dd e86e45fdff CALL runtime.printlock(SB) x.go:4 0x10501e2 48c7042403000000 MOVQ $0x3, 0(SP) x.go:4 0x10501ea e8e14cfdff CALL runtime.printint(SB) x.go:4 0x10501ef e8ec47fdff CALL runtime.printnl(SB) x.go:4 0x10501f4 e8d745fdff CALL runtime.printunlock(SB) x.go:5 0x10501f9 488b6c2408 MOVQ 0x8(SP), BP x.go:5 0x10501fe 4883c410 ADDQ $0x10, SP x.go:5 0x1050202 c3 RET x.go:3 0x1050203 e83882ffff CALL runtime.morestack_noctxt(SB) x.go:3 0x1050208 ebb6 JMP main.main(SB) </pre> <h3 id="constants">Constants</h3> <p> Although the assembler takes its guidance from the Plan 9 assemblers, it is a distinct program, so there are some differences. One is in constant evaluation. Constant expressions in the assembler are parsed using Go's operator precedence, not the C-like precedence of the original. Thus <code>3&1<<2</code> is 4, not 0—it parses as <code>(3&1)<<2</code> not <code>3&(1<<2)</code>. Also, constants are always evaluated as 64-bit unsigned integers. Thus <code>-2</code> is not the integer value minus two, but the unsigned 64-bit integer with the same bit pattern. The distinction rarely matters but to avoid ambiguity, division or right shift where the right operand's high bit is set is rejected. </p> <h3 id="symbols">Symbols</h3> <p> Some symbols, such as <code>R1</code> or <code>LR</code>, are predefined and refer to registers. The exact set depends on the architecture. </p> <p> There are four predeclared symbols that refer to pseudo-registers. These are not real registers, but rather virtual registers maintained by the toolchain, such as a frame pointer. The set of pseudo-registers is the same for all architectures: </p> <ul> <li> <code>FP</code>: Frame pointer: arguments and locals. </li> <li> <code>PC</code>: Program counter: jumps and branches. </li> <li> <code>SB</code>: Static base pointer: global symbols. </li> <li> <code>SP</code>: Stack pointer: the highest address within the local stack frame. </li> </ul> <p> All user-defined symbols are written as offsets to the pseudo-registers <code>FP</code> (arguments and locals) and <code>SB</code> (globals). </p> <p> The <code>SB</code> pseudo-register can be thought of as the origin of memory, so the symbol <code>foo(SB)</code> is the name <code>foo</code> as an address in memory. This form is used to name global functions and data. Adding <code><></code> to the name, as in <span style="white-space: nowrap"><code>foo<>(SB)</code></span>, makes the name visible only in the current source file, like a top-level <code>static</code> declaration in a C file. Adding an offset to the name refers to that offset from the symbol's address, so <code>foo+4(SB)</code> is four bytes past the start of <code>foo</code>. </p> <p> The <code>FP</code> pseudo-register is a virtual frame pointer used to refer to function arguments. The compilers maintain a virtual frame pointer and refer to the arguments on the stack as offsets from that pseudo-register. Thus <code>0(FP)</code> is the first argument to the function, <code>8(FP)</code> is the second (on a 64-bit machine), and so on. However, when referring to a function argument this way, it is necessary to place a name at the beginning, as in <code>first_arg+0(FP)</code> and <code>second_arg+8(FP)</code>. (The meaning of the offset—offset from the frame pointer—distinct from its use with <code>SB</code>, where it is an offset from the symbol.) The assembler enforces this convention, rejecting plain <code>0(FP)</code> and <code>8(FP)</code>. The actual name is semantically irrelevant but should be used to document the argument's name. It is worth stressing that <code>FP</code> is always a pseudo-register, not a hardware register, even on architectures with a hardware frame pointer. </p> <p> For assembly functions with Go prototypes, <code>go</code> <code>vet</code> will check that the argument names and offsets match. On 32-bit systems, the low and high 32 bits of a 64-bit value are distinguished by adding a <code>_lo</code> or <code>_hi</code> suffix to the name, as in <code>arg_lo+0(FP)</code> or <code>arg_hi+4(FP)</code>. If a Go prototype does not name its result, the expected assembly name is <code>ret</code>. </p> <p> The <code>SP</code> pseudo-register is a virtual stack pointer used to refer to frame-local variables and the arguments being prepared for function calls. It points to the highest address within the local stack frame, so references should use negative offsets in the range [−framesize, 0): <code>x-8(SP)</code>, <code>y-4(SP)</code>, and so on. </p> <p> On architectures with a hardware register named <code>SP</code>, the name prefix distinguishes references to the virtual stack pointer from references to the architectural <code>SP</code> register. That is, <code>x-8(SP)</code> and <code>-8(SP)</code> are different memory locations: the first refers to the virtual stack pointer pseudo-register, while the second refers to the hardware's <code>SP</code> register. </p> <p> On machines where <code>SP</code> and <code>PC</code> are traditionally aliases for a physical, numbered register, in the Go assembler the names <code>SP</code> and <code>PC</code> are still treated specially; for instance, references to <code>SP</code> require a symbol, much like <code>FP</code>. To access the actual hardware register use the true <code>R</code> name. For example, on the ARM architecture the hardware <code>SP</code> and <code>PC</code> are accessible as <code>R13</code> and <code>R15</code>. </p> <p> Branches and direct jumps are always written as offsets to the PC, or as jumps to labels: </p> <pre> label: MOVW $0, R1 JMP label </pre> <p> Each label is visible only within the function in which it is defined. It is therefore permitted for multiple functions in a file to define and use the same label names. Direct jumps and call instructions can target text symbols, such as <code>name(SB)</code>, but not offsets from symbols, such as <code>name+4(SB)</code>. </p> <p> Instructions, registers, and assembler directives are always in UPPER CASE to remind you that assembly programming is a fraught endeavor. (Exception: the <code>g</code> register renaming on ARM.) </p> <p> In Go object files and binaries, the full name of a symbol is the package path followed by a period and the symbol name: <code>fmt.Printf</code> or <code>math/rand.Int</code>. Because the assembler's parser treats period and slash as punctuation, those strings cannot be used directly as identifier names. Instead, the assembler allows the middle dot character U+00B7 and the division slash U+2215 in identifiers and rewrites them to plain period and slash. Within an assembler source file, the symbols above are written as <code>fmt·Printf</code> and <code>math∕rand·Int</code>. The assembly listings generated by the compilers when using the <code>-S</code> flag show the period and slash directly instead of the Unicode replacements required by the assemblers. </p> <p> Most hand-written assembly files do not include the full package path in symbol names, because the linker inserts the package path of the current object file at the beginning of any name starting with a period: in an assembly source file within the math/rand package implementation, the package's Int function can be referred to as <code>·Int</code>. This convention avoids the need to hard-code a package's import path in its own source code, making it easier to move the code from one location to another. </p> <h3 id="directives">Directives</h3> <p> The assembler uses various directives to bind text and data to symbol names. For example, here is a simple complete function definition. The <code>TEXT</code> directive declares the symbol <code>runtime·profileloop</code> and the instructions that follow form the body of the function. The last instruction in a <code>TEXT</code> block must be some sort of jump, usually a <code>RET</code> (pseudo-)instruction. (If it's not, the linker will append a jump-to-itself instruction; there is no fallthrough in <code>TEXTs</code>.) After the symbol, the arguments are flags (see below) and the frame size, a constant (but see below): </p> <pre> TEXT runtime·profileloop(SB),NOSPLIT,$8 MOVQ $runtime·profileloop1(SB), CX MOVQ CX, 0(SP) CALL runtime·externalthreadhandler(SB) RET </pre> <p> In the general case, the frame size is followed by an argument size, separated by a minus sign. (It's not a subtraction, just idiosyncratic syntax.) The frame size <code>$24-8</code> states that the function has a 24-byte frame and is called with 8 bytes of argument, which live on the caller's frame. If <code>NOSPLIT</code> is not specified for the <code>TEXT</code>, the argument size must be provided. For assembly functions with Go prototypes, <code>go</code> <code>vet</code> will check that the argument size is correct. </p> <p> Note that the symbol name uses a middle dot to separate the components and is specified as an offset from the static base pseudo-register <code>SB</code>. This function would be called from Go source for package <code>runtime</code> using the simple name <code>profileloop</code>. </p> <p> Global data symbols are defined by a sequence of initializing <code>DATA</code> directives followed by a <code>GLOBL</code> directive. Each <code>DATA</code> directive initializes a section of the corresponding memory. The memory not explicitly initialized is zeroed. The general form of the <code>DATA</code> directive is <pre> DATA symbol+offset(SB)/width, value </pre> <p> which initializes the symbol memory at the given offset and width with the given value. The <code>DATA</code> directives for a given symbol must be written with increasing offsets. </p> <p> The <code>GLOBL</code> directive declares a symbol to be global. The arguments are optional flags and the size of the data being declared as a global, which will have initial value all zeros unless a <code>DATA</code> directive has initialized it. The <code>GLOBL</code> directive must follow any corresponding <code>DATA</code> directives. </p> <p> For example, </p> <pre> DATA divtab<>+0x00(SB)/4, $0xf4f8fcff DATA divtab<>+0x04(SB)/4, $0xe6eaedf0 ... DATA divtab<>+0x3c(SB)/4, $0x81828384 GLOBL divtab<>(SB), RODATA, $64 GLOBL runtime·tlsoffset(SB), NOPTR, $4 </pre> <p> declares and initializes <code>divtab<></code>, a read-only 64-byte table of 4-byte integer values, and declares <code>runtime·tlsoffset</code>, a 4-byte, implicitly zeroed variable that contains no pointers. </p> <p> There may be one or two arguments to the directives. If there are two, the first is a bit mask of flags, which can be written as numeric expressions, added or or-ed together, or can be set symbolically for easier absorption by a human. Their values, defined in the standard <code>#include</code> file <code>textflag.h</code>, are: </p> <ul> <li> <code>NOPROF</code> = 1 <br> (For <code>TEXT</code> items.) Don't profile the marked function. This flag is deprecated. </li> <li> <code>DUPOK</code> = 2 <br> It is legal to have multiple instances of this symbol in a single binary. The linker will choose one of the duplicates to use. </li> <li> <code>NOSPLIT</code> = 4 <br> (For <code>TEXT</code> items.) Don't insert the preamble to check if the stack must be split. The frame for the routine, plus anything it calls, must fit in the spare space remaining in the current stack segment. Used to protect routines such as the stack splitting code itself. </li> <li> <code>RODATA</code> = 8 <br> (For <code>DATA</code> and <code>GLOBL</code> items.) Put this data in a read-only section. </li> <li> <code>NOPTR</code> = 16 <br> (For <code>DATA</code> and <code>GLOBL</code> items.) This data contains no pointers and therefore does not need to be scanned by the garbage collector. </li> <li> <code>WRAPPER</code> = 32 <br> (For <code>TEXT</code> items.) This is a wrapper function and should not count as disabling <code>recover</code>. </li> <li> <code>NEEDCTXT</code> = 64 <br> (For <code>TEXT</code> items.) This function is a closure so it uses its incoming context register. </li> <li> <code>LOCAL</code> = 128 <br> This symbol is local to the dynamic shared object. </li> <li> <code>TLSBSS</code> = 256 <br> (For <code>DATA</code> and <code>GLOBL</code> items.) Put this data in thread local storage. </li> <li> <code>NOFRAME</code> = 512 <br> (For <code>TEXT</code> items.) Do not insert instructions to allocate a stack frame and save/restore the return address, even if this is not a leaf function. Only valid on functions that declare a frame size of 0. </li> <li> <code>TOPFRAME</code> = 2048 <br> (For <code>TEXT</code> items.) Function is the outermost frame of the call stack. Traceback should stop at this function. </li> </ul> <h3 id="special-instructions">Special instructions</h3> <p> The <code>PCALIGN</code> pseudo-instruction is used to indicate that the next instruction should be aligned to a specified boundary by padding with no-op instructions. </p> <p> It is currently supported on arm64, amd64, ppc64, loong64 and riscv64. For example, the start of the <code>MOVD</code> instruction below is aligned to 32 bytes: <pre> PCALIGN $32 MOVD $2, R0 </pre> </p> <h3 id="data-offsets">Interacting with Go types and constants</h3> <p> If a package has any .s files, then <code>go build</code> will direct the compiler to emit a special header called <code>go_asm.h</code>, which the .s files can then <code>#include</code>. The file contains symbolic <code>#define</code> constants for the offsets of Go struct fields, the sizes of Go struct types, and most Go <code>const</code> declarations defined in the current package. Go assembly should avoid making assumptions about the layout of Go types and instead use these constants. This improves the readability of assembly code, and keeps it robust to changes in data layout either in the Go type definitions or in the layout rules used by the Go compiler. </p> <p> Constants are of the form <code>const_<i>name</i></code>. For example, given the Go declaration <code>const bufSize = 1024</code>, assembly code can refer to the value of this constant as <code>const_bufSize</code>. </p> <p> Field offsets are of the form <code><i>type</i>_<i>field</i></code>. Struct sizes are of the form <code><i>type</i>__size</code>. For example, consider the following Go definition: </p> <pre> type reader struct { buf [bufSize]byte r int } </pre> <p> Assembly can refer to the size of this struct as <code>reader__size</code> and the offsets of the two fields as <code>reader_buf</code> and <code>reader_r</code>. Hence, if register <code>R1</code> contains a pointer to a <code>reader</code>, assembly can reference the <code>r</code> field as <code>reader_r(R1)</code>. </p> <p> If any of these <code>#define</code> names are ambiguous (for example, a struct with a <code>_size</code> field), <code>#include "go_asm.h"</code> will fail with a "redefinition of macro" error. </p> <h3 id="runtime">Runtime Coordination</h3> <p> For garbage collection to run correctly, the runtime must know the location of pointers in all global data and in most stack frames. The Go compiler emits this information when compiling Go source files, but assembly programs must define it explicitly. </p> <p> A data symbol marked with the <code>NOPTR</code> flag (see above) is treated as containing no pointers to runtime-allocated data. A data symbol with the <code>RODATA</code> flag is allocated in read-only memory and is therefore treated as implicitly marked <code>NOPTR</code>. A data symbol with a total size smaller than a pointer is also treated as implicitly marked <code>NOPTR</code>. It is not possible to define a symbol containing pointers in an assembly source file; such a symbol must be defined in a Go source file instead. Assembly source can still refer to the symbol by name even without <code>DATA</code> and <code>GLOBL</code> directives. A good general rule of thumb is to define all non-<code>RODATA</code> symbols in Go instead of in assembly. </p> <p> Each function also needs annotations giving the location of live pointers in its arguments, results, and local stack frame. For an assembly function with no pointer results and either no local stack frame or no function calls, the only requirement is to define a Go prototype for the function in a Go source file in the same package. The name of the assembly function must not contain the package name component (for example, function <code>Syscall</code> in package <code>syscall</code> should use the name <code>·Syscall</code> instead of the equivalent name <code>syscall·Syscall</code> in its <code>TEXT</code> directive). For more complex situations, explicit annotation is needed. These annotations use pseudo-instructions defined in the standard <code>#include</code> file <code>funcdata.h</code>. </p> <p> If a function has no arguments and no results, the pointer information can be omitted. This is indicated by an argument size annotation of <code>$<i>n</i>-0</code> on the <code>TEXT</code> instruction. Otherwise, pointer information must be provided by a Go prototype for the function in a Go source file, even for assembly functions not called directly from Go. (The prototype will also let <code>go</code> <code>vet</code> check the argument references.) At the start of the function, the arguments are assumed to be initialized but the results are assumed uninitialized. If the results will hold live pointers during a call instruction, the function should start by zeroing the results and then executing the pseudo-instruction <code>GO_RESULTS_INITIALIZED</code>. This instruction records that the results are now initialized and should be scanned during stack movement and garbage collection. It is typically easier to arrange that assembly functions do not return pointers or do not contain call instructions; no assembly functions in the standard library use <code>GO_RESULTS_INITIALIZED</code>. </p> <p> If a function has no local stack frame, the pointer information can be omitted. This is indicated by a local frame size annotation of <code>$0-<i>n</i></code> on the <code>TEXT</code> instruction. The pointer information can also be omitted if the function contains no call instructions. Otherwise, the local stack frame must not contain pointers, and the assembly must confirm this fact by executing the pseudo-instruction <code>NO_LOCAL_POINTERS</code>. Because stack resizing is implemented by moving the stack, the stack pointer may change during any function call: even pointers to stack data must not be kept in local variables. </p> <p> Assembly functions should always be given Go prototypes, both to provide pointer information for the arguments and results and to let <code>go</code> <code>vet</code> check that the offsets being used to access them are correct. </p> <h2 id="architectures">Architecture-specific details</h2> <p> It is impractical to list all the instructions and other details for each machine. To see what instructions are defined for a given machine, say ARM, look in the source for the <code>obj</code> support library for that architecture, located in the directory <code>src/cmd/internal/obj/arm</code>. In that directory is a file <code>a.out.go</code>; it contains a long list of constants starting with <code>A</code>, like this: </p> <pre> const ( AAND = obj.ABaseARM + obj.A_ARCHSPECIFIC + iota AEOR ASUB ARSB AADD ... </pre> <p> This is the list of instructions and their spellings as known to the assembler and linker for that architecture. Each instruction begins with an initial capital <code>A</code> in this list, so <code>AAND</code> represents the bitwise and instruction, <code>AND</code> (without the leading <code>A</code>), and is written in assembly source as <code>AND</code>. The enumeration is mostly in alphabetical order. (The architecture-independent <code>AXXX</code>, defined in the <code>cmd/internal/obj</code> package, represents an invalid instruction). The sequence of the <code>A</code> names has nothing to do with the actual encoding of the machine instructions. The <code>cmd/internal/obj</code> package takes care of that detail. </p> <p> The instructions for both the 386 and AMD64 architectures are listed in <code>cmd/internal/obj/x86/a.out.go</code>. </p> <p> The architectures share syntax for common addressing modes such as <code>(R1)</code> (register indirect), <code>4(R1)</code> (register indirect with offset), and <code>$foo(SB)</code> (absolute address). The assembler also supports some (not necessarily all) addressing modes specific to each architecture. The sections below list these. </p> <p> One detail evident in the examples from the previous sections is that data in the instructions flows from left to right: <code>MOVQ</code> <code>$0,</code> <code>CX</code> clears <code>CX</code>. This rule applies even on architectures where the conventional notation uses the opposite direction. </p> <p> Here follow some descriptions of key Go-specific details for the supported architectures. </p> <h3 id="x86">32-bit Intel 386</h3> <p> The runtime pointer to the <code>g</code> structure is maintained through the value of an otherwise unused (as far as Go is concerned) register in the MMU. In the runtime package, assembly code can include <code>go_tls.h</code>, which defines an OS- and architecture-dependent macro <code>get_tls</code> for accessing this register. The <code>get_tls</code> macro takes one argument, which is the register to load the <code>g</code> pointer into. </p> <p> For example, the sequence to load <code>g</code> and <code>m</code> using <code>CX</code> looks like this: </p> <pre> #include "go_tls.h" #include "go_asm.h" ... get_tls(CX) MOVL g(CX), AX // Move g into AX. MOVL g_m(AX), BX // Move g.m into BX. </pre> <p> The <code>get_tls</code> macro is also defined on <a href="#amd64">amd64</a>. </p> <p> Addressing modes: </p> <ul> <li> <code>(DI)(BX2)</code>: The location at address <code>DI</code> plus <code>BX2</code>. </li> <li> <code>64(DI)(BX2)</code>: The location at address <code>DI</code> plus <code>BX2</code> plus 64. These modes accept only 1, 2, 4, and 8 as scale factors. </li> </ul> <p> When using the compiler and assembler's <code>-dynlink</code> or <code>-shared</code> modes, any load or store of a fixed memory location such as a global variable must be assumed to overwrite <code>CX</code>. Therefore, to be safe for use with these modes, assembly sources should typically avoid CX except between memory references. </p> <h3 id="amd64">64-bit Intel 386 (a.k.a. amd64)</h3> <p> The two architectures behave largely the same at the assembler level. Assembly code to access the <code>m</code> and <code>g</code> pointers on the 64-bit version is the same as on the 32-bit 386, except it uses <code>MOVQ</code> rather than <code>MOVL</code>: </p> <pre> get_tls(CX) MOVQ g(CX), AX // Move g into AX. MOVQ g_m(AX), BX // Move g.m into BX. </pre> <p> Register <code>BP</code> is callee-save. The assembler automatically inserts <code>BP</code> save/restore when frame size is larger than zero. Using <code>BP</code> as a general purpose register is allowed, however it can interfere with sampling-based profiling. </p> <h3 id="arm">ARM</h3> <p> The registers <code>R10</code> and <code>R11</code> are reserved by the compiler and linker. </p> <p> <code>R10</code> points to the <code>g</code> (goroutine) structure. Within assembler source code, this pointer must be referred to as <code>g</code>; the name <code>R10</code> is not recognized. </p> <p> To make it easier for people and compilers to write assembly, the ARM linker allows general addressing forms and pseudo-operations like <code>DIV</code> or <code>MOD</code> that may not be expressible using a single hardware instruction. It implements these forms as multiple instructions, often using the <code>R11</code> register to hold temporary values. Hand-written assembly can use <code>R11</code>, but doing so requires being sure that the linker is not also using it to implement any of the other instructions in the function. </p> <p> When defining a <code>TEXT</code>, specifying frame size <code>$-4</code> tells the linker that this is a leaf function that does not need to save <code>LR</code> on entry. </p> <p> The name <code>SP</code> always refers to the virtual stack pointer described earlier. For the hardware register, use <code>R13</code>. </p> <p> Condition code syntax is to append a period and the one- or two-letter code to the instruction, as in <code>MOVW.EQ</code>. Multiple codes may be appended: <code>MOVM.IA.W</code>. The order of the code modifiers is irrelevant. </p> <p> Addressing modes: </p> <ul> <li> <code>R0->16</code> <br> <code>R0>>16</code> <br> <code>R0<<16</code> <br> <code>R0@>16</code>: For <code><<</code>, left shift <code>R0</code> by 16 bits. The other codes are <code>-></code> (arithmetic right shift), <code>>></code> (logical right shift), and <code>@></code> (rotate right). </li> <li> <code>R0->R1</code> <br> <code>R0>>R1</code> <br> <code>R0<<R1</code> <br> <code>R0@>R1</code>: For <code><<</code>, left shift <code>R0</code> by the count in <code>R1</code>. The other codes are <code>-></code> (arithmetic right shift), <code>>></code> (logical right shift), and <code>@></code> (rotate right). </li> <li> <code>[R0,g,R12-R15]</code>: For multi-register instructions, the set comprising <code>R0</code>, <code>g</code>, and <code>R12</code> through <code>R15</code> inclusive. </li> <li> <code>(R5, R6)</code>: Destination register pair. </li> </ul> <h3 id="arm64">ARM64</h3> <p> <code>R18</code> is the "platform register", reserved on the Apple platform. To prevent accidental misuse, the register is named <code>R18_PLATFORM</code>. <code>R27</code> and <code>R28</code> are reserved by the compiler and linker. <code>R29</code> is the frame pointer. <code>R30</code> is the link register. </p> <p> Instruction modifiers are appended to the instruction following a period. The only modifiers are <code>P</code> (postincrement) and <code>W</code> (preincrement): <code>MOVW.P</code>, <code>MOVW.W</code> </p> <p> Addressing modes: </p> <ul> <li> <code>R0->16</code> <br> <code>R0>>16</code> <br> <code>R0<<16</code> <br> <code>R0@>16</code>: These are the same as on the 32-bit ARM. </li> <li> <code>$(8<<12)</code>: Left shift the immediate value <code>8</code> by <code>12</code> bits. </li> <li> <code>8(R0)</code>: Add the value of <code>R0</code> and <code>8</code>. </li> <li> <code>(R2)(R0)</code>: The location at <code>R0</code> plus <code>R2</code>. </li> <li> <code>R0.UXTB</code> <br> <code>R0.UXTB<<imm</code>: <code>UXTB</code>: extract an 8-bit value from the low-order bits of <code>R0</code> and zero-extend it to the size of <code>R0</code>. <code>R0.UXTB<<imm</code>: left shift the result of <code>R0.UXTB</code> by <code>imm</code> bits. The <code>imm</code> value can be 0, 1, 2, 3, or 4. The other extensions include <code>UXTH</code> (16-bit), <code>UXTW</code> (32-bit), and <code>UXTX</code> (64-bit). </li> <li> <code>R0.SXTB</code> <br> <code>R0.SXTB<<imm</code>: <code>SXTB</code>: extract an 8-bit value from the low-order bits of <code>R0</code> and sign-extend it to the size of <code>R0</code>. <code>R0.SXTB<<imm</code>: left shift the result of <code>R0.SXTB</code> by <code>imm</code> bits. The <code>imm</code> value can be 0, 1, 2, 3, or 4. The other extensions include <code>SXTH</code> (16-bit), <code>SXTW</code> (32-bit), and <code>SXTX</code> (64-bit). </li> <li> <code>(R5, R6)</code>: Register pair for <code>LDAXP</code>/<code>LDP</code>/<code>LDXP</code>/<code>STLXP</code>/<code>STP</code>/<code>STP</code>. </li> </ul> <p> Reference: <a href="/pkg/cmd/internal/obj/arm64">Go ARM64 Assembly Instructions Reference Manual</a> </p> <h3 id="ppc64">PPC64</h3> <p> This assembler is used by GOARCH values ppc64 and ppc64le. </p> <p> Reference: <a href="/pkg/cmd/internal/obj/ppc64">Go PPC64 Assembly Instructions Reference Manual</a> </p> <h3 id="s390x">IBM z/Architecture, a.k.a. s390x</h3> <p> The registers <code>R10</code> and <code>R11</code> are reserved. The assembler uses them to hold temporary values when assembling some instructions. </p> <p> <code>R13</code> points to the <code>g</code> (goroutine) structure. This register must be referred to as <code>g</code>; the name <code>R13</code> is not recognized. </p> <p> <code>R15</code> points to the stack frame and should typically only be accessed using the virtual registers <code>SP</code> and <code>FP</code>. </p> <p> Load- and store-multiple instructions operate on a range of registers. The range of registers is specified by a start register and an end register. For example, <code>LMG</code> <code>(R9),</code> <code>R5,</code> <code>R7</code> would load <code>R5</code>, <code>R6</code> and <code>R7</code> with the 64-bit values at <code>0(R9)</code>, <code>8(R9)</code> and <code>16(R9)</code> respectively. </p> <p> Storage-and-storage instructions such as <code>MVC</code> and <code>XC</code> are written with the length as the first argument. For example, <code>XC</code> <code>$8,</code> <code>(R9),</code> <code>(R9)</code> would clear eight bytes at the address specified in <code>R9</code>. </p> <p> If a vector instruction takes a length or an index as an argument then it will be the first argument. For example, <code>VLEIF</code> <code>$1,</code> <code>$16,</code> <code>V2</code> will load the value sixteen into index one of <code>V2</code>. Care should be taken when using vector instructions to ensure that they are available at runtime. To use vector instructions a machine must have both the vector facility (bit 129 in the facility list) and kernel support. Without kernel support a vector instruction will have no effect (it will be equivalent to a <code>NOP</code> instruction). </p> <p> Addressing modes: </p> <ul> <li> <code>(R5)(R61)</code>: The location at <code>R5</code> plus <code>R6</code>. It is a scaled mode as on the x86, but the only scale allowed is <code>1</code>. </li> </ul> <h3 id="mips">MIPS, MIPS64</h3> <p> General purpose registers are named <code>R0</code> through <code>R31</code>, floating point registers are <code>F0</code> through <code>F31</code>. </p> <p> <code>R30</code> is reserved to point to <code>g</code>. <code>R23</code> is used as a temporary register. </p> <p> In a <code>TEXT</code> directive, the frame size <code>$-4</code> for MIPS or <code>$-8</code> for MIPS64 instructs the linker not to save <code>LR</code>. </p> <p> <code>SP</code> refers to the virtual stack pointer. For the hardware register, use <code>R29</code>. </p> <p> Addressing modes: </p> <ul> <li> <code>16(R1)</code>: The location at <code>R1</code> plus 16. </li> <li> <code>(R1)</code>: Alias for <code>0(R1)</code>. </li> </ul> <p> The value of <code>GOMIPS</code> environment variable (<code>hardfloat</code> or <code>softfloat</code>) is made available to assembly code by predefining either <code>GOMIPS_hardfloat</code> or <code>GOMIPS_softfloat</code>. </p> <p> The value of <code>GOMIPS64</code> environment variable (<code>hardfloat</code> or <code>softfloat</code>) is made available to assembly code by predefining either <code>GOMIPS64_hardfloat</code> or <code>GOMIPS64_softfloat</code>. </p> <h3 id="unsupported_opcodes">Unsupported opcodes</h3> <p> The assemblers are designed to support the compiler so not all hardware instructions are defined for all architectures: if the compiler doesn't generate it, it might not be there. If you need to use a missing instruction, there are two ways to proceed. One is to update the assembler to support that instruction, which is straightforward but only worthwhile if it's likely the instruction will be used again. Instead, for simple one-off cases, it's possible to use the <code>BYTE</code> and <code>WORD</code> directives to lay down explicit data into the instruction stream within a <code>TEXT</code>. Here's how the 386 runtime defines the 64-bit atomic load function. </p> <pre> // uint64 atomicload64(uint64 volatile addr); // so actually // void atomicload64(uint64 res, uint64 volatile addr); TEXT runtime·atomicload64(SB), NOSPLIT, $0-12 MOVL ptr+0(FP), AX TESTL $7, AX JZ 2(PC) MOVL 0, AX // crash with nil ptr deref LEAL ret_lo+4(FP), BX // MOVQ (%EAX), %MM0 BYTE $0x0f; BYTE $0x6f; BYTE $0x00 // MOVQ %MM0, 0(%EBX) BYTE $0x0f; BYTE $0x7f; BYTE $0x03 // EMMS BYTE $0x0F; BYTE $0x77 RET </pre>	36.33 KB	2024-02-02 18:09
📄 go1.17_spec.html <!--{ "Title": "The Go Programming Language Specification", "Subtitle": "Version of Oct 15, 2021", "Path": "/ref/spec" }--> <h2 id="Introduction">Introduction</h2> <p> This is the reference manual for the Go programming language as it was for language version 1.17, in October 2021, before the introduction of generics. It is provided for historical interest. The current reference manual can be found <a href="/doc/go_spec.html">here</a>. For more information and other documents, see <a href="/">go.dev</a>. </p> <p> Go is a general-purpose language designed with systems programming in mind. It is strongly typed and garbage-collected and has explicit support for concurrent programming. Programs are constructed from <i>packages</i>, whose properties allow efficient management of dependencies. </p> <p> The grammar is compact and simple to parse, allowing for easy analysis by automatic tools such as integrated development environments. </p> <h2 id="Notation">Notation</h2> <p> The syntax is specified using Extended Backus-Naur Form (EBNF): </p> <pre class="grammar"> Production = production_name "=" [ Expression ] "." . Expression = Alternative { "\|" Alternative } . Alternative = Term { Term } . Term = production_name \| token [ "…" token ] \| Group \| Option \| Repetition . Group = "(" Expression ")" . Option = "[" Expression "]" . Repetition = "{" Expression "}" . </pre> <p> Productions are expressions constructed from terms and the following operators, in increasing precedence: </p> <pre class="grammar"> \| alternation () grouping [] option (0 or 1 times) {} repetition (0 to n times) </pre> <p> Lower-case production names are used to identify lexical tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes <code>""</code> or back quotes <code>``</code>. </p> <p> The form <code>a … b</code> represents the set of characters from <code>a</code> through <code>b</code> as alternatives. The horizontal ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character <code>…</code> (as opposed to the three characters <code>...</code>) is not a token of the Go language. </p> <h2 id="Source_code_representation">Source code representation</h2> <p> Source code is Unicode text encoded in <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term <i>character</i> to refer to a Unicode code point in the source text. </p> <p> Each code point is distinct; for instance, upper and lower case letters are different characters. </p> <p> Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text. </p> <p> Implementation restriction: For compatibility with other tools, a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF) if it is the first Unicode code point in the source text. A byte order mark may be disallowed anywhere else in the source. </p> <h3 id="Characters">Characters</h3> <p> The following terms are used to denote specific Unicode character classes: </p> <pre class="ebnf"> newline = /* the Unicode code point U+000A / . unicode_char = / an arbitrary Unicode code point except newline / . unicode_letter = / a Unicode code point classified as "Letter" / . unicode_digit = / a Unicode code point classified as "Number, decimal digit" / . </pre> <p> In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>, Section 4.5 "General Category" defines a set of character categories. Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in the Number category Nd as Unicode digits. </p> <h3 id="Letters_and_digits">Letters and digits</h3> <p> The underscore character <code>_</code> (U+005F) is considered a letter. </p> <pre class="ebnf"> letter = unicode_letter \| "_" . decimal_digit = "0" … "9" . binary_digit = "0" \| "1" . octal_digit = "0" … "7" . hex_digit = "0" … "9" \| "A" … "F" \| "a" … "f" . </pre> <h2 id="Lexical_elements">Lexical elements</h2> <h3 id="Comments">Comments</h3> <p> Comments serve as program documentation. There are two forms: </p> <ol> <li> <i>Line comments</i> start with the character sequence <code>//</code> and stop at the end of the line. </li> <li> <i>General comments</i> start with the character sequence <code>/</code> and stop with the first subsequent character sequence <code>/</code>. </li> </ol> <p> A comment cannot start inside a <a href="#Rune_literals">rune</a> or <a href="#String_literals">string literal</a>, or inside a comment. A general comment containing no newlines acts like a space. Any other comment acts like a newline. </p> <h3 id="Tokens">Tokens</h3> <p> Tokens form the vocabulary of the Go language. There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from spaces (U+0020), horizontal tabs (U+0009), carriage returns (U+000D), and newlines (U+000A), is ignored except as it separates tokens that would otherwise combine into a single token. Also, a newline or end of file may trigger the insertion of a <a href="#Semicolons">semicolon</a>. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token. </p> <h3 id="Semicolons">Semicolons</h3> <p> The formal grammar uses semicolons <code>";"</code> as terminators in a number of productions. Go programs may omit most of these semicolons using the following two rules: </p> <ol> <li> When the input is broken into tokens, a semicolon is automatically inserted into the token stream immediately after a line's final token if that token is <ul> <li>an <a href="#Identifiers">identifier</a> </li> <li>an <a href="#Integer_literals">integer</a>, <a href="#Floating-point_literals">floating-point</a>, <a href="#Imaginary_literals">imaginary</a>, <a href="#Rune_literals">rune</a>, or <a href="#String_literals">string</a> literal </li> <li>one of the <a href="#Keywords">keywords</a> <code>break</code>, <code>continue</code>, <code>fallthrough</code>, or <code>return</code> </li> <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a> <code>++</code>, <code>--</code>, <code>)</code>, <code>]</code>, or <code>}</code> </li> </ul> </li> <li> To allow complex statements to occupy a single line, a semicolon may be omitted before a closing <code>")"</code> or <code>"}"</code>. </li> </ol> <p> To reflect idiomatic use, code examples in this document elide semicolons using these rules. </p> <h3 id="Identifiers">Identifiers</h3> <p> Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters and digits. The first character in an identifier must be a letter. </p> <pre class="ebnf"> identifier = letter { letter \| unicode_digit } . </pre> <pre> a _x9 ThisVariableIsExported αβ </pre> <p> Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>. </p> <h3 id="Keywords">Keywords</h3> <p> The following keywords are reserved and may not be used as identifiers. </p> <pre class="grammar"> break default func interface select case defer go map struct chan else goto package switch const fallthrough if range type continue for import return var </pre> <h3 id="Operators_and_punctuation">Operators and punctuation</h3> <p> The following character sequences represent <a href="#Operators">operators</a> (including <a href="#Assignments">assignment operators</a>) and punctuation: </p> <pre class="grammar"> + & += &= && == != ( ) - \| -= \|= \|\| < <= [ ] ^ = ^= <- > >= { } / << /= <<= ++ = := , ; % >> %= >>= -- ! ... . : &^ &^= </pre> <h3 id="Integer_literals">Integer literals</h3> <p> An integer literal is a sequence of digits representing an <a href="#Constants">integer constant</a>. An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code> for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal, and <code>0x</code> or <code>0X</code> for hexadecimal. A single <code>0</code> is considered a decimal zero. In hexadecimal literals, letters <code>a</code> through <code>f</code> and <code>A</code> through <code>F</code> represent values 10 through 15. </p> <p> For readability, an underscore character <code>_</code> may appear after a base prefix or between successive digits; such underscores do not change the literal's value. </p> <pre class="ebnf"> int_lit = decimal_lit \| binary_lit \| octal_lit \| hex_lit . decimal_lit = "0" \| ( "1" … "9" ) [ [ "_" ] decimal_digits ] . binary_lit = "0" ( "b" \| "B" ) [ "_" ] binary_digits . octal_lit = "0" [ "o" \| "O" ] [ "_" ] octal_digits . hex_lit = "0" ( "x" \| "X" ) [ "_" ] hex_digits . decimal_digits = decimal_digit { [ "_" ] decimal_digit } . binary_digits = binary_digit { [ "_" ] binary_digit } . octal_digits = octal_digit { [ "_" ] octal_digit } . hex_digits = hex_digit { [ "_" ] hex_digit } . </pre> <pre> 42 4_2 0600 0_600 0o600 0O600 // second character is capital letter 'O' 0xBadFace 0xBad_Face 0x_67_7a_2f_cc_40_c6 170141183460469231731687303715884105727 170_141183_460469_231731_687303_715884_105727 _42 // an identifier, not an integer literal 42_ // invalid: _ must separate successive digits 4__2 // invalid: only one _ at a time 0_xBadFace // invalid: _ must separate successive digits </pre> <h3 id="Floating-point_literals">Floating-point literals</h3> <p> A floating-point literal is a decimal or hexadecimal representation of a <a href="#Constants">floating-point constant</a>. </p> <p> A decimal floating-point literal consists of an integer part (decimal digits), a decimal point, a fractional part (decimal digits), and an exponent part (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; one of the decimal point or the exponent part may be elided. An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>. </p> <p> A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code> prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits), and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; the radix point may be elided as well, but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.) An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup>. </p> <p> For readability, an underscore character <code>_</code> may appear after a base prefix or between successive digits; such underscores do not change the literal value. </p> <pre class="ebnf"> float_lit = decimal_float_lit \| hex_float_lit . decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] \| decimal_digits decimal_exponent \| "." decimal_digits [ decimal_exponent ] . decimal_exponent = ( "e" \| "E" ) [ "+" \| "-" ] decimal_digits . hex_float_lit = "0" ( "x" \| "X" ) hex_mantissa hex_exponent . hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] \| [ "_" ] hex_digits \| "." hex_digits . hex_exponent = ( "p" \| "P" ) [ "+" \| "-" ] decimal_digits . </pre> <pre> 0. 72.40 072.40 // == 72.40 2.71828 1.e+0 6.67428e-11 1E6 .25 .12345E+5 1_5. // == 15.0 0.15e+0_2 // == 15.0 0x1p-2 // == 0.25 0x2.p10 // == 2048.0 0x1.Fp+0 // == 1.9375 0X.8p-0 // == 0.5 0X_1FFFP-16 // == 0.1249847412109375 0x15e-2 // == 0x15e - 2 (integer subtraction) 0x.p1 // invalid: mantissa has no digits 1p-2 // invalid: p exponent requires hexadecimal mantissa 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent 1_.5 // invalid: _ must separate successive digits 1._5 // invalid: _ must separate successive digits 1.5_e1 // invalid: _ must separate successive digits 1.5e_1 // invalid: _ must separate successive digits 1.5e1_ // invalid: _ must separate successive digits </pre> <h3 id="Imaginary_literals">Imaginary literals</h3> <p> An imaginary literal represents the imaginary part of a <a href="#Constants">complex constant</a>. It consists of an <a href="#Integer_literals">integer</a> or <a href="#Floating-point_literals">floating-point</a> literal followed by the lower-case letter <code>i</code>. The value of an imaginary literal is the value of the respective integer or floating-point literal multiplied by the imaginary unit <i>i</i>. </p> <pre class="ebnf"> imaginary_lit = (decimal_digits \| int_lit \| float_lit) "i" . </pre> <p> For backward compatibility, an imaginary literal's integer part consisting entirely of decimal digits (and possibly underscores) is considered a decimal integer, even if it starts with a leading <code>0</code>. </p> <pre> 0i 0123i // == 123i for backward-compatibility 0o123i // == 0o123 1i == 83i 0xabci // == 0xabc * 1i == 2748i 0.i 2.71828i 1.e+0i 6.67428e-11i 1E6i .25i .12345E+5i 0x1p-2i // == 0x1p-2 * 1i == 0.25i </pre> <h3 id="Rune_literals">Rune literals</h3> <p> A rune literal represents a <a href="#Constants">rune constant</a>, an integer value identifying a Unicode code point. A rune literal is expressed as one or more characters enclosed in single quotes, as in <code>'x'</code> or <code>'\n'</code>. Within the quotes, any character may appear except newline and unescaped single quote. A single quoted character represents the Unicode value of the character itself, while multi-character sequences beginning with a backslash encode values in various formats. </p> <p> The simplest form represents the single character within the quotes; since Go source text is Unicode characters encoded in UTF-8, multiple UTF-8-encoded bytes may represent a single integer value. For instance, the literal <code>'a'</code> holds a single byte representing a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>. </p> <p> Several backslash escapes allow arbitrary values to be encoded as ASCII text. There are four ways to represent the integer value as a numeric constant: <code>\x</code> followed by exactly two hexadecimal digits; <code>\u</code> followed by exactly four hexadecimal digits; <code>\U</code> followed by exactly eight hexadecimal digits, and a plain backslash <code>\</code> followed by exactly three octal digits. In each case the value of the literal is the value represented by the digits in the corresponding base. </p> <p> Although these representations all result in an integer, they have different valid ranges. Octal escapes must represent a value between 0 and 255 inclusive. Hexadecimal escapes satisfy this condition by construction. The escapes <code>\u</code> and <code>\U</code> represent Unicode code points so within them some values are illegal, in particular those above <code>0x10FFFF</code> and surrogate halves. </p> <p> After a backslash, certain single-character escapes represent special values: </p> <pre class="grammar"> \a U+0007 alert or bell \b U+0008 backspace \f U+000C form feed \n U+000A line feed or newline \r U+000D carriage return \t U+0009 horizontal tab \v U+000B vertical tab \\ U+005C backslash \' U+0027 single quote (valid escape only within rune literals) \" U+0022 double quote (valid escape only within string literals) </pre> <p> All other sequences starting with a backslash are illegal inside rune literals. </p> <pre class="ebnf"> rune_lit = "'" ( unicode_value \| byte_value ) "'" . unicode_value = unicode_char \| little_u_value \| big_u_value \| escaped_char . byte_value = octal_byte_value \| hex_byte_value . octal_byte_value = `\` octal_digit octal_digit octal_digit . hex_byte_value = `\` "x" hex_digit hex_digit . little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit . big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit . escaped_char = `\` ( "a" \| "b" \| "f" \| "n" \| "r" \| "t" \| "v" \| `\` \| "'" \| `"` ) . </pre> <pre> 'a' 'ä' '本' '\t' '\000' '\007' '\377' '\x07' '\xff' '\u12e4' '\U00101234' '\'' // rune literal containing single quote character 'aa' // illegal: too many characters '\xa' // illegal: too few hexadecimal digits '\0' // illegal: too few octal digits '\uDFFF' // illegal: surrogate half '\U00110000' // illegal: invalid Unicode code point </pre> <h3 id="String_literals">String literals</h3> <p> A string literal represents a <a href="#Constants">string constant</a> obtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals. </p> <p> Raw string literals are character sequences between back quotes, as in <code>`foo`</code>. Within the quotes, any character may appear except back quote. The value of a raw string literal is the string composed of the uninterpreted (implicitly UTF-8-encoded) characters between the quotes; in particular, backslashes have no special meaning and the string may contain newlines. Carriage return characters ('\r') inside raw string literals are discarded from the raw string value. </p> <p> Interpreted string literals are character sequences between double quotes, as in <code>"bar"</code>. Within the quotes, any character may appear except newline and unescaped double quote. The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and <code>\"</code> is legal), with the same restrictions. The three-digit octal (<code>\</code><i>nnn</i>) and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual <i>bytes</i> of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>. Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>, <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character U+00FF. </p> <pre class="ebnf"> string_lit = raw_string_lit \| interpreted_string_lit . raw_string_lit = "`" { unicode_char \| newline } "`" . interpreted_string_lit = `"` { unicode_value \| byte_value } `"` . </pre> <pre> `abc` // same as "abc" `\n \n` // same as "\\n\n\\n" "\n" "\"" // same as `"` "Hello, world!\n" "日本語" "\u65e5本\U00008a9e" "\xff\u00FF" "\uD800" // illegal: surrogate half "\U00110000" // illegal: invalid Unicode code point </pre> <p> These examples all represent the same string: </p> <pre> "日本語" // UTF-8 input text `日本語` // UTF-8 input text as a raw literal "\u65e5\u672c\u8a9e" // the explicit Unicode code points "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes </pre> <p> If the source code represents a character as two code points, such as a combining form involving an accent and a letter, the result will be an error if placed in a rune literal (it is not a single code point), and will appear as two code points if placed in a string literal. </p> <h2 id="Constants">Constants</h2> <p>There are <i>boolean constants</i>, <i>rune constants</i>, <i>integer constants</i>, <i>floating-point constants</i>, <i>complex constants</i>, and <i>string constants</i>. Rune, integer, floating-point, and complex constants are collectively called <i>numeric constants</i>. </p> <p> A constant value is represented by a <a href="#Rune_literals">rune</a>, <a href="#Integer_literals">integer</a>, <a href="#Floating-point_literals">floating-point</a>, <a href="#Imaginary_literals">imaginary</a>, or <a href="#String_literals">string</a> literal, an identifier denoting a constant, a <a href="#Constant_expressions">constant expression</a>, a <a href="#Conversions">conversion</a> with a result that is a constant, or the result value of some built-in functions such as <code>unsafe.Sizeof</code> applied to any value, <code>cap</code> or <code>len</code> applied to <a href="#Length_and_capacity">some expressions</a>, <code>real</code> and <code>imag</code> applied to a complex constant and <code>complex</code> applied to numeric constants. The boolean truth values are represented by the predeclared constants <code>true</code> and <code>false</code>. The predeclared identifier <a href="#Iota">iota</a> denotes an integer constant. </p> <p> In general, complex constants are a form of <a href="#Constant_expressions">constant expression</a> and are discussed in that section. </p> <p> Numeric constants represent exact values of arbitrary precision and do not overflow. Consequently, there are no constants denoting the IEEE-754 negative zero, infinity, and not-a-number values. </p> <p> Constants may be <a href="#Types">typed</a> or <i>untyped</i>. Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>, and certain <a href="#Constant_expressions">constant expressions</a> containing only untyped constant operands are untyped. </p> <p> A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a> or <a href="#Conversions">conversion</a>, or implicitly when used in a <a href="#Variable_declarations">variable declaration</a> or an <a href="#Assignments">assignment</a> or as an operand in an <a href="#Expressions">expression</a>. It is an error if the constant value cannot be <a href="#Representability">represented</a> as a value of the respective type. </p> <p> An untyped constant has a <i>default type</i> which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a <a href="#Short_variable_declarations">short variable declaration</a> such as <code>i := 0</code> where there is no explicit type. The default type of an untyped constant is <code>bool</code>, <code>rune</code>, <code>int</code>, <code>float64</code>, <code>complex128</code> or <code>string</code> respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant. </p> <p> Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must: </p> <ul> <li>Represent integer constants with at least 256 bits.</li> <li>Represent floating-point constants, including the parts of a complex constant, with a mantissa of at least 256 bits and a signed binary exponent of at least 16 bits.</li> <li>Give an error if unable to represent an integer constant precisely.</li> <li>Give an error if unable to represent a floating-point or complex constant due to overflow.</li> <li>Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.</li> </ul> <p> These requirements apply both to literal constants and to the result of evaluating <a href="#Constant_expressions">constant expressions</a>. </p> <h2 id="Variables">Variables</h2> <p> A variable is a storage location for holding a <i>value</i>. The set of permissible values is determined by the variable's <i><a href="#Types">type</a></i>. </p> <p> A <a href="#Variable_declarations">variable declaration</a> or, for function parameters and results, the signature of a <a href="#Function_declarations">function declaration</a> or <a href="#Function_literals">function literal</a> reserves storage for a named variable. Calling the built-in function <a href="#Allocation"><code>new</code></a> or taking the address of a <a href="#Composite_literals">composite literal</a> allocates storage for a variable at run time. Such an anonymous variable is referred to via a (possibly implicit) <a href="#Address_operators">pointer indirection</a>. </p> <p> <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>, and <a href="#Struct_types">struct</a> types have elements and fields that may be <a href="#Address_operators">addressed</a> individually. Each such element acts like a variable. </p> <p> The <i>static type</i> (or just <i>type</i>) of a variable is the type given in its declaration, the type provided in the <code>new</code> call or composite literal, or the type of an element of a structured variable. Variables of interface type also have a distinct <i>dynamic type</i>, which is the concrete type of the value assigned to the variable at run time (unless the value is the predeclared identifier <code>nil</code>, which has no type). The dynamic type may vary during execution but values stored in interface variables are always <a href="#Assignability">assignable</a> to the static type of the variable. </p> <pre> var x interface{} // x is nil and has static type interface{} var v T // v has value nil, static type T x = 42 // x has value 42 and dynamic type int x = v // x has value (T)(nil) and dynamic type T </pre> <p> A variable's value is retrieved by referring to the variable in an <a href="#Expressions">expression</a>; it is the most recent value <a href="#Assignments">assigned</a> to the variable. If a variable has not yet been assigned a value, its value is the <a href="#The_zero_value">zero value</a> for its type. </p> <h2 id="Types">Types</h2> <p> A type determines a set of values together with operations and methods specific to those values. A type may be denoted by a <i>type name</i>, if it has one, or specified using a <i>type literal</i>, which composes a type from existing types. </p> <pre class="ebnf"> Type = TypeName \| TypeLit \| "(" Type ")" . TypeName = identifier \| QualifiedIdent . TypeLit = ArrayType \| StructType \| PointerType \| FunctionType \| InterfaceType \| SliceType \| MapType \| ChannelType . </pre> <p> The language <a href="#Predeclared_identifiers">predeclares</a> certain type names. Others are introduced with <a href="#Type_declarations">type declarations</a>. <i>Composite types</i>—array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals. </p> <p> Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code> is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is <code>T</code> itself. Otherwise, <code>T</code>'s underlying type is the underlying type of the type to which <code>T</code> refers in its <a href="#Type_declarations">type declaration</a>. </p> <pre> type ( A1 = string A2 = A1 ) type ( B1 string B2 B1 B3 []B1 B4 B3 ) </pre> <p> The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>, and <code>B2</code> is <code>string</code>. The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>. </p> <h3 id="Method_sets">Method sets</h3> <p> A type has a (possibly empty) <i>method set</i> associated with it. The method set of an <a href="#Interface_types">interface type</a> is its interface. The method set of any other type <code>T</code> consists of all <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>. The method set of the corresponding <a href="#Pointer_types">pointer type</a> <code>T</code> is the set of all methods declared with receiver <code>T</code> or <code>T</code> (that is, it also contains the method set of <code>T</code>). Further rules apply to structs containing embedded fields, as described in the section on <a href="#Struct_types">struct types</a>. Any other type has an empty method set. In a method set, each method must have a <a href="#Uniqueness_of_identifiers">unique</a> non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>. </p> <p> The method set of a type determines the interfaces that the type <a href="#Interface_types">implements</a> and the methods that can be <a href="#Calls">called</a> using a receiver of that type. </p> <h3 id="Boolean_types">Boolean types</h3> <p> A <i>boolean type</i> represents the set of Boolean truth values denoted by the predeclared constants <code>true</code> and <code>false</code>. The predeclared boolean type is <code>bool</code>; it is a <a href="#Type_definitions">defined type</a>. </p> <h3 id="Numeric_types">Numeric types</h3> <p> A <i>numeric type</i> represents sets of integer or floating-point values. The predeclared architecture-independent numeric types are: </p> <pre class="grammar"> uint8 the set of all unsigned 8-bit integers (0 to 255) uint16 the set of all unsigned 16-bit integers (0 to 65535) uint32 the set of all unsigned 32-bit integers (0 to 4294967295) uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615) int8 the set of all signed 8-bit integers (-128 to 127) int16 the set of all signed 16-bit integers (-32768 to 32767) int32 the set of all signed 32-bit integers (-2147483648 to 2147483647) int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807) float32 the set of all IEEE-754 32-bit floating-point numbers float64 the set of all IEEE-754 64-bit floating-point numbers complex64 the set of all complex numbers with float32 real and imaginary parts complex128 the set of all complex numbers with float64 real and imaginary parts byte alias for uint8 rune alias for int32 </pre> <p> The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>. </p> <p> There is also a set of predeclared numeric types with implementation-specific sizes: </p> <pre class="grammar"> uint either 32 or 64 bits int same size as uint uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value </pre> <p> To avoid portability issues all numeric types are <a href="#Type_definitions">defined types</a> and thus distinct except <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and <code>rune</code>, which is an alias for <code>int32</code>. Explicit conversions are required when different numeric types are mixed in an expression or assignment. For instance, <code>int32</code> and <code>int</code> are not the same type even though they may have the same size on a particular architecture. </p> <h3 id="String_types">String types</h3> <p> A <i>string type</i> represents the set of string values. A string value is a (possibly empty) sequence of bytes. The number of bytes is called the length of the string and is never negative. Strings are immutable: once created, it is impossible to change the contents of a string. The predeclared string type is <code>string</code>; it is a <a href="#Type_definitions">defined type</a>. </p> <p> The length of a string <code>s</code> can be discovered using the built-in function <a href="#Length_and_capacity"><code>len</code></a>. The length is a compile-time constant if the string is a constant. A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a> 0 through <code>len(s)-1</code>. It is illegal to take the address of such an element; if <code>s[i]</code> is the <code>i</code>'th byte of a string, <code>&s[i]</code> is invalid. </p> <h3 id="Array_types">Array types</h3> <p> An array is a numbered sequence of elements of a single type, called the element type. The number of elements is called the length of the array and is never negative. </p> <pre class="ebnf"> ArrayType = "[" ArrayLength "]" ElementType . ArrayLength = Expression . ElementType = Type . </pre> <p> The length is part of the array's type; it must evaluate to a non-negative <a href="#Constants">constant</a> <a href="#Representability">representable</a> by a value of type <code>int</code>. The length of array <code>a</code> can be discovered using the built-in function <a href="#Length_and_capacity"><code>len</code></a>. The elements can be addressed by integer <a href="#Index_expressions">indices</a> 0 through <code>len(a)-1</code>. Array types are always one-dimensional but may be composed to form multi-dimensional types. </p> <pre> [32]byte [2N] struct { x, y int32 } [1000]float64 [3][5]int [2][2][2]float64 // same as [2]([2]([2]float64)) </pre> <h3 id="Slice_types">Slice types</h3> <p> A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and provides access to a numbered sequence of elements from that array. A slice type denotes the set of all slices of arrays of its element type. The number of elements is called the length of the slice and is never negative. The value of an uninitialized slice is <code>nil</code>. </p> <pre class="ebnf"> SliceType = "[" "]" ElementType . </pre> <p> The length of a slice <code>s</code> can be discovered by the built-in function <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a> 0 through <code>len(s)-1</code>. The slice index of a given element may be less than the index of the same element in the underlying array. </p> <p> A slice, once initialized, is always associated with an underlying array that holds its elements. A slice therefore shares storage with its array and with other slices of the same array; by contrast, distinct arrays always represent distinct storage. </p> <p> The array underlying a slice may extend past the end of the slice. The <i>capacity</i> is a measure of that extent: it is the sum of the length of the slice and the length of the array beyond the slice; a slice of length up to that capacity can be created by <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice. The capacity of a slice <code>a</code> can be discovered using the built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>. </p> <p> A new, initialized slice value for a given element type <code>T</code> is made using the built-in function <a href="#Making_slices_maps_and_channels"><code>make</code></a>, which takes a slice type and parameters specifying the length and optionally the capacity. A slice created with <code>make</code> always allocates a new, hidden array to which the returned slice value refers. That is, executing </p> <pre> make([]T, length, capacity) </pre> <p> produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a> it, so these two expressions are equivalent: </p> <pre> make([]int, 50, 100) new([100]int)[0:50] </pre> <p> Like arrays, slices are always one-dimensional but may be composed to construct higher-dimensional objects. With arrays of arrays, the inner arrays are, by construction, always the same length; however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. Moreover, the inner slices must be initialized individually. </p> <h3 id="Struct_types">Struct types</h3> <p> A struct is a sequence of named elements, called fields, each of which has a name and a type. Field names may be specified explicitly (IdentifierList) or implicitly (EmbeddedField). Within a struct, non-<a href="#Blank_identifier">blank</a> field names must be <a href="#Uniqueness_of_identifiers">unique</a>. </p> <pre class="ebnf"> StructType = "struct" "{" { FieldDecl ";" } "}" . FieldDecl = (IdentifierList Type \| EmbeddedField) [ Tag ] . EmbeddedField = [ "" ] TypeName . Tag = string_lit . </pre> <pre> // An empty struct. struct {} // A struct with 6 fields. struct { x, y int u float32 _ float32 // padding A []int F func() } </pre> <p> A field declared with a type but no explicit field name is called an <i>embedded field</i>. An embedded field must be specified as a type name <code>T</code> or as a pointer to a non-interface type name <code>T</code>, and <code>T</code> itself may not be a pointer type. The unqualified type name acts as the field name. </p> <pre> // A struct with four embedded fields of types T1, T2, P.T3 and P.T4 struct { T1 // field name is T1 T2 // field name is T2 P.T3 // field name is T3 P.T4 // field name is T4 x, y int // field names are x and y } </pre> <p> The following declaration is illegal because field names must be unique in a struct type: </p> <pre> struct { T // conflicts with embedded field T and P.T T // conflicts with embedded field T and P.T P.T // conflicts with embedded field T and T } </pre> <p> A field or <a href="#Method_declarations">method</a> <code>f</code> of an embedded field in a struct <code>x</code> is called <i>promoted</i> if <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes that field or method <code>f</code>. </p> <p> Promoted fields act like ordinary fields of a struct except that they cannot be used as field names in <a href="#Composite_literals">composite literals</a> of the struct. </p> <p> Given a struct type <code>S</code> and a <a href="#Type_definitions">defined type</a> <code>T</code>, promoted methods are included in the method set of the struct as follows: </p> <ul> <li> If <code>S</code> contains an embedded field <code>T</code>, the <a href="#Method_sets">method sets</a> of <code>S</code> and <code>S</code> both include promoted methods with receiver <code>T</code>. The method set of <code>S</code> also includes promoted methods with receiver <code>T</code>. </li> <li> If <code>S</code> contains an embedded field <code>T</code>, the method sets of <code>S</code> and <code>S</code> both include promoted methods with receiver <code>T</code> or <code>T</code>. </li> </ul> <p> A field declaration may be followed by an optional string literal <i>tag</i>, which becomes an attribute for all the fields in the corresponding field declaration. An empty tag string is equivalent to an absent tag. The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a> and take part in <a href="#Type_identity">type identity</a> for structs but are otherwise ignored. </p> <pre> struct { x, y float64 "" // an empty tag string is like an absent tag name string "any string is permitted as a tag" _ [4]byte "ceci n'est pas un champ de structure" } // A struct corresponding to a TimeStamp protocol buffer. // The tag strings define the protocol buffer field numbers; // they follow the convention outlined by the reflect package. struct { microsec uint64 `protobuf:"1"` serverIP6 uint64 `protobuf:"2"` } </pre> <h3 id="Pointer_types">Pointer types</h3> <p> A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given type, called the <i>base type</i> of the pointer. The value of an uninitialized pointer is <code>nil</code>. </p> <pre class="ebnf"> PointerType = "" BaseType . BaseType = Type . </pre> <pre> Point [4]int </pre> <h3 id="Function_types">Function types</h3> <p> A function type denotes the set of all functions with the same parameter and result types. The value of an uninitialized variable of function type is <code>nil</code>. </p> <pre class="ebnf"> FunctionType = "func" Signature . Signature = Parameters [ Result ] . Result = Parameters \| Type . Parameters = "(" [ ParameterList [ "," ] ] ")" . ParameterList = ParameterDecl { "," ParameterDecl } . ParameterDecl = [ IdentifierList ] [ "..." ] Type . </pre> <p> Within a list of parameters or results, the names (IdentifierList) must either all be present or all be absent. If present, each name stands for one item (parameter or result) of the specified type and all non-<a href="#Blank_identifier">blank</a> names in the signature must be <a href="#Uniqueness_of_identifiers">unique</a>. If absent, each type stands for one item of that type. Parameter and result lists are always parenthesized except that if there is exactly one unnamed result it may be written as an unparenthesized type. </p> <p> The final incoming parameter in a function signature may have a type prefixed with <code>...</code>. A function with such a parameter is called <i>variadic</i> and may be invoked with zero or more arguments for that parameter. </p> <pre> func() func(x int) int func(a, _ int, z float32) bool func(a, b int, z float32) (bool) func(prefix string, values ...int) func(a, b int, z float64, opt ...interface{}) (success bool) func(int, int, float64) (float64, []int) func(n int) func(p T) </pre> <h3 id="Interface_types">Interface types</h3> <p> An interface type specifies a <a href="#Method_sets">method set</a> called its <i>interface</i>. A variable of interface type can store a value of any type with a method set that is any superset of the interface. Such a type is said to <i>implement the interface</i>. The value of an uninitialized variable of interface type is <code>nil</code>. </p> <pre class="ebnf"> InterfaceType = "interface" "{" { ( MethodSpec \| InterfaceTypeName ) ";" } "}" . MethodSpec = MethodName Signature . MethodName = identifier . InterfaceTypeName = TypeName . </pre> <p> An interface type may specify methods <i>explicitly</i> through method specifications, or it may <i>embed</i> methods of other interfaces through interface type names. </p> <pre> // A simple File interface. interface { Read([]byte) (int, error) Write([]byte) (int, error) Close() error } </pre> <p> The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a> and not <a href="#Blank_identifier">blank</a>. </p> <pre> interface { String() string String() string // illegal: String not unique _(x int) // illegal: method must have non-blank name } </pre> <p> More than one type may implement an interface. For instance, if two types <code>S1</code> and <code>S2</code> have the method set </p> <pre> func (p T) Read(p []byte) (n int, err error) func (p T) Write(p []byte) (n int, err error) func (p T) Close() error </pre> <p> (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>) then the <code>File</code> interface is implemented by both <code>S1</code> and <code>S2</code>, regardless of what other methods <code>S1</code> and <code>S2</code> may have or share. </p> <p> A type implements any interface comprising any subset of its methods and may therefore implement several distinct interfaces. For instance, all types implement the <i>empty interface</i>: </p> <pre> interface{} </pre> <p> Similarly, consider this interface specification, which appears within a <a href="#Type_declarations">type declaration</a> to define an interface called <code>Locker</code>: </p> <pre> type Locker interface { Lock() Unlock() } </pre> <p> If <code>S1</code> and <code>S2</code> also implement </p> <pre> func (p T) Lock() { … } func (p T) Unlock() { … } </pre> <p> they implement the <code>Locker</code> interface as well as the <code>File</code> interface. </p> <p> An interface <code>T</code> may use a (possibly qualified) interface type name <code>E</code> in place of a method specification. This is called <i>embedding</i> interface <code>E</code> in <code>T</code>. The <a href="#Method_sets">method set</a> of <code>T</code> is the <i>union</i> of the method sets of <code>T</code>’s explicitly declared methods and of <code>T</code>’s embedded interfaces. </p> <pre> type Reader interface { Read(p []byte) (n int, err error) Close() error } type Writer interface { Write(p []byte) (n int, err error) Close() error } // ReadWriter's methods are Read, Write, and Close. type ReadWriter interface { Reader // includes methods of Reader in ReadWriter's method set Writer // includes methods of Writer in ReadWriter's method set } </pre> <p> A <i>union</i> of method sets contains the (exported and non-exported) methods of each method set exactly once, and methods with the <a href="#Uniqueness_of_identifiers">same</a> names must have <a href="#Type_identity">identical</a> signatures. </p> <pre> type ReadCloser interface { Reader // includes methods of Reader in ReadCloser's method set Close() // illegal: signatures of Reader.Close and Close are different } </pre> <p> An interface type <code>T</code> may not embed itself or any interface type that embeds <code>T</code>, recursively. </p> <pre> // illegal: Bad cannot embed itself type Bad interface { Bad } // illegal: Bad1 cannot embed itself using Bad2 type Bad1 interface { Bad2 } type Bad2 interface { Bad1 } </pre> <h3 id="Map_types">Map types</h3> <p> A map is an unordered group of elements of one type, called the element type, indexed by a set of unique <i>keys</i> of another type, called the key type. The value of an uninitialized map is <code>nil</code>. </p> <pre class="ebnf"> MapType = "map" "[" KeyType "]" ElementType . KeyType = Type . </pre> <p> The <a href="#Comparison_operators">comparison operators</a> <code>==</code> and <code>!=</code> must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a <a href="#Run_time_panics">run-time panic</a>. </p> <pre> map[string]int map[T]struct{ x, y float64 } map[string]interface{} </pre> <p> The number of map elements is called its length. For a map <code>m</code>, it can be discovered using the built-in function <a href="#Length_and_capacity"><code>len</code></a> and may change during execution. Elements may be added during execution using <a href="#Assignments">assignments</a> and retrieved with <a href="#Index_expressions">index expressions</a>; they may be removed with the <a href="#Deletion_of_map_elements"><code>delete</code></a> built-in function. </p> <p> A new, empty map value is made using the built-in function <a href="#Making_slices_maps_and_channels"><code>make</code></a>, which takes the map type and an optional capacity hint as arguments: </p> <pre> make(map[string]int) make(map[string]int, 100) </pre> <p> The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of <code>nil</code> maps. A <code>nil</code> map is equivalent to an empty map except that no elements may be added. </p> <h3 id="Channel_types">Channel types</h3> <p> A channel provides a mechanism for <a href="#Go_statements">concurrently executing functions</a> to communicate by <a href="#Send_statements">sending</a> and <a href="#Receive_operator">receiving</a> values of a specified element type. The value of an uninitialized channel is <code>nil</code>. </p> <pre class="ebnf"> ChannelType = ( "chan" \| "chan" "<-" \| "<-" "chan" ) ElementType . </pre> <p> The optional <code><-</code> operator specifies the channel <i>direction</i>, <i>send</i> or <i>receive</i>. If no direction is given, the channel is <i>bidirectional</i>. A channel may be constrained only to send or only to receive by <a href="#Assignments">assignment</a> or explicit <a href="#Conversions">conversion</a>. </p> <pre> chan T // can be used to send and receive values of type T chan<- float64 // can only be used to send float64s <-chan int // can only be used to receive ints </pre> <p> The <code><-</code> operator associates with the leftmost <code>chan</code> possible: </p> <pre> chan<- chan int // same as chan<- (chan int) chan<- <-chan int // same as chan<- (<-chan int) <-chan <-chan int // same as <-chan (<-chan int) chan (<-chan int) </pre> <p> A new, initialized channel value can be made using the built-in function <a href="#Making_slices_maps_and_channels"><code>make</code></a>, which takes the channel type and an optional <i>capacity</i> as arguments: </p> <pre> make(chan int, 100) </pre> <p> The capacity, in number of elements, sets the size of the buffer in the channel. If the capacity is zero or absent, the channel is unbuffered and communication succeeds only when both a sender and receiver are ready. Otherwise, the channel is buffered and communication succeeds without blocking if the buffer is not full (sends) or not empty (receives). A <code>nil</code> channel is never ready for communication. </p> <p> A channel may be closed with the built-in function <a href="#Close"><code>close</code></a>. The multi-valued assignment form of the <a href="#Receive_operator">receive operator</a> reports whether a received value was sent before the channel was closed. </p> <p> A single channel may be used in <a href="#Send_statements">send statements</a>, <a href="#Receive_operator">receive operations</a>, and calls to the built-in functions <a href="#Length_and_capacity"><code>cap</code></a> and <a href="#Length_and_capacity"><code>len</code></a> by any number of goroutines without further synchronization. Channels act as first-in-first-out queues. For example, if one goroutine sends values on a channel and a second goroutine receives them, the values are received in the order sent. </p> <h2 id="Properties_of_types_and_values">Properties of types and values</h2> <h3 id="Type_identity">Type identity</h3> <p> Two types are either <i>identical</i> or <i>different</i>. </p> <p> A <a href="#Type_definitions">defined type</a> is always different from any other type. Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are structurally equivalent; that is, they have the same literal structure and corresponding components have identical types. In detail: </p> <ul> <li>Two array types are identical if they have identical element types and the same array length.</li> <li>Two slice types are identical if they have identical element types.</li> <li>Two struct types are identical if they have the same sequence of fields, and if corresponding fields have the same names, and identical types, and identical tags. <a href="#Exported_identifiers">Non-exported</a> field names from different packages are always different.</li> <li>Two pointer types are identical if they have identical base types.</li> <li>Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is. Parameter and result names are not required to match.</li> <li>Two interface types are identical if they have the same set of methods with the same names and identical function types. <a href="#Exported_identifiers">Non-exported</a> method names from different packages are always different. The order of the methods is irrelevant.</li> <li>Two map types are identical if they have identical key and element types.</li> <li>Two channel types are identical if they have identical element types and the same direction.</li> </ul> <p> Given the declarations </p> <pre> type ( A0 = []string A1 = A0 A2 = struct{ a, b int } A3 = int A4 = func(A3, float64) A0 A5 = func(x int, _ float64) []string ) type ( B0 A0 B1 []string B2 struct{ a, b int } B3 struct{ a, c int } B4 func(int, float64) B0 B5 func(x int, y float64) A1 ) type C0 = B0 </pre> <p> these types are identical: </p> <pre> A0, A1, and []string A2 and struct{ a, b int } A3 and int A4, func(int, float64) []string, and A5 B0 and C0 []int and []int struct{ a, b T5 } and struct{ a, b T5 } func(x int, y float64) []string, func(int, float64) (result []string), and A5 </pre> <p> <code>B0</code> and <code>B1</code> are different because they are new types created by distinct <a href="#Type_definitions">type definitions</a>; <code>func(int, float64) B0</code> and <code>func(x int, y float64) []string</code> are different because <code>B0</code> is different from <code>[]string</code>. </p> <h3 id="Assignability">Assignability</h3> <p> A value <code>x</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code> ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies: </p> <ul> <li> <code>x</code>'s type is identical to <code>T</code>. </li> <li> <code>x</code>'s type <code>V</code> and <code>T</code> have identical <a href="#Types">underlying types</a> and at least one of <code>V</code> or <code>T</code> is not a <a href="#Type_definitions">defined</a> type. </li> <li> <code>T</code> is an interface type and <code>x</code> <a href="#Interface_types">implements</a> <code>T</code>. </li> <li> <code>x</code> is a bidirectional channel value, <code>T</code> is a channel type, <code>x</code>'s type <code>V</code> and <code>T</code> have identical element types, and at least one of <code>V</code> or <code>T</code> is not a defined type. </li> <li> <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code> is a pointer, function, slice, map, channel, or interface type. </li> <li> <code>x</code> is an untyped <a href="#Constants">constant</a> <a href="#Representability">representable</a> by a value of type <code>T</code>. </li> </ul> <h3 id="Representability">Representability</h3> <p> A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i> by a value of type <code>T</code> if one of the following conditions applies: </p> <ul> <li> <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>. </li> <li> <code>T</code> is a floating-point type and <code>x</code> can be rounded to <code>T</code>'s precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE negative zero further simplified to an unsigned zero. Note that constant values never result in an IEEE negative zero, NaN, or infinity. </li> <li> <code>T</code> is a complex type, and <code>x</code>'s <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code> are representable by values of <code>T</code>'s component type (<code>float32</code> or <code>float64</code>). </li> </ul> <pre> x T x is representable by a value of T because 'a' byte 97 is in the set of byte values 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers "foo" string "foo" is in the set of string values 1024 int16 1024 is in the set of 16-bit integers 42.0 byte 42 is in the set of unsigned 8-bit integers 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0 0i int 0 is an integer value (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values </pre> <pre> x T x is not representable by a value of T because 0 bool 0 is not in the set of boolean values 'a' string 'a' is a rune, it is not in the set of string values 1024 byte 1024 is not in the set of unsigned 8-bit integers -1 uint16 -1 is not in the set of unsigned 16-bit integers 1.1 int 1.1 is not an integer value 42i float32 (0 + 42i) is not in the set of float32 values 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding </pre> <h2 id="Blocks">Blocks</h2> <p> A <i>block</i> is a possibly empty sequence of declarations and statements within matching brace brackets. </p> <pre class="ebnf"> Block = "{" StatementList "}" . StatementList = { Statement ";" } . </pre> <p> In addition to explicit blocks in the source code, there are implicit blocks: </p> <ol> <li>The <i>universe block</i> encompasses all Go source text.</li> <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all Go source text for that package.</li> <li>Each file has a <i>file block</i> containing all Go source text in that file.</li> <li>Each <a href="#If_statements">"if"</a>, <a href="#For_statements">"for"</a>, and <a href="#Switch_statements">"switch"</a> statement is considered to be in its own implicit block.</li> <li>Each clause in a <a href="#Switch_statements">"switch"</a> or <a href="#Select_statements">"select"</a> statement acts as an implicit block.</li> </ol> <p> Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>. </p> <h2 id="Declarations_and_scope">Declarations and scope</h2> <p> A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a <a href="#Constant_declarations">constant</a>, <a href="#Type_declarations">type</a>, <a href="#Variable_declarations">variable</a>, <a href="#Function_declarations">function</a>, <a href="#Labeled_statements">label</a>, or <a href="#Import_declarations">package</a>. Every identifier in a program must be declared. No identifier may be declared twice in the same block, and no identifier may be declared in both the file and package block. </p> <p> The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier in a declaration, but it does not introduce a binding and thus is not declared. In the package block, the identifier <code>init</code> may only be used for <a href="#Package_initialization"><code>init</code> function</a> declarations, and like the blank identifier it does not introduce a new binding. </p> <pre class="ebnf"> Declaration = ConstDecl \| TypeDecl \| VarDecl . TopLevelDecl = Declaration \| FunctionDecl \| MethodDecl . </pre> <p> The <i>scope</i> of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package. </p> <p> Go is lexically scoped using <a href="#Blocks">blocks</a>: </p> <ol> <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li> <li>The scope of an identifier denoting a constant, type, variable, or function (but not method) declared at top level (outside any function) is the package block.</li> <li>The scope of the package name of an imported package is the file block of the file containing the import declaration.</li> <li>The scope of an identifier denoting a method receiver, function parameter, or result variable is the function body.</li> <li>The scope of a constant or variable identifier declared inside a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl for short variable declarations) and ends at the end of the innermost containing block.</li> <li>The scope of a type identifier declared inside a function begins at the identifier in the TypeSpec and ends at the end of the innermost containing block.</li> </ol> <p> An identifier declared in a block may be redeclared in an inner block. While the identifier of the inner declaration is in scope, it denotes the entity declared by the inner declaration. </p> <p> The <a href="#Package_clause">package clause</a> is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same <a href="#Packages">package</a> and to specify the default package name for import declarations. </p> <h3 id="Label_scopes">Label scopes</h3> <p> Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are used in the <a href="#Break_statements">"break"</a>, <a href="#Continue_statements">"continue"</a>, and <a href="#Goto_statements">"goto"</a> statements. It is illegal to define a label that is never used. In contrast to other identifiers, labels are not block scoped and do not conflict with identifiers that are not labels. The scope of a label is the body of the function in which it is declared and excludes the body of any nested function. </p> <h3 id="Blank_identifier">Blank identifier</h3> <p> The <i>blank identifier</i> is represented by the underscore character <code>_</code>. It serves as an anonymous placeholder instead of a regular (non-blank) identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>, as an <a href="#Operands">operand</a>, and in <a href="#Assignments">assignments</a>. </p> <h3 id="Predeclared_identifiers">Predeclared identifiers</h3> <p> The following identifiers are implicitly declared in the <a href="#Blocks">universe block</a>: </p> <pre class="grammar"> Types: bool byte complex64 complex128 error float32 float64 int int8 int16 int32 int64 rune string uint uint8 uint16 uint32 uint64 uintptr Constants: true false iota Zero value: nil Functions: append cap close complex copy delete imag len make new panic print println real recover </pre> <h3 id="Exported_identifiers">Exported identifiers</h3> <p> An identifier may be <i>exported</i> to permit access to it from another package. An identifier is exported if both: </p> <ol> <li>the first character of the identifier's name is a Unicode upper case letter (Unicode class "Lu"); and</li> <li>the identifier is declared in the <a href="#Blocks">package block</a> or it is a <a href="#Struct_types">field name</a> or <a href="#MethodName">method name</a>.</li> </ol> <p> All other identifiers are not exported. </p> <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3> <p> Given a set of identifiers, an identifier is called <i>unique</i> if it is <i>different</i> from every other in the set. Two identifiers are different if they are spelled differently, or if they appear in different <a href="#Packages">packages</a> and are not <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same. </p> <h3 id="Constant_declarations">Constant declarations</h3> <p> A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>. The number of identifiers must be equal to the number of expressions, and the <i>n</i>th identifier on the left is bound to the value of the <i>n</i>th expression on the right. </p> <pre class="ebnf"> ConstDecl = "const" ( ConstSpec \| "(" { ConstSpec ";" } ")" ) . ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] . IdentifierList = identifier { "," identifier } . ExpressionList = Expression { "," Expression } . </pre> <p> If the type is present, all constants take the type specified, and the expressions must be <a href="#Assignability">assignable</a> to that type. If the type is omitted, the constants take the individual types of the corresponding expressions. If the expression values are untyped <a href="#Constants">constants</a>, the declared constants remain untyped and the constant identifiers denote the constant values. For instance, if the expression is a floating-point literal, the constant identifier denotes a floating-point constant, even if the literal's fractional part is zero. </p> <pre> const Pi float64 = 3.14159265358979323846 const zero = 0.0 // untyped floating-point constant const ( size int64 = 1024 eof = -1 // untyped integer constant ) const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants const u, v float32 = 0, 3 // u = 0.0, v = 3.0 </pre> <p> Within a parenthesized <code>const</code> declaration list the expression list may be omitted from any but the first ConstSpec. Such an empty list is equivalent to the textual substitution of the first preceding non-empty expression list and its type if any. Omitting the list of expressions is therefore equivalent to repeating the previous list. The number of identifiers must be equal to the number of expressions in the previous list. Together with the <a href="#Iota"><code>iota</code> constant generator</a> this mechanism permits light-weight declaration of sequential values: </p> <pre> const ( Sunday = iota Monday Tuesday Wednesday Thursday Friday Partyday numberOfDays // this constant is not exported ) </pre> <h3 id="Iota">Iota</h3> <p> Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier <code>iota</code> represents successive untyped integer <a href="#Constants"> constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a> in that constant declaration, starting at zero. It can be used to construct a set of related constants: </p> <pre> const ( c0 = iota // c0 == 0 c1 = iota // c1 == 1 c2 = iota // c2 == 2 ) const ( a = 1 << iota // a == 1 (iota == 0) b = 1 << iota // b == 2 (iota == 1) c = 3 // c == 3 (iota == 2, unused) d = 1 << iota // d == 8 (iota == 3) ) const ( u = iota * 42 // u == 0 (untyped integer constant) v float64 = iota * 42 // v == 42.0 (float64 constant) w = iota * 42 // w == 84 (untyped integer constant) ) const x = iota // x == 0 const y = iota // y == 0 </pre> <p> By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value: </p> <pre> const ( bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0) bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1) _, _ // (iota == 2, unused) bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3) ) </pre> <p> This last example exploits the <a href="#Constant_declarations">implicit repetition</a> of the last non-empty expression list. </p> <h3 id="Type_declarations">Type declarations</h3> <p> A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>. Type declarations come in two forms: alias declarations and type definitions. </p> <pre class="ebnf"> TypeDecl = "type" ( TypeSpec \| "(" { TypeSpec ";" } ")" ) . TypeSpec = AliasDecl \| TypeDef . </pre> <h4 id="Alias_declarations">Alias declarations</h4> <p> An alias declaration binds an identifier to the given type. </p> <pre class="ebnf"> AliasDecl = identifier "=" Type . </pre> <p> Within the <a href="#Declarations_and_scope">scope</a> of the identifier, it serves as an <i>alias</i> for the type. </p> <pre> type ( nodeList = []Node // nodeList and []Node are identical types Polar = polar // Polar and polar denote identical types ) </pre> <h4 id="Type_definitions">Type definitions</h4> <p> A type definition creates a new, distinct type with the same <a href="#Types">underlying type</a> and operations as the given type, and binds an identifier to it. </p> <pre class="ebnf"> TypeDef = identifier Type . </pre> <p> The new type is called a <i>defined type</i>. It is <a href="#Type_identity">different</a> from any other type, including the type it is created from. </p> <pre> type ( Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types polar Point // polar and Point denote different types ) type TreeNode struct { left, right TreeNode value Comparable } type Block interface { BlockSize() int Encrypt(src, dst []byte) Decrypt(src, dst []byte) } </pre> <p> A defined type may have <a href="#Method_declarations">methods</a> associated with it. It does not inherit any methods bound to the given type, but the <a href="#Method_sets">method set</a> of an interface type or of elements of a composite type remains unchanged: </p> <pre> // A Mutex is a data type with two methods, Lock and Unlock. type Mutex struct { /* Mutex fields / } func (m Mutex) Lock() { /* Lock implementation / } func (m Mutex) Unlock() { /* Unlock implementation / } // NewMutex has the same composition as Mutex but its method set is empty. type NewMutex Mutex // The method set of PtrMutex's underlying type Mutex remains unchanged, // but the method set of PtrMutex is empty. type PtrMutex Mutex // The method set of PrintableMutex contains the methods // Lock and Unlock bound to its embedded field Mutex. type PrintableMutex struct { Mutex } // MyBlock is an interface type that has the same method set as Block. type MyBlock Block </pre> <p> Type definitions may be used to define different boolean, numeric, or string types and associate methods with them: </p> <pre> type TimeZone int const ( EST TimeZone = -(5 + iota) CST MST PST ) func (tz TimeZone) String() string { return fmt.Sprintf("GMT%+dh", tz) } </pre> <h3 id="Variable_declarations">Variable declarations</h3> <p> A variable declaration creates one or more <a href="#Variables">variables</a>, binds corresponding identifiers to them, and gives each a type and an initial value. </p> <pre class="ebnf"> VarDecl = "var" ( VarSpec \| "(" { VarSpec ";" } ")" ) . VarSpec = IdentifierList ( Type [ "=" ExpressionList ] \| "=" ExpressionList ) . </pre> <pre> var i int var U, V, W float64 var k = 0 var x, y float32 = -1, -2 var ( i int u, v, s = 2.0, 3.0, "bar" ) var re, im = complexSqrt(-1) var _, found = entries[name] // map lookup; only interested in "found" </pre> <p> If a list of expressions is given, the variables are initialized with the expressions following the rules for <a href="#Assignments">assignments</a>. Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>. </p> <p> If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment. If that value is an untyped constant, it is first implicitly <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>; if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>. The predeclared value <code>nil</code> cannot be used to initialize a variable with no explicit type. </p> <pre> var d = math.Sin(0.5) // d is float64 var i = 42 // i is int var t, ok = x.(T) // t is T, ok is bool var n = nil // illegal </pre> <p> Implementation restriction: A compiler may make it illegal to declare a variable inside a <a href="#Function_declarations">function body</a> if the variable is never used. </p> <h3 id="Short_variable_declarations">Short variable declarations</h3> <p> A <i>short variable declaration</i> uses the syntax: </p> <pre class="ebnf"> ShortVarDecl = IdentifierList ":=" ExpressionList . </pre> <p> It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a> with initializer expressions but no types: </p> <pre class="grammar"> "var" IdentifierList = ExpressionList . </pre> <pre> i, j := 0, 10 f := func() int { return 7 } ch := make(chan int) r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate </pre> <p> Unlike regular variable declarations, a short variable declaration may <i>redeclare</i> variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original. </p> <pre> field1, offset := nextField(str, 0) field2, offset := nextField(str, offset) // redeclares offset a, a := 1, 2 // illegal: double declaration of a or no new variable if a was declared elsewhere </pre> <p> Short variable declarations may appear only inside functions. In some contexts such as the initializers for <a href="#If_statements">"if"</a>, <a href="#For_statements">"for"</a>, or <a href="#Switch_statements">"switch"</a> statements, they can be used to declare local temporary variables. </p> <h3 id="Function_declarations">Function declarations</h3> <p> A function declaration binds an identifier, the <i>function name</i>, to a function. </p> <pre class="ebnf"> FunctionDecl = "func" FunctionName Signature [ FunctionBody ] . FunctionName = identifier . FunctionBody = Block . </pre> <p> If the function's <a href="#Function_types">signature</a> declares result parameters, the function body's statement list must end in a <a href="#Terminating_statements">terminating statement</a>. </p> <pre> func IndexRune(s string, r rune) int { for i, c := range s { if c == r { return i } } // invalid: missing return statement } </pre> <p> A function declaration may omit the body. Such a declaration provides the signature for a function implemented outside Go, such as an assembly routine. </p> <pre> func min(x int, y int) int { if x < y { return x } return y } func flushICache(begin, end uintptr) // implemented externally </pre> <h3 id="Method_declarations">Method declarations</h3> <p> A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>. A method declaration binds an identifier, the <i>method name</i>, to a method, and associates the method with the receiver's <i>base type</i>. </p> <pre class="ebnf"> MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] . Receiver = Parameters . </pre> <p> The receiver is specified via an extra parameter section preceding the method name. That parameter section must declare a single non-variadic parameter, the receiver. Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a pointer to a defined type <code>T</code>. <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be a pointer or interface type and it must be defined in the same package as the method. The method is said to be <i>bound</i> to its receiver base type and the method name is visible only within <a href="#Selectors">selectors</a> for type <code>T</code> or <code>T</code>. </p> <p> A non-<a href="#Blank_identifier">blank</a> receiver identifier must be <a href="#Uniqueness_of_identifiers">unique</a> in the method signature. If the receiver's value is not referenced inside the body of the method, its identifier may be omitted in the declaration. The same applies in general to parameters of functions and methods. </p> <p> For a base type, the non-blank names of methods bound to it must be unique. If the base type is a <a href="#Struct_types">struct type</a>, the non-blank method and field names must be distinct. </p> <p> Given defined type <code>Point</code>, the declarations </p> <pre> func (p Point) Length() float64 { return math.Sqrt(p.x * p.x + p.y * p.y) } func (p Point) Scale(factor float64) { p.x = factor p.y = factor } </pre> <p> bind the methods <code>Length</code> and <code>Scale</code>, with receiver type <code>Point</code>, to the base type <code>Point</code>. </p> <p> The type of a method is the type of a function with the receiver as first argument. For instance, the method <code>Scale</code> has type </p> <pre> func(p Point, factor float64) </pre> <p> However, a function declared this way is not a method. </p> <h2 id="Expressions">Expressions</h2> <p> An expression specifies the computation of a value by applying operators and functions to operands. </p> <h3 id="Operands">Operands</h3> <p> Operands denote the elementary values in an expression. An operand may be a literal, a (possibly <a href="#Qualified_identifiers">qualified</a>) non-<a href="#Blank_identifier">blank</a> identifier denoting a <a href="#Constant_declarations">constant</a>, <a href="#Variable_declarations">variable</a>, or <a href="#Function_declarations">function</a>, or a parenthesized expression. </p> <p> The <a href="#Blank_identifier">blank identifier</a> may appear as an operand only on the left-hand side of an <a href="#Assignments">assignment</a>. </p> <pre class="ebnf"> Operand = Literal \| OperandName \| "(" Expression ")" . Literal = BasicLit \| CompositeLit \| FunctionLit . BasicLit = int_lit \| float_lit \| imaginary_lit \| rune_lit \| string_lit . OperandName = identifier \| QualifiedIdent . </pre> <h3 id="Qualified_identifiers">Qualified identifiers</h3> <p> A qualified identifier is an identifier qualified with a package name prefix. Both the package name and the identifier must not be <a href="#Blank_identifier">blank</a>. </p> <pre class="ebnf"> QualifiedIdent = PackageName "." identifier . </pre> <p> A qualified identifier accesses an identifier in a different package, which must be <a href="#Import_declarations">imported</a>. The identifier must be <a href="#Exported_identifiers">exported</a> and declared in the <a href="#Blocks">package block</a> of that package. </p> <pre> math.Sin // denotes the Sin function in package math </pre> <h3 id="Composite_literals">Composite literals</h3> <p> Composite literals construct values for structs, arrays, slices, and maps and create a new value each time they are evaluated. They consist of the type of the literal followed by a brace-bound list of elements. Each element may optionally be preceded by a corresponding key. </p> <pre class="ebnf"> CompositeLit = LiteralType LiteralValue . LiteralType = StructType \| ArrayType \| "[" "..." "]" ElementType \| SliceType \| MapType \| TypeName . LiteralValue = "{" [ ElementList [ "," ] ] "}" . ElementList = KeyedElement { "," KeyedElement } . KeyedElement = [ Key ":" ] Element . Key = FieldName \| Expression \| LiteralValue . FieldName = identifier . Element = Expression \| LiteralValue . </pre> <p> The LiteralType's underlying type must be a struct, array, slice, or map type (the grammar enforces this constraint except when the type is given as a TypeName). The types of the elements and keys must be <a href="#Assignability">assignable</a> to the respective field, element, and key types of the literal type; there is no additional conversion. The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key. It is an error to specify multiple elements with the same field name or constant key value. For non-constant map keys, see the section on <a href="#Order_of_evaluation">evaluation order</a>. </p> <p> For struct literals the following rules apply: </p> <ul> <li>A key must be a field name declared in the struct type. </li> <li>An element list that does not contain any keys must list an element for each struct field in the order in which the fields are declared. </li> <li>If any element has a key, every element must have a key. </li> <li>An element list that contains keys does not need to have an element for each struct field. Omitted fields get the zero value for that field. </li> <li>A literal may omit the element list; such a literal evaluates to the zero value for its type. </li> <li>It is an error to specify an element for a non-exported field of a struct belonging to a different package. </li> </ul> <p> Given the declarations </p> <pre> type Point3D struct { x, y, z float64 } type Line struct { p, q Point3D } </pre> <p> one may write </p> <pre> origin := Point3D{} // zero value for Point3D line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x </pre> <p> For array and slice literals the following rules apply: </p> <ul> <li>Each element has an associated integer index marking its position in the array. </li> <li>An element with a key uses the key as its index. The key must be a non-negative constant <a href="#Representability">representable</a> by a value of type <code>int</code>; and if it is typed it must be of integer type. </li> <li>An element without a key uses the previous element's index plus one. If the first element has no key, its index is zero. </li> </ul> <p> <a href="#Address_operators">Taking the address</a> of a composite literal generates a pointer to a unique <a href="#Variables">variable</a> initialized with the literal's value. </p> <pre> var pointer Point3D = &Point3D{y: 1000} </pre> <p> Note that the <a href="#The_zero_value">zero value</a> for a slice or map type is not the same as an initialized but empty value of the same type. Consequently, taking the address of an empty slice or map composite literal does not have the same effect as allocating a new slice or map value with <a href="#Allocation">new</a>. </p> <pre> p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0 </pre> <p> The length of an array literal is the length specified in the literal type. If fewer elements than the length are provided in the literal, the missing elements are set to the zero value for the array element type. It is an error to provide elements with index values outside the index range of the array. The notation <code>...</code> specifies an array length equal to the maximum element index plus one. </p> <pre> buffer := [10]string{} // len(buffer) == 10 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6 days := [...]string{"Sat", "Sun"} // len(days) == 2 </pre> <p> A slice literal describes the entire underlying array literal. Thus the length and capacity of a slice literal are the maximum element index plus one. A slice literal has the form </p> <pre> []T{x1, x2, … xn} </pre> <p> and is shorthand for a slice operation applied to an array: </p> <pre> tmp := [n]T{x1, x2, … xn} tmp[0 : n] </pre> <p> Within a composite literal of array, slice, or map type <code>T</code>, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of <code>T</code>. Similarly, elements or keys that are addresses of composite literals may elide the <code>&T</code> when the element or key type is <code>T</code>. </p> <pre> [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}} [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}} [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}} map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}} map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"} type PPoint Point [2]Point{{1.5, -3.5}, {}} // same as [2]Point{&Point{1.5, -3.5}, &Point{}} [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})} </pre> <p> A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the <a href="#Keywords">keyword</a> and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses. </p> <pre> if x == (T{a,b,c}[i]) { … } if (x == T{a,b,c}[i]) { … } </pre> <p> Examples of valid array, slice, and map literals: </p> <pre> // list of prime numbers primes := []int{2, 3, 5, 7, 9, 2147483647} // vowels[ch] is true if ch is a vowel vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true} // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1} filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1} // frequencies in Hz for equal-tempered scale (A4 = 440Hz) noteFrequency := map[string]float32{ "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83, "G0": 24.50, "A0": 27.50, "B0": 30.87, } </pre> <h3 id="Function_literals">Function literals</h3> <p> A function literal represents an anonymous <a href="#Function_declarations">function</a>. </p> <pre class="ebnf"> FunctionLit = "func" Signature FunctionBody . </pre> <pre> func(a, b int, z float64) bool { return ab < int(z) } </pre> <p> A function literal can be assigned to a variable or invoked directly. </p> <pre> f := func(x, y int) int { return x + y } func(ch chan int) { ch <- ACK }(replyChan) </pre> <p> Function literals are <i>closures</i>: they may refer to variables defined in a surrounding function. Those variables are then shared between the surrounding function and the function literal, and they survive as long as they are accessible. </p> <h3 id="Primary_expressions">Primary expressions</h3> <p> Primary expressions are the operands for unary and binary expressions. </p> <pre class="ebnf"> PrimaryExpr = Operand \| Conversion \| MethodExpr \| PrimaryExpr Selector \| PrimaryExpr Index \| PrimaryExpr Slice \| PrimaryExpr TypeAssertion \| PrimaryExpr Arguments . Selector = "." identifier . Index = "[" Expression "]" . Slice = "[" [ Expression ] ":" [ Expression ] "]" \| "[" [ Expression ] ":" Expression ":" Expression "]" . TypeAssertion = "." "(" Type ")" . Arguments = "(" [ ( ExpressionList \| Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" . </pre> <pre> x 2 (s + ".txt") f(3.1415, true) Point{1, 2} m["foo"] s[i : j + 1] obj.color f.p[i].x() </pre> <h3 id="Selectors">Selectors</h3> <p> For a <a href="#Primary_expressions">primary expression</a> <code>x</code> that is not a <a href="#Package_clause">package name</a>, the <i>selector expression</i> </p> <pre> x.f </pre> <p> denotes the field or method <code>f</code> of the value <code>x</code> (or sometimes <code>x</code>; see below). The identifier <code>f</code> is called the (field or method) <i>selector</i>; it must not be the <a href="#Blank_identifier">blank identifier</a>. The type of the selector expression is the type of <code>f</code>. If <code>x</code> is a package name, see the section on <a href="#Qualified_identifiers">qualified identifiers</a>. </p> <p> A selector <code>f</code> may denote a field or method <code>f</code> of a type <code>T</code>, or it may refer to a field or method <code>f</code> of a nested <a href="#Struct_types">embedded field</a> of <code>T</code>. The number of embedded fields traversed to reach <code>f</code> is called its <i>depth</i> in <code>T</code>. The depth of a field or method <code>f</code> declared in <code>T</code> is zero. The depth of a field or method <code>f</code> declared in an embedded field <code>A</code> in <code>T</code> is the depth of <code>f</code> in <code>A</code> plus one. </p> <p> The following rules apply to selectors: </p> <ol> <li> For a value <code>x</code> of type <code>T</code> or <code>T</code> where <code>T</code> is not a pointer or interface type, <code>x.f</code> denotes the field or method at the shallowest depth in <code>T</code> where there is such an <code>f</code>. If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a> with shallowest depth, the selector expression is illegal. </li> <li> For a value <code>x</code> of type <code>I</code> where <code>I</code> is an interface type, <code>x.f</code> denotes the actual method with name <code>f</code> of the dynamic value of <code>x</code>. If there is no method with name <code>f</code> in the <a href="#Method_sets">method set</a> of <code>I</code>, the selector expression is illegal. </li> <li> As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a> pointer type and <code>(x).f</code> is a valid selector expression denoting a field (but not a method), <code>x.f</code> is shorthand for <code>(x).f</code>. </li> <li> In all other cases, <code>x.f</code> is illegal. </li> <li> If <code>x</code> is of pointer type and has the value <code>nil</code> and <code>x.f</code> denotes a struct field, assigning to or evaluating <code>x.f</code> causes a <a href="#Run_time_panics">run-time panic</a>. </li> <li> If <code>x</code> is of interface type and has the value <code>nil</code>, <a href="#Calls">calling</a> or <a href="#Method_values">evaluating</a> the method <code>x.f</code> causes a <a href="#Run_time_panics">run-time panic</a>. </li> </ol> <p> For example, given the declarations: </p> <pre> type T0 struct { x int } func (T0) M0() type T1 struct { y int } func (T1) M1() type T2 struct { z int T1 T0 } func (T2) M2() type Q T2 var t T2 // with t.T0 != nil var p T2 // with p != nil and (p).T0 != nil var q Q = p </pre> <p> one may write: </p> <pre> t.z // t.z t.y // t.T1.y t.x // (t.T0).x p.z // (p).z p.y // (p).T1.y p.x // ((p).T0).x q.x // ((q).T0).x (q).x is a valid field selector p.M0() // ((p).T0).M0() M0 expects T0 receiver p.M1() // ((p).T1).M1() M1 expects T1 receiver p.M2() // p.M2() M2 expects T2 receiver t.M2() // (&t).M2() M2 expects T2 receiver, see section on Calls </pre> <p> but the following is invalid: </p> <pre> q.M0() // (q).M0 is valid but not a field selector </pre> <h3 id="Method_expressions">Method expressions</h3> <p> If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>, <code>T.M</code> is a function that is callable as a regular function with the same arguments as <code>M</code> prefixed by an additional argument that is the receiver of the method. </p> <pre class="ebnf"> MethodExpr = ReceiverType "." MethodName . ReceiverType = Type . </pre> <p> Consider a struct type <code>T</code> with two methods, <code>Mv</code>, whose receiver is of type <code>T</code>, and <code>Mp</code>, whose receiver is of type <code>T</code>. </p> <pre> type T struct { a int } func (tv T) Mv(a int) int { return 0 } // value receiver func (tp T) Mp(f float32) float32 { return 1 } // pointer receiver var t T </pre> <p> The expression </p> <pre> T.Mv </pre> <p> yields a function equivalent to <code>Mv</code> but with an explicit receiver as its first argument; it has signature </p> <pre> func(tv T, a int) int </pre> <p> That function may be called normally with an explicit receiver, so these five invocations are equivalent: </p> <pre> t.Mv(7) T.Mv(t, 7) (T).Mv(t, 7) f1 := T.Mv; f1(t, 7) f2 := (T).Mv; f2(t, 7) </pre> <p> Similarly, the expression </p> <pre> (T).Mp </pre> <p> yields a function value representing <code>Mp</code> with signature </p> <pre> func(tp T, f float32) float32 </pre> <p> For a method with a value receiver, one can derive a function with an explicit pointer receiver, so </p> <pre> (T).Mv </pre> <p> yields a function value representing <code>Mv</code> with signature </p> <pre> func(tv T, a int) int </pre> <p> Such a function indirects through the receiver to create a value to pass as the receiver to the underlying method; the method does not overwrite the value whose address is passed in the function call. </p> <p> The final case, a value-receiver function for a pointer-receiver method, is illegal because pointer-receiver methods are not in the method set of the value type. </p> <p> Function values derived from methods are called with function call syntax; the receiver is provided as the first argument to the call. That is, given <code>f := T.Mv</code>, <code>f</code> is invoked as <code>f(t, 7)</code> not <code>t.f(7)</code>. To construct a function that binds the receiver, use a <a href="#Function_literals">function literal</a> or <a href="#Method_values">method value</a>. </p> <p> It is legal to derive a function value from a method of an interface type. The resulting function takes an explicit receiver of that interface type. </p> <h3 id="Method_values">Method values</h3> <p> If the expression <code>x</code> has static type <code>T</code> and <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>, <code>x.M</code> is called a <i>method value</i>. The method value <code>x.M</code> is a function value that is callable with the same arguments as a method call of <code>x.M</code>. The expression <code>x</code> is evaluated and saved during the evaluation of the method value; the saved copy is then used as the receiver in any calls, which may be executed later. </p> <pre> type S struct { T } type T int func (t T) M() { print(t) } t := new(T) s := S{T: t} f := t.M // receiver t is evaluated and stored in f g := s.M // receiver (s.T) is evaluated and stored in g t = 42 // does not affect stored receivers in f and g </pre> <p> The type <code>T</code> may be an interface or non-interface type. </p> <p> As in the discussion of <a href="#Method_expressions">method expressions</a> above, consider a struct type <code>T</code> with two methods, <code>Mv</code>, whose receiver is of type <code>T</code>, and <code>Mp</code>, whose receiver is of type <code>T</code>. </p> <pre> type T struct { a int } func (tv T) Mv(a int) int { return 0 } // value receiver func (tp T) Mp(f float32) float32 { return 1 } // pointer receiver var t T var pt T func makeT() T </pre> <p> The expression </p> <pre> t.Mv </pre> <p> yields a function value of type </p> <pre> func(int) int </pre> <p> These two invocations are equivalent: </p> <pre> t.Mv(7) f := t.Mv; f(7) </pre> <p> Similarly, the expression </p> <pre> pt.Mp </pre> <p> yields a function value of type </p> <pre> func(float32) float32 </pre> <p> As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(pt).Mv</code>. </p> <p> As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>. </p> <pre> f := t.Mv; f(7) // like t.Mv(7) f := pt.Mp; f(7) // like pt.Mp(7) f := pt.Mv; f(7) // like (pt).Mv(7) f := t.Mp; f(7) // like (&t).Mp(7) f := makeT().Mp // invalid: result of makeT() is not addressable </pre> <p> Although the examples above use non-interface types, it is also legal to create a method value from a value of interface type. </p> <pre> var i interface { M(int) } = myVal f := i.M; f(7) // like i.M(7) </pre> <h3 id="Index_expressions">Index expressions</h3> <p> A primary expression of the form </p> <pre> a[x] </pre> <p> denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>. The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively. The following rules apply: </p> <p> If <code>a</code> is not a map: </p> <ul> <li>the index <code>x</code> must be of integer type or an untyped constant</li> <li>a constant index must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code></li> <li>a constant index that is untyped is given type <code>int</code></li> <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>, otherwise it is <i>out of range</i></li> </ul> <p> For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>: </p> <ul> <li>a <a href="#Constants">constant</a> index must be in range</li> <li>if <code>x</code> is out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs</li> <li><code>a[x]</code> is the array element at index <code>x</code> and the type of <code>a[x]</code> is the element type of <code>A</code></li> </ul> <p> For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type: </p> <ul> <li><code>a[x]</code> is shorthand for <code>(a)[x]</code></li> </ul> <p> For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>: </p> <ul> <li>if <code>x</code> is out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs</li> <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of <code>a[x]</code> is the element type of <code>S</code></li> </ul> <p> For <code>a</code> of <a href="#String_types">string type</a>: </p> <ul> <li>a <a href="#Constants">constant</a> index must be in range if the string <code>a</code> is also constant</li> <li>if <code>x</code> is out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs</li> <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of <code>a[x]</code> is <code>byte</code></li> <li><code>a[x]</code> may not be assigned to</li> </ul> <p> For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>: </p> <ul> <li><code>x</code>'s type must be <a href="#Assignability">assignable</a> to the key type of <code>M</code></li> <li>if the map contains an entry with key <code>x</code>, <code>a[x]</code> is the map element with key <code>x</code> and the type of <code>a[x]</code> is the element type of <code>M</code></li> <li>if the map is <code>nil</code> or does not contain such an entry, <code>a[x]</code> is the <a href="#The_zero_value">zero value</a> for the element type of <code>M</code></li> </ul> <p> Otherwise <code>a[x]</code> is illegal. </p> <p> An index expression on a map <code>a</code> of type <code>map[K]V</code> used in an <a href="#Assignments">assignment</a> or initialization of the special form </p> <pre> v, ok = a[x] v, ok := a[x] var v, ok = a[x] </pre> <p> yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code> if the key <code>x</code> is present in the map, and <code>false</code> otherwise. </p> <p> Assigning to an element of a <code>nil</code> map causes a <a href="#Run_time_panics">run-time panic</a>. </p> <h3 id="Slice_expressions">Slice expressions</h3> <p> Slice expressions construct a substring or slice from a string, array, pointer to array, or slice. There are two variants: a simple form that specifies a low and high bound, and a full form that also specifies a bound on the capacity. </p> <h4>Simple slice expressions</h4> <p> For a string, array, pointer to array, or slice <code>a</code>, the primary expression </p> <pre> a[low : high] </pre> <p> constructs a substring or slice. The <i>indices</i> <code>low</code> and <code>high</code> select which elements of operand <code>a</code> appear in the result. The result has indices starting at 0 and length equal to <code>high</code> - <code>low</code>. After slicing the array <code>a</code> </p> <pre> a := [5]int{1, 2, 3, 4, 5} s := a[1:4] </pre> <p> the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements </p> <pre> s[0] == 2 s[1] == 3 s[2] == 4 </pre> <p> For convenience, any of the indices may be omitted. A missing <code>low</code> index defaults to zero; a missing <code>high</code> index defaults to the length of the sliced operand: </p> <pre> a[2:] // same as a[2 : len(a)] a[:3] // same as a[0 : 3] a[:] // same as a[0 : len(a)] </pre> <p> If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for <code>(a)[low : high]</code>. </p> <p> For arrays or strings, the indices are <i>in range</i> if <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>, otherwise they are <i>out of range</i>. For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length. A <a href="#Constants">constant</a> index must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; for arrays or constant strings, constant indices must also be in range. If both indices are constant, they must satisfy <code>low <= high</code>. If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <p> Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice, the result of the slice operation is a non-constant value of the same type as the operand. For untyped string operands the result is a non-constant value of type <code>string</code>. If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a> and the result of the slice operation is a slice with the same element type as the array. </p> <p> If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying array with the operand. </p> <pre> var a [10]int s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5] s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5] s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element </pre> <h4>Full slice expressions</h4> <p> For an array, pointer to array, or slice <code>a</code> (but not a string), the primary expression </p> <pre> a[low : high : max] </pre> <p> constructs a slice of the same type, and with the same length and elements as the simple slice expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0. After slicing the array <code>a</code> </p> <pre> a := [5]int{1, 2, 3, 4, 5} t := a[1:3:5] </pre> <p> the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements </p> <pre> t[0] == 2 t[1] == 3 </pre> <p> As for simple slice expressions, if <code>a</code> is a pointer to an array, <code>a[low : high : max]</code> is shorthand for <code>(a)[low : high : max]</code>. If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>. </p> <p> The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>, otherwise they are <i>out of range</i>. A <a href="#Constants">constant</a> index must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; for arrays, constant indices must also be in range. If multiple indices are constant, the constants that are present must be in range relative to each other. If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <h3 id="Type_assertions">Type assertions</h3> <p> For an expression <code>x</code> of <a href="#Interface_types">interface type</a> and a type <code>T</code>, the primary expression </p> <pre> x.(T) </pre> <p> asserts that <code>x</code> is not <code>nil</code> and that the value stored in <code>x</code> is of type <code>T</code>. The notation <code>x.(T)</code> is called a <i>type assertion</i>. </p> <p> More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a> to the type <code>T</code>. In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>; otherwise the type assertion is invalid since it is not possible for <code>x</code> to store a value of type <code>T</code>. If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type of <code>x</code> implements the interface <code>T</code>. </p> <p> If the type assertion holds, the value of the expression is the value stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false, a <a href="#Run_time_panics">run-time panic</a> occurs. In other words, even though the dynamic type of <code>x</code> is known only at run time, the type of <code>x.(T)</code> is known to be <code>T</code> in a correct program. </p> <pre> var x interface{} = 7 // x has dynamic type int and value 7 i := x.(int) // i has type int and value 7 type I interface { m() } func f(y I) { s := y.(string) // illegal: string does not implement I (missing method m) r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader … } </pre> <p> A type assertion used in an <a href="#Assignments">assignment</a> or initialization of the special form </p> <pre> v, ok = x.(T) v, ok := x.(T) var v, ok = x.(T) var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool </pre> <p> yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code> if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is the <a href="#The_zero_value">zero value</a> for type <code>T</code>. No <a href="#Run_time_panics">run-time panic</a> occurs in this case. </p> <h3 id="Calls">Calls</h3> <p> Given an expression <code>f</code> of function type <code>F</code>, </p> <pre> f(a1, a2, … an) </pre> <p> calls <code>f</code> with arguments <code>a1, a2, … an</code>. Except for one special case, arguments must be single-valued expressions <a href="#Assignability">assignable</a> to the parameter types of <code>F</code> and are evaluated before the function is called. The type of the expression is the result type of <code>F</code>. A method invocation is similar but the method itself is specified as a selector upon a value of the receiver type for the method. </p> <pre> math.Atan2(x, y) // function call var pt Point pt.Scale(3.5) // method call with receiver pt </pre> <p> In a function call, the function value and arguments are evaluated in <a href="#Order_of_evaluation">the usual order</a>. After they are evaluated, the parameters of the call are passed by value to the function and the called function begins execution. The return parameters of the function are passed by value back to the caller when the function returns. </p> <p> Calling a <code>nil</code> function value causes a <a href="#Run_time_panics">run-time panic</a>. </p> <p> As a special case, if the return values of a function or method <code>g</code> are equal in number and individually assignable to the parameters of another function or method <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code> will invoke <code>f</code> after binding the return values of <code>g</code> to the parameters of <code>f</code> in order. The call of <code>f</code> must contain no parameters other than the call of <code>g</code>, and <code>g</code> must have at least one return value. If <code>f</code> has a final <code>...</code> parameter, it is assigned the return values of <code>g</code> that remain after assignment of regular parameters. </p> <pre> func Split(s string, pos int) (string, string) { return s[0:pos], s[pos:] } func Join(s, t string) string { return s + t } if Join(Split(value, len(value)/2)) != value { log.Panic("test fails") } </pre> <p> A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a> of (the type of) <code>x</code> contains <code>m</code> and the argument list can be assigned to the parameter list of <code>m</code>. If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method set contains <code>m</code>, <code>x.m()</code> is shorthand for <code>(&x).m()</code>: </p> <pre> var p Point p.Scale(3.5) </pre> <p> There is no distinct method type and there are no method literals. </p> <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3> <p> If <code>f</code> is <a href="#Function_types">variadic</a> with a final parameter <code>p</code> of type <code>...T</code>, then within <code>f</code> the type of <code>p</code> is equivalent to type <code>[]T</code>. If <code>f</code> is invoked with no actual arguments for <code>p</code>, the value passed to <code>p</code> is <code>nil</code>. Otherwise, the value passed is a new slice of type <code>[]T</code> with a new underlying array whose successive elements are the actual arguments, which all must be <a href="#Assignability">assignable</a> to <code>T</code>. The length and capacity of the slice is therefore the number of arguments bound to <code>p</code> and may differ for each call site. </p> <p> Given the function and calls </p> <pre> func Greeting(prefix string, who ...string) Greeting("nobody") Greeting("hello:", "Joe", "Anna", "Eileen") </pre> <p> within <code>Greeting</code>, <code>who</code> will have the value <code>nil</code> in the first call, and <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second. </p> <p> If the final argument is assignable to a slice type <code>[]T</code> and is followed by <code>...</code>, it is passed unchanged as the value for a <code>...T</code> parameter. In this case no new slice is created. </p> <p> Given the slice <code>s</code> and call </p> <pre> s := []string{"James", "Jasmine"} Greeting("goodbye:", s...) </pre> <p> within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code> with the same underlying array. </p> <h3 id="Operators">Operators</h3> <p> Operators combine operands into expressions. </p> <pre class="ebnf"> Expression = UnaryExpr \| Expression binary_op Expression . UnaryExpr = PrimaryExpr \| unary_op UnaryExpr . binary_op = "\|\|" \| "&&" \| rel_op \| add_op \| mul_op . rel_op = "==" \| "!=" \| "<" \| "<=" \| ">" \| ">=" . add_op = "+" \| "-" \| "\|" \| "^" . mul_op = "" \| "/" \| "%" \| "<<" \| ">>" \| "&" \| "&^" . unary_op = "+" \| "-" \| "!" \| "^" \| "" \| "&" \| "<-" . </pre> <p> Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>. For other binary operators, the operand types must be <a href="#Type_identity">identical</a> unless the operation involves shifts or untyped <a href="#Constants">constants</a>. For operations involving constants only, see the section on <a href="#Constant_expressions">constant expressions</a>. </p> <p> Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a> and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a> to the type of the other operand. </p> <p> The right operand in a shift expression must have integer type or be an untyped constant <a href="#Representability">representable</a> by a value of type <code>uint</code>. If the left operand of a non-constant shift expression is an untyped constant, it is first implicitly converted to the type it would assume if the shift expression were replaced by its left operand alone. </p> <pre> var a [1024]byte var s uint = 33 // The results of the following examples are given for 64-bit ints. var i = 1<<s // 1 has type int var j int32 = 1<<s // 1 has type int32; j == 0 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33 var m int = 1.0<<s // 1.0 has type int; m == 1<<33 var n = 1.0<<s == j // 1.0 has type int32; n == true var o = 1<<s == 2<<s // 1 and 2 have type int; o == false var p = 1<<s == 1<<33 // 1 has type int; p == true var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift var v float32 = 1<<s // illegal: 1 has type float32, cannot shift var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33 // The results of the following examples are given for 32-bit ints, // which means the shifts will overflow. var mm int = 1.0<<s // 1.0 has type int; mm == 0 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int var xx = a[1.0<<s] // 1.0 has type int; xx == a[0] var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0 </pre> <h4 id="Operator_precedence">Operator precedence</h4> <p> Unary operators have the highest precedence. As the <code>++</code> and <code>--</code> operators form statements, not expressions, they fall outside the operator hierarchy. As a consequence, statement <code>p++</code> is the same as <code>(p)++</code>. </p> <p> There are five precedence levels for binary operators. Multiplication operators bind strongest, followed by addition operators, comparison operators, <code>&&</code> (logical AND), and finally <code>\|\|</code> (logical OR): </p> <pre class="grammar"> Precedence Operator 5 * / % << >> & &^ 4 + - \| ^ 3 == != < <= > >= 2 && 1 \|\| </pre> <p> Binary operators of the same precedence associate from left to right. For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>. </p> <pre> +x 23 + 3x[i] x <= f() ^a >> b f() \|\| g() x == y+1 && <-chanInt > 0 </pre> <h3 id="Arithmetic_operators">Arithmetic operators</h3> <p> Arithmetic operators apply to numeric values and yield a result of the same type as the first operand. The four standard arithmetic operators (<code>+</code>, <code>-</code>, <code></code>, <code>/</code>) apply to integer, floating-point, and complex types; <code>+</code> also applies to strings. The bitwise logical and shift operators apply to integers only. </p> <pre class="grammar"> + sum integers, floats, complex values, strings - difference integers, floats, complex values * product integers, floats, complex values / quotient integers, floats, complex values % remainder integers & bitwise AND integers \| bitwise OR integers ^ bitwise XOR integers &^ bit clear (AND NOT) integers << left shift integer << integer >= 0 >> right shift integer >> integer >= 0 </pre> <h4 id="Integer_operators">Integer operators</h4> <p> For two integer values <code>x</code> and <code>y</code>, the integer quotient <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following relationships: </p> <pre> x = qy + r and \|r\| < \|y\| </pre> <p> with <code>x / y</code> truncated towards zero (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>). </p> <pre> x y x / y x % y 5 3 1 2 -5 3 -1 -2 5 -3 -1 2 -5 -3 1 -2 </pre> <p> The one exception to this rule is that if the dividend <code>x</code> is the most negative value for the int type of <code>x</code>, the quotient <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>) due to two's-complement <a href="#Integer_overflow">integer overflow</a>: </p> <pre> x, q int8 -128 int16 -32768 int32 -2147483648 int64 -9223372036854775808 </pre> <p> If the divisor is a <a href="#Constants">constant</a>, it must not be zero. If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. If the dividend is non-negative and the divisor is a constant power of 2, the division may be replaced by a right shift, and computing the remainder may be replaced by a bitwise AND operation: </p> <pre> x x / 4 x % 4 x >> 2 x & 3 11 2 3 2 3 -11 -2 -3 -3 1 </pre> <p> The shift operators shift the left operand by the shift count specified by the right operand, which must be non-negative. If the shift count is negative at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. The shift operators implement arithmetic shifts if the left operand is a signed integer and logical shifts if it is an unsigned integer. There is no upper limit on the shift count. Shifts behave as if the left operand is shifted <code>n</code> times by 1 for a shift count of <code>n</code>. As a result, <code>x << 1</code> is the same as <code>x2</code> and <code>x >> 1</code> is the same as <code>x/2</code> but truncated towards negative infinity. </p> <p> For integer operands, the unary operators <code>+</code>, <code>-</code>, and <code>^</code> are defined as follows: </p> <pre class="grammar"> +x is 0 + x -x negation is 0 - x ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x and m = -1 for signed x </pre> <h4 id="Integer_overflow">Integer overflow</h4> <p> For unsigned integer values, the operations <code>+</code>, <code>-</code>, <code></code>, and <code><<</code> are computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of the <a href="#Numeric_types">unsigned integer</a>'s type. Loosely speaking, these unsigned integer operations discard high bits upon overflow, and programs may rely on "wrap around". </p> <p> For signed integers, the operations <code>+</code>, <code>-</code>, <code></code>, <code>/</code>, and <code><<</code> may legally overflow and the resulting value exists and is deterministically defined by the signed integer representation, the operation, and its operands. Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>. A compiler may not optimize code under the assumption that overflow does not occur. For instance, it may not assume that <code>x < x + 1</code> is always true. </p> <h4 id="Floating_point_operators">Floating-point operators</h4> <p> For floating-point and complex numbers, <code>+x</code> is the same as <code>x</code>, while <code>-x</code> is the negation of <code>x</code>. The result of a floating-point or complex division by zero is not specified beyond the IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a> occurs is implementation-specific. </p> <p> An implementation may combine multiple floating-point operations into a single fused operation, possibly across statements, and produce a result that differs from the value obtained by executing and rounding the instructions individually. An explicit floating-point type <a href="#Conversions">conversion</a> rounds to the precision of the target type, preventing fusion that would discard that rounding. </p> <p> For instance, some architectures provide a "fused multiply and add" (FMA) instruction that computes <code>xy + z</code> without rounding the intermediate result <code>xy</code>. These examples show when a Go implementation can use that instruction: </p> <pre> // FMA allowed for computing r, because xy is not explicitly rounded: r = xy + z r = z; r += xy t = xy; r = t + z p = xy; r = p + z r = xy + float64(z) // FMA disallowed for computing r, because it would omit rounding of xy: r = float64(xy) + z r = z; r += float64(xy) t = float64(xy); r = t + z </pre> <h4 id="String_concatenation">String concatenation</h4> <p> Strings can be concatenated using the <code>+</code> operator or the <code>+=</code> assignment operator: </p> <pre> s := "hi" + string(c) s += " and good bye" </pre> <p> String addition creates a new string by concatenating the operands. </p> <h3 id="Comparison_operators">Comparison operators</h3> <p> Comparison operators compare two operands and yield an untyped boolean value. </p> <pre class="grammar"> == equal != not equal < less <= less or equal > greater >= greater or equal </pre> <p> In any comparison, the first operand must be <a href="#Assignability">assignable</a> to the type of the second operand, or vice versa. </p> <p> The equality operators <code>==</code> and <code>!=</code> apply to operands that are <i>comparable</i>. The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code> apply to operands that are <i>ordered</i>. These terms and the result of the comparisons are defined as follows: </p> <ul> <li> Boolean values are comparable. Two boolean values are equal if they are either both <code>true</code> or both <code>false</code>. </li> <li> Integer values are comparable and ordered, in the usual way. </li> <li> Floating-point values are comparable and ordered, as defined by the IEEE-754 standard. </li> <li> Complex values are comparable. Two complex values <code>u</code> and <code>v</code> are equal if both <code>real(u) == real(v)</code> and <code>imag(u) == imag(v)</code>. </li> <li> String values are comparable and ordered, lexically byte-wise. </li> <li> Pointer values are comparable. Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>. Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal. </li> <li> Channel values are comparable. Two channel values are equal if they were created by the same call to <a href="#Making_slices_maps_and_channels"><code>make</code></a> or if both have value <code>nil</code>. </li> <li> Interface values are comparable. Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types and equal dynamic values or if both have value <code>nil</code>. </li> <li> A value <code>x</code> of non-interface type <code>X</code> and a value <code>t</code> of interface type <code>T</code> are comparable when values of type <code>X</code> are comparable and <code>X</code> implements <code>T</code>. They are equal if <code>t</code>'s dynamic type is identical to <code>X</code> and <code>t</code>'s dynamic value is equal to <code>x</code>. </li> <li> Struct values are comparable if all their fields are comparable. Two struct values are equal if their corresponding non-<a href="#Blank_identifier">blank</a> fields are equal. </li> <li> Array values are comparable if values of the array element type are comparable. Two array values are equal if their corresponding elements are equal. </li> </ul> <p> A comparison of two interface values with identical dynamic types causes a <a href="#Run_time_panics">run-time panic</a> if values of that type are not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields. </p> <p> Slice, map, and function values are not comparable. However, as a special case, a slice, map, or function value may be compared to the predeclared identifier <code>nil</code>. Comparison of pointer, channel, and interface values to <code>nil</code> is also allowed and follows from the general rules above. </p> <pre> const c = 3 < 4 // c is the untyped boolean constant true type MyBool bool var x, y int var ( // The result of a comparison is an untyped boolean. // The usual assignment rules apply. b3 = x == y // b3 has type bool b4 bool = x == y // b4 has type bool b5 MyBool = x == y // b5 has type MyBool ) </pre> <h3 id="Logical_operators">Logical operators</h3> <p> Logical operators apply to <a href="#Boolean_types">boolean</a> values and yield a result of the same type as the operands. The right operand is evaluated conditionally. </p> <pre class="grammar"> && conditional AND p && q is "if p then q else false" \|\| conditional OR p \|\| q is "if p then true else q" ! NOT !p is "not p" </pre> <h3 id="Address_operators">Address operators</h3> <p> For an operand <code>x</code> of type <code>T</code>, the address operation <code>&x</code> generates a pointer of type <code>T</code> to <code>x</code>. The operand must be <i>addressable</i>, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, <code>x</code> may also be a (possibly parenthesized) <a href="#Composite_literals">composite literal</a>. If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>, then the evaluation of <code>&x</code> does too. </p> <p> For an operand <code>x</code> of pointer type <code>T</code>, the pointer indirection <code>x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed to by <code>x</code>. If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>x</code> will cause a <a href="#Run_time_panics">run-time panic</a>. </p> <pre> &x &a[f(2)] &Point{2, 3} p pf(x) var x int = nil x // causes a run-time panic &x // causes a run-time panic </pre> <h3 id="Receive_operator">Receive operator</h3> <p> For an operand <code>ch</code> of <a href="#Channel_types">channel type</a>, the value of the receive operation <code><-ch</code> is the value received from the channel <code>ch</code>. The channel direction must permit receive operations, and the type of the receive operation is the element type of the channel. The expression blocks until a value is available. Receiving from a <code>nil</code> channel blocks forever. A receive operation on a <a href="#Close">closed</a> channel can always proceed immediately, yielding the element type's <a href="#The_zero_value">zero value</a> after any previously sent values have been received. </p> <pre> v1 := <-ch v2 = <-ch f(<-ch) <-strobe // wait until clock pulse and discard received value </pre> <p> A receive expression used in an <a href="#Assignments">assignment</a> or initialization of the special form </p> <pre> x, ok = <-ch x, ok := <-ch var x, ok = <-ch var x, ok T = <-ch </pre> <p> yields an additional untyped boolean result reporting whether the communication succeeded. The value of <code>ok</code> is <code>true</code> if the value received was delivered by a successful send operation to the channel, or <code>false</code> if it is a zero value generated because the channel is closed and empty. </p> <h3 id="Conversions">Conversions</h3> <p> A conversion changes the <a href="#Types">type</a> of an expression to the type specified by the conversion. A conversion may appear literally in the source, or it may be <i>implied</i> by the context in which an expression appears. </p> <p> An <i>explicit</i> conversion is an expression of the form <code>T(x)</code> where <code>T</code> is a type and <code>x</code> is an expression that can be converted to type <code>T</code>. </p> <pre class="ebnf"> Conversion = Type "(" Expression [ "," ] ")" . </pre> <p> If the type starts with the operator <code></code> or <code><-</code>, or if the type starts with the keyword <code>func</code> and has no result list, it must be parenthesized when necessary to avoid ambiguity: </p> <pre> Point(p) // same as (Point(p)) (Point)(p) // p is converted to Point <-chan int(c) // same as <-(chan int(c)) (<-chan int)(c) // c is converted to <-chan int func()(x) // function signature func() x (func())(x) // x is converted to func() (func() int)(x) // x is converted to func() int func() int(x) // x is converted to func() int (unambiguous) </pre> <p> A <a href="#Constants">constant</a> value <code>x</code> can be converted to type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a> by a value of <code>T</code>. As a special case, an integer constant <code>x</code> can be explicitly converted to a <a href="#String_types">string type</a> using the <a href="#Conversions_to_and_from_a_string_type">same rule</a> as for non-constant <code>x</code>. </p> <p> Converting a constant yields a typed constant as result. </p> <pre> uint(iota) // iota value of type uint float32(2.718281828) // 2.718281828 of type float32 complex128(1) // 1.0 + 0.0i of type complex128 float32(0.49999999) // 0.5 of type float32 float64(-1e-1000) // 0.0 of type float64 string('x') // "x" of type string string(0x266c) // "♬" of type string MyString("foo" + "bar") // "foobar" of type MyString string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant (int)(nil) // not a constant: nil is not a constant, int is not a boolean, numeric, or string type int(1.2) // illegal: 1.2 cannot be represented as an int string(65.0) // illegal: 65.0 is not an integer constant </pre> <p> A non-constant value <code>x</code> can be converted to type <code>T</code> in any of these cases: </p> <ul> <li> <code>x</code> is <a href="#Assignability">assignable</a> to <code>T</code>. </li> <li> ignoring struct tags (see below), <code>x</code>'s type and <code>T</code> have <a href="#Type_identity">identical</a> <a href="#Types">underlying types</a>. </li> <li> ignoring struct tags (see below), <code>x</code>'s type and <code>T</code> are pointer types that are not <a href="#Type_definitions">defined types</a>, and their pointer base types have identical underlying types. </li> <li> <code>x</code>'s type and <code>T</code> are both integer or floating point types. </li> <li> <code>x</code>'s type and <code>T</code> are both complex types. </li> <li> <code>x</code> is an integer or a slice of bytes or runes and <code>T</code> is a string type. </li> <li> <code>x</code> is a string and <code>T</code> is a slice of bytes or runes. </li> <li> <code>x</code> is a slice, <code>T</code> is a pointer to an array, and the slice and array types have <a href="#Type_identity">identical</a> element types. </li> </ul> <p> <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types for identity for the purpose of conversion: </p> <pre> type Person struct { Name string Address struct { Street string City string } } var data struct { Name string `json:"name"` Address struct { Street string `json:"street"` City string `json:"city"` } `json:"address"` } var person = (Person)(data) // ignoring tags, the underlying types are identical </pre> <p> Specific rules apply to (non-constant) conversions between numeric types or to and from a string type. These conversions may change the representation of <code>x</code> and incur a run-time cost. All other conversions only change the type but not the representation of <code>x</code>. </p> <p> There is no linguistic mechanism to convert between pointers and integers. The package <a href="#Package_unsafe"><code>unsafe</code></a> implements this functionality under restricted circumstances. </p> <h4>Conversions between numeric types</h4> <p> For the conversion of non-constant numeric values, the following rules apply: </p> <ol> <li> When converting between integer types, if the value is a signed integer, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type's size. For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>. The conversion always yields a valid value; there is no indication of overflow. </li> <li> When converting a floating-point number to an integer, the fraction is discarded (truncation towards zero). </li> <li> When converting an integer or floating-point number to a floating-point type, or a complex number to another complex type, the result value is rounded to the precision specified by the destination type. For instance, the value of a variable <code>x</code> of type <code>float32</code> may be stored using additional precision beyond that of an IEEE-754 32-bit number, but float32(x) represents the result of rounding <code>x</code>'s value to 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits of precision, but <code>float32(x + 0.1)</code> does not. </li> </ol> <p> In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent. </p> <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4> <ol> <li> Converting a signed or unsigned integer value to a string type yields a string containing the UTF-8 representation of the integer. Values outside the range of valid Unicode code points are converted to <code>"\uFFFD"</code>. <pre> string('a') // "a" string(-1) // "\ufffd" == "\xef\xbf\xbd" string(0xf8) // "\u00f8" == "ø" == "\xc3\xb8" type MyString string MyString(0x65e5) // "\u65e5" == "日" == "\xe6\x97\xa5" </pre> </li> <li> Converting a slice of bytes to a string type yields a string whose successive bytes are the elements of the slice. <pre> string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø" string([]byte{}) // "" string([]byte(nil)) // "" type MyBytes []byte string(MyBytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø" </pre> </li> <li> Converting a slice of runes to a string type yields a string that is the concatenation of the individual rune values converted to strings. <pre> string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔" string([]rune{}) // "" string([]rune(nil)) // "" type MyRunes []rune string(MyRunes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔" </pre> </li> <li> Converting a value of a string type to a slice of bytes type yields a slice whose successive elements are the bytes of the string. <pre> []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'} []byte("") // []byte{} MyBytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'} </pre> </li> <li> Converting a value of a string type to a slice of runes type yields a slice containing the individual Unicode code points of the string. <pre> []rune(MyString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4} []rune("") // []rune{} MyRunes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4} </pre> </li> </ol> <h4 id="Conversions_from_slice_to_array_pointer">Conversions from slice to array pointer</h4> <p> Converting a slice to an array pointer yields a pointer to the underlying array of the slice. If the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <pre> s := make([]byte, 2, 4) s0 := ([0]byte)(s) // s0 != nil s1 := ([1]byte)(s[1:]) // &s1[0] == &s[1] s2 := ([2]byte)(s) // &s2[0] == &s[0] s4 := ([4]byte)(s) // panics: len([4]byte) > len(s) var t []string t0 := ([0]string)(t) // t0 == nil t1 := ([1]string)(t) // panics: len([1]string) > len(t) u := make([]byte, 0) u0 := ([0]byte)(u) // u0 != nil </pre> <h3 id="Constant_expressions">Constant expressions</h3> <p> Constant expressions may contain only <a href="#Constants">constant</a> operands and are evaluated at compile time. </p> <p> Untyped boolean, numeric, and string constants may be used as operands wherever it is legal to use an operand of boolean, numeric, or string type, respectively. </p> <p> A constant <a href="#Comparison_operators">comparison</a> always yields an untyped boolean constant. If the left operand of a constant <a href="#Operators">shift expression</a> is an untyped constant, the result is an integer constant; otherwise it is a constant of the same type as the left operand, which must be of <a href="#Numeric_types">integer type</a>. </p> <p> Any other operation on untyped constants results in an untyped constant of the same kind; that is, a boolean, integer, floating-point, complex, or string constant. If the untyped operands of a binary operation (other than a shift) are of different kinds, the result is of the operand's kind that appears later in this list: integer, rune, floating-point, complex. For example, an untyped integer constant divided by an untyped complex constant yields an untyped complex constant. </p> <pre> const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant) const b = 15 / 4 // b == 3 (untyped integer constant) const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant) const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division) const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division) const d = 1 << 3.0 // d == 8 (untyped integer constant) const e = 1.0 << 3 // e == 8 (untyped integer constant) const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32) const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant) const h = "foo" > "bar" // h == true (untyped boolean constant) const j = true // j == true (untyped boolean constant) const k = 'w' + 1 // k == 'x' (untyped rune constant) const l = "hi" // l == "hi" (untyped string constant) const m = string(k) // m == "x" (type string) const Σ = 1 - 0.707i // (untyped complex constant) const Δ = Σ + 2.0e-4 // (untyped complex constant) const Φ = iota1i - 1/1i // (untyped complex constant) </pre> <p> Applying the built-in function <code>complex</code> to untyped integer, rune, or floating-point constants yields an untyped complex constant. </p> <pre> const ic = complex(0, c) // ic == 3.75i (untyped complex constant) const iΘ = complex(0, Θ) // iΘ == 1i (type complex128) </pre> <p> Constant expressions are always evaluated exactly; intermediate values and the constants themselves may require precision significantly larger than supported by any predeclared type in the language. The following are legal declarations: </p> <pre> const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant) const Four int8 = Huge >> 98 // Four == 4 (type int8) </pre> <p> The divisor of a constant division or remainder operation must not be zero: </p> <pre> 3.14 / 0.0 // illegal: division by zero </pre> <p> The values of <i>typed</i> constants must always be accurately <a href="#Representability">representable</a> by values of the constant type. The following constant expressions are illegal: </p> <pre> uint(-1) // -1 cannot be represented as a uint int(3.14) // 3.14 cannot be represented as an int int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64 Four * 300 // operand 300 cannot be represented as an int8 (type of Four) Four * 100 // product 400 cannot be represented as an int8 (type of Four) </pre> <p> The mask used by the unary bitwise complement operator <code>^</code> matches the rule for non-constants: the mask is all 1s for unsigned constants and -1 for signed and untyped constants. </p> <pre> ^1 // untyped integer constant, equal to -2 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE) int8(^1) // same as int8(-2) ^int8(1) // same as -1 ^ int8(1) = -2 </pre> <p> Implementation restriction: A compiler may use rounding while computing untyped floating-point or complex constant expressions; see the implementation restriction in the section on <a href="#Constants">constants</a>. This rounding may cause a floating-point constant expression to be invalid in an integer context, even if it would be integral when calculated using infinite precision, and vice versa. </p> <h3 id="Order_of_evaluation">Order of evaluation</h3> <p> At package level, <a href="#Package_initialization">initialization dependencies</a> determine the evaluation order of individual initialization expressions in <a href="#Variable_declarations">variable declarations</a>. Otherwise, when evaluating the <a href="#Operands">operands</a> of an expression, assignment, or <a href="#Return_statements">return statement</a>, all function calls, method calls, and communication operations are evaluated in lexical left-to-right order. </p> <p> For example, in the (function-local) assignment </p> <pre> y[f()], ok = g(h(), i()+x[j()], <-c), k() </pre> <p> the function calls and communication happen in the order <code>f()</code>, <code>h()</code>, <code>i()</code>, <code>j()</code>, <code><-c</code>, <code>g()</code>, and <code>k()</code>. However, the order of those events compared to the evaluation and indexing of <code>x</code> and the evaluation of <code>y</code> is not specified. </p> <pre> a := 1 f := func() int { a++; return a } x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified m := map[int]int{a: 1, a: 2} // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified n := map[int]int{a: f()} // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified </pre> <p> At package level, initialization dependencies override the left-to-right rule for individual initialization expressions, but not for operands within each expression: </p> <pre> var a, b, c = f() + v(), g(), sqr(u()) + v() func f() int { return c } func g() int { return a } func sqr(x int) int { return xx } // functions u and v are independent of all other variables and functions </pre> <p> The function calls happen in the order <code>u()</code>, <code>sqr()</code>, <code>v()</code>, <code>f()</code>, <code>v()</code>, and <code>g()</code>. </p> <p> Floating-point operations within a single expression are evaluated according to the associativity of the operators. Explicit parentheses affect the evaluation by overriding the default associativity. In the expression <code>x + (y + z)</code> the addition <code>y + z</code> is performed before adding <code>x</code>. </p> <h2 id="Statements">Statements</h2> <p> Statements control execution. </p> <pre class="ebnf"> Statement = Declaration \| LabeledStmt \| SimpleStmt \| GoStmt \| ReturnStmt \| BreakStmt \| ContinueStmt \| GotoStmt \| FallthroughStmt \| Block \| IfStmt \| SwitchStmt \| SelectStmt \| ForStmt \| DeferStmt . SimpleStmt = EmptyStmt \| ExpressionStmt \| SendStmt \| IncDecStmt \| Assignment \| ShortVarDecl . </pre> <h3 id="Terminating_statements">Terminating statements</h3> <p> A <i>terminating statement</i> interrupts the regular flow of control in a <a href="#Blocks">block</a>. The following statements are terminating: </p> <ol> <li> A <a href="#Return_statements">"return"</a> or <a href="#Goto_statements">"goto"</a> statement. <!-- ul below only for regular layout --> <ul> </ul> </li> <li> A call to the built-in function <a href="#Handling_panics"><code>panic</code></a>. <!-- ul below only for regular layout --> <ul> </ul> </li> <li> A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement. <!-- ul below only for regular layout --> <ul> </ul> </li> <li> An <a href="#If_statements">"if" statement</a> in which: <ul> <li>the "else" branch is present, and</li> <li>both branches are terminating statements.</li> </ul> </li> <li> A <a href="#For_statements">"for" statement</a> in which: <ul> <li>there are no "break" statements referring to the "for" statement, and</li> <li>the loop condition is absent, and</li> <li>the "for" statement does not use a range clause.</li> </ul> </li> <li> A <a href="#Switch_statements">"switch" statement</a> in which: <ul> <li>there are no "break" statements referring to the "switch" statement,</li> <li>there is a default case, and</li> <li>the statement lists in each case, including the default, end in a terminating statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough" statement</a>.</li> </ul> </li> <li> A <a href="#Select_statements">"select" statement</a> in which: <ul> <li>there are no "break" statements referring to the "select" statement, and</li> <li>the statement lists in each case, including the default if present, end in a terminating statement.</li> </ul> </li> <li> A <a href="#Labeled_statements">labeled statement</a> labeling a terminating statement. </li> </ol> <p> All other statements are not terminating. </p> <p> A <a href="#Blocks">statement list</a> ends in a terminating statement if the list is not empty and its final non-empty statement is terminating. </p> <h3 id="Empty_statements">Empty statements</h3> <p> The empty statement does nothing. </p> <pre class="ebnf"> EmptyStmt = . </pre> <h3 id="Labeled_statements">Labeled statements</h3> <p> A labeled statement may be the target of a <code>goto</code>, <code>break</code> or <code>continue</code> statement. </p> <pre class="ebnf"> LabeledStmt = Label ":" Statement . Label = identifier . </pre> <pre> Error: log.Panic("error encountered") </pre> <h3 id="Expression_statements">Expression statements</h3> <p> With the exception of specific built-in functions, function and method <a href="#Calls">calls</a> and <a href="#Receive_operator">receive operations</a> can appear in statement context. Such statements may be parenthesized. </p> <pre class="ebnf"> ExpressionStmt = Expression . </pre> <p> The following built-in functions are not permitted in statement context: </p> <pre> append cap complex imag len make new real unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice </pre> <pre> h(x+y) f.Close() <-ch (<-ch) len("foo") // illegal if len is the built-in function </pre> <h3 id="Send_statements">Send statements</h3> <p> A send statement sends a value on a channel. The channel expression must be of <a href="#Channel_types">channel type</a>, the channel direction must permit send operations, and the type of the value to be sent must be <a href="#Assignability">assignable</a> to the channel's element type. </p> <pre class="ebnf"> SendStmt = Channel "<-" Expression . Channel = Expression . </pre> <p> Both the channel and the value expression are evaluated before communication begins. Communication blocks until the send can proceed. A send on an unbuffered channel can proceed if a receiver is ready. A send on a buffered channel can proceed if there is room in the buffer. A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>. A send on a <code>nil</code> channel blocks forever. </p> <pre> ch <- 3 // send value 3 to channel ch </pre> <h3 id="IncDec_statements">IncDec statements</h3> <p> The "++" and "--" statements increment or decrement their operands by the untyped <a href="#Constants">constant</a> <code>1</code>. As with an assignment, the operand must be <a href="#Address_operators">addressable</a> or a map index expression. </p> <pre class="ebnf"> IncDecStmt = Expression ( "++" \| "--" ) . </pre> <p> The following <a href="#Assignments">assignment statements</a> are semantically equivalent: </p> <pre class="grammar"> IncDec statement Assignment x++ x += 1 x-- x -= 1 </pre> <h3 id="Assignments">Assignments</h3> <pre class="ebnf"> Assignment = ExpressionList assign_op ExpressionList . assign_op = [ add_op \| mul_op ] "=" . </pre> <p> Each left-hand side operand must be <a href="#Address_operators">addressable</a>, a map index expression, or (for <code>=</code> assignments only) the <a href="#Blank_identifier">blank identifier</a>. Operands may be parenthesized. </p> <pre> x = 1 p = f() a[i] = 23 (k) = <-ch // same as: k = <-ch </pre> <p> An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code> <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a> is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i> <code>(y)</code> but evaluates <code>x</code> only once. The <i>op</i><code>=</code> construct is a single token. In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression, and the left-hand expression must not be the blank identifier. </p> <pre> a[i] <<= 2 i &^= 1<<n </pre> <p> A tuple assignment assigns the individual elements of a multi-valued operation to a list of variables. There are two forms. In the first, the right hand operand is a single multi-valued expression such as a function call, a <a href="#Channel_types">channel</a> or <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>. The number of operands on the left hand side must match the number of values. For instance, if <code>f</code> is a function returning two values, </p> <pre> x, y = f() </pre> <p> assigns the first value to <code>x</code> and the second to <code>y</code>. In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the <i>n</i>th expression on the right is assigned to the <i>n</i>th operand on the left: </p> <pre> one, two, three = '一', '二', '三' </pre> <p> The <a href="#Blank_identifier">blank identifier</a> provides a way to ignore right-hand side values in an assignment: </p> <pre> _ = x // evaluate x but ignore it x, _ = f() // evaluate f() but ignore second result value </pre> <p> The assignment proceeds in two phases. First, the operands of <a href="#Index_expressions">index expressions</a> and <a href="#Address_operators">pointer indirections</a> (including implicit pointer indirections in <a href="#Selectors">selectors</a>) on the left and the expressions on the right are all <a href="#Order_of_evaluation">evaluated in the usual order</a>. Second, the assignments are carried out in left-to-right order. </p> <pre> a, b = b, a // exchange a and b x := []int{1, 2, 3} i := 0 i, x[i] = 1, 2 // set i = 1, x[0] = 2 i = 0 x[i], i = 2, 1 // set x[0] = 2, i = 1 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end) x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5. type Point struct { x, y int } var p Point x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7 i = 2 x = []int{3, 5, 7} for i, x[i] = range x { // set i, x[2] = 0, x[0] break } // after this loop, i == 0 and x == []int{3, 5, 3} </pre> <p> In assignments, each value must be <a href="#Assignability">assignable</a> to the type of the operand to which it is assigned, with the following special cases: </p> <ol> <li> Any typed value may be assigned to the blank identifier. </li> <li> If an untyped constant is assigned to a variable of interface type or the blank identifier, the constant is first implicitly <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>. </li> <li> If an untyped boolean value is assigned to a variable of interface type or the blank identifier, it is first implicitly converted to type <code>bool</code>. </li> </ol> <h3 id="If_statements">If statements</h3> <p> "If" statements specify the conditional execution of two branches according to the value of a boolean expression. If the expression evaluates to true, the "if" branch is executed, otherwise, if present, the "else" branch is executed. </p> <pre class="ebnf"> IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt \| Block ) ] . </pre> <pre> if x > max { x = max } </pre> <p> The expression may be preceded by a simple statement, which executes before the expression is evaluated. </p> <pre> if x := f(); x < y { return x } else if x > z { return z } else { return y } </pre> <h3 id="Switch_statements">Switch statements</h3> <p> "Switch" statements provide multi-way execution. An expression or type is compared to the "cases" inside the "switch" to determine which branch to execute. </p> <pre class="ebnf"> SwitchStmt = ExprSwitchStmt \| TypeSwitchStmt . </pre> <p> There are two forms: expression switches and type switches. In an expression switch, the cases contain expressions that are compared against the value of the switch expression. In a type switch, the cases contain types that are compared against the type of a specially annotated switch expression. The switch expression is evaluated exactly once in a switch statement. </p> <h4 id="Expression_switches">Expression switches</h4> <p> In an expression switch, the switch expression is evaluated and the case expressions, which need not be constants, are evaluated left-to-right and top-to-bottom; the first one that equals the switch expression triggers execution of the statements of the associated case; the other cases are skipped. If no case matches and there is a "default" case, its statements are executed. There can be at most one default case and it may appear anywhere in the "switch" statement. A missing switch expression is equivalent to the boolean value <code>true</code>. </p> <pre class="ebnf"> ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" . ExprCaseClause = ExprSwitchCase ":" StatementList . ExprSwitchCase = "case" ExpressionList \| "default" . </pre> <p> If the switch expression evaluates to an untyped constant, it is first implicitly <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>. The predeclared untyped value <code>nil</code> cannot be used as a switch expression. The switch expression type must be <a href="#Comparison_operators">comparable</a>. </p> <p> If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a> to the type of the switch expression. For each (possibly converted) case expression <code>x</code> and the value <code>t</code> of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>. </p> <p> In other words, the switch expression is treated as if it were used to declare and initialize a temporary variable <code>t</code> without explicit type; it is that value of <code>t</code> against which each case expression <code>x</code> is tested for equality. </p> <p> In a case or default clause, the last non-empty statement may be a (possibly <a href="#Labeled_statements">labeled</a>) <a href="#Fallthrough_statements">"fallthrough" statement</a> to indicate that control should flow from the end of this clause to the first statement of the next clause. Otherwise control flows to the end of the "switch" statement. A "fallthrough" statement may appear as the last statement of all but the last clause of an expression switch. </p> <p> The switch expression may be preceded by a simple statement, which executes before the expression is evaluated. </p> <pre> switch tag { default: s3() case 0, 1, 2, 3: s1() case 4, 5, 6, 7: s2() } switch x := f(); { // missing switch expression means "true" case x < 0: return -x default: return x } switch { case x < y: f1() case x < z: f2() case x == 4: f3() } </pre> <p> Implementation restriction: A compiler may disallow multiple case expressions evaluating to the same constant. For instance, the current compilers disallow duplicate integer, floating point, or string constants in case expressions. </p> <h4 id="Type_switches">Type switches</h4> <p> A type switch compares types rather than values. It is otherwise similar to an expression switch. It is marked by a special switch expression that has the form of a <a href="#Type_assertions">type assertion</a> using the keyword <code>type</code> rather than an actual type: </p> <pre> switch x.(type) { // cases } </pre> <p> Cases then match actual types <code>T</code> against the dynamic type of the expression <code>x</code>. As with type assertions, <code>x</code> must be of <a href="#Interface_types">interface type</a>, and each non-interface type <code>T</code> listed in a case must implement the type of <code>x</code>. The types listed in the cases of a type switch must all be <a href="#Type_identity">different</a>. </p> <pre class="ebnf"> TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" . TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" . TypeCaseClause = TypeSwitchCase ":" StatementList . TypeSwitchCase = "case" TypeList \| "default" . TypeList = Type { "," Type } . </pre> <p> The TypeSwitchGuard may include a <a href="#Short_variable_declarations">short variable declaration</a>. When that form is used, the variable is declared at the end of the TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard. </p> <p> Instead of a type, a case may use the predeclared identifier <a href="#Predeclared_identifiers"><code>nil</code></a>; that case is selected when the expression in the TypeSwitchGuard is a <code>nil</code> interface value. There may be at most one <code>nil</code> case. </p> <p> Given an expression <code>x</code> of type <code>interface{}</code>, the following type switch: </p> <pre> switch i := x.(type) { case nil: printString("x is nil") // type of i is type of x (interface{}) case int: printInt(i) // type of i is int case float64: printFloat64(i) // type of i is float64 case func(int) float64: printFunction(i) // type of i is func(int) float64 case bool, string: printString("type is bool or string") // type of i is type of x (interface{}) default: printString("don't know the type") // type of i is type of x (interface{}) } </pre> <p> could be rewritten: </p> <pre> v := x // x is evaluated exactly once if v == nil { i := v // type of i is type of x (interface{}) printString("x is nil") } else if i, isInt := v.(int); isInt { printInt(i) // type of i is int } else if i, isFloat64 := v.(float64); isFloat64 { printFloat64(i) // type of i is float64 } else if i, isFunc := v.(func(int) float64); isFunc { printFunction(i) // type of i is func(int) float64 } else { _, isBool := v.(bool) _, isString := v.(string) if isBool \|\| isString { i := v // type of i is type of x (interface{}) printString("type is bool or string") } else { i := v // type of i is type of x (interface{}) printString("don't know the type") } } </pre> <p> The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated. </p> <p> The "fallthrough" statement is not permitted in a type switch. </p> <h3 id="For_statements">For statements</h3> <p> A "for" statement specifies repeated execution of a block. There are three forms: The iteration may be controlled by a single condition, a "for" clause, or a "range" clause. </p> <pre class="ebnf"> ForStmt = "for" [ Condition \| ForClause \| RangeClause ] Block . Condition = Expression . </pre> <h4 id="For_condition">For statements with single condition</h4> <p> In its simplest form, a "for" statement specifies the repeated execution of a block as long as a boolean condition evaluates to true. The condition is evaluated before each iteration. If the condition is absent, it is equivalent to the boolean value <code>true</code>. </p> <pre> for a < b { a = 2 } </pre> <h4 id="For_clause">For statements with <code>for</code> clause</h4> <p> A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an <i>init</i> and a <i>post</i> statement, such as an assignment, an increment or decrement statement. The init statement may be a <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not. Variables declared by the init statement are re-used in each iteration. </p> <pre class="ebnf"> ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] . InitStmt = SimpleStmt . PostStmt = SimpleStmt . </pre> <pre> for i := 0; i < 10; i++ { f(i) } </pre> <p> If non-empty, the init statement is executed once before evaluating the condition for the first iteration; the post statement is executed after each execution of the block (and only if the block was executed). Any element of the ForClause may be empty but the <a href="#Semicolons">semicolons</a> are required unless there is only a condition. If the condition is absent, it is equivalent to the boolean value <code>true</code>. </p> <pre> for cond { S() } is the same as for ; cond ; { S() } for { S() } is the same as for true { S() } </pre> <h4 id="For_range">For statements with <code>range</code> clause</h4> <p> A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, or values received on a channel. For each entry it assigns <i>iteration values</i> to corresponding <i>iteration variables</i> if present and then executes the block. </p> <pre class="ebnf"> RangeClause = [ ExpressionList "=" \| IdentifierList ":=" ] "range" Expression . </pre> <p> The expression on the right in the "range" clause is called the <i>range expression</i>, which may be an array, pointer to an array, slice, string, map, or channel permitting <a href="#Receive_operator">receive operations</a>. As with an assignment, if present the operands on the left must be <a href="#Address_operators">addressable</a> or map index expressions; they denote the iteration variables. If the range expression is a channel, at most one iteration variable is permitted, otherwise there may be up to two. If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>, the range clause is equivalent to the same clause without that identifier. </p> <p> The range expression <code>x</code> is evaluated once before beginning the loop, with one exception: if at most one iteration variable is present and <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>, the range expression is not evaluated. </p> <p> Function calls on the left are evaluated once per iteration. For each iteration, iteration values are produced as follows if the respective iteration variables are present: </p> <pre class="grammar"> Range expression 1st value 2nd value array or slice a [n]E, [n]E, or []E index i int a[i] E string s string type index i int see below rune map m map[K]V key k K m[k] V channel c chan E, <-chan E element e E </pre> <ol> <li> For an array, pointer to array, or slice value <code>a</code>, the index iteration values are produced in increasing order, starting at element index 0. If at most one iteration variable is present, the range loop produces iteration values from 0 up to <code>len(a)-1</code> and does not index into the array or slice itself. For a <code>nil</code> slice, the number of iterations is 0. </li> <li> For a string value, the "range" clause iterates over the Unicode code points in the string starting at byte index 0. On successive iterations, the index value will be the index of the first byte of successive UTF-8-encoded code points in the string, and the second value, of type <code>rune</code>, will be the value of the corresponding code point. If the iteration encounters an invalid UTF-8 sequence, the second value will be <code>0xFFFD</code>, the Unicode replacement character, and the next iteration will advance a single byte in the string. </li> <li> The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If a map entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced. If a map entry is created during iteration, that entry may be produced during the iteration or may be skipped. The choice may vary for each entry created and from one iteration to the next. If the map is <code>nil</code>, the number of iterations is 0. </li> <li> For channels, the iteration values produced are the successive values sent on the channel until the channel is <a href="#Close">closed</a>. If the channel is <code>nil</code>, the range expression blocks forever. </li> </ol> <p> The iteration values are assigned to the respective iteration variables as in an <a href="#Assignments">assignment statement</a>. </p> <p> The iteration variables may be declared by the "range" clause using a form of <a href="#Short_variable_declarations">short variable declaration</a> (<code>:=</code>). In this case their types are set to the types of the respective iteration values and their <a href="#Declarations_and_scope">scope</a> is the block of the "for" statement; they are re-used in each iteration. If the iteration variables are declared outside the "for" statement, after execution their values will be those of the last iteration. </p> <pre> var testdata struct { a [7]int } for i, _ := range testdata.a { // testdata.a is never evaluated; len(testdata.a) is constant // i ranges from 0 to 6 f(i) } var a [10]string for i, s := range a { // type of i is int // type of s is string // s == a[i] g(i, s) } var key string var val interface{} // element type of m is assignable to val m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6} for key, val = range m { h(key, val) } // key == last map key encountered in iteration // val == map[key] var ch chan Work = producer() for w := range ch { doWork(w) } // empty a channel for range ch {} </pre> <h3 id="Go_statements">Go statements</h3> <p> A "go" statement starts the execution of a function call as an independent concurrent thread of control, or <i>goroutine</i>, within the same address space. </p> <pre class="ebnf"> GoStmt = "go" Expression . </pre> <p> The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for <a href="#Expression_statements">expression statements</a>. </p> <p> The function value and parameters are <a href="#Calls">evaluated as usual</a> in the calling goroutine, but unlike with a regular call, program execution does not wait for the invoked function to complete. Instead, the function begins executing independently in a new goroutine. When the function terminates, its goroutine also terminates. If the function has any return values, they are discarded when the function completes. </p> <pre> go Server() go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c) </pre> <h3 id="Select_statements">Select statements</h3> <p> A "select" statement chooses which of a set of possible <a href="#Send_statements">send</a> or <a href="#Receive_operator">receive</a> operations will proceed. It looks similar to a <a href="#Switch_statements">"switch"</a> statement but with the cases all referring to communication operations. </p> <pre class="ebnf"> SelectStmt = "select" "{" { CommClause } "}" . CommClause = CommCase ":" StatementList . CommCase = "case" ( SendStmt \| RecvStmt ) \| "default" . RecvStmt = [ ExpressionList "=" \| IdentifierList ":=" ] RecvExpr . RecvExpr = Expression . </pre> <p> A case with a RecvStmt may assign the result of a RecvExpr to one or two variables, which may be declared using a <a href="#Short_variable_declarations">short variable declaration</a>. The RecvExpr must be a (possibly parenthesized) receive operation. There can be at most one default case and it may appear anywhere in the list of cases. </p> <p> Execution of a "select" statement proceeds in several steps: </p> <ol> <li> For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the "select" statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed. Expressions on the left-hand side of a RecvStmt with a short variable declaration or assignment are not yet evaluated. </li> <li> If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed. </li> <li> Unless the selected case is the default case, the respective communication operation is executed. </li> <li> If the selected case is a RecvStmt with a short variable declaration or an assignment, the left-hand side expressions are evaluated and the received value (or values) are assigned. </li> <li> The statement list of the selected case is executed. </li> </ol> <p> Since communication on <code>nil</code> channels can never proceed, a select with only <code>nil</code> channels and no default case blocks forever. </p> <pre> var a []int var c, c1, c2, c3, c4 chan int var i1, i2 int select { case i1 = <-c1: print("received ", i1, " from c1\n") case c2 <- i2: print("sent ", i2, " to c2\n") case i3, ok := (<-c3): // same as: i3, ok := <-c3 if ok { print("received ", i3, " from c3\n") } else { print("c3 is closed\n") } case a[f()] = <-c4: // same as: // case t := <-c4 // a[f()] = t default: print("no communication\n") } for { // send random sequence of bits to c select { case c <- 0: // note: no statement, no fallthrough, no folding of cases case c <- 1: } } select {} // block forever </pre> <h3 id="Return_statements">Return statements</h3> <p> A "return" statement in a function <code>F</code> terminates the execution of <code>F</code>, and optionally provides one or more result values. Any functions <a href="#Defer_statements">deferred</a> by <code>F</code> are executed before <code>F</code> returns to its caller. </p> <pre class="ebnf"> ReturnStmt = "return" [ ExpressionList ] . </pre> <p> In a function without a result type, a "return" statement must not specify any result values. </p> <pre> func noResult() { return } </pre> <p> There are three ways to return values from a function with a result type: </p> <ol> <li>The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and <a href="#Assignability">assignable</a> to the corresponding element of the function's result type. <pre> func simpleF() int { return 2 } func complexF1() (re float64, im float64) { return -7.0, -4.0 } </pre> </li> <li>The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply. <pre> func complexF2() (re float64, im float64) { return complexF1() } </pre> </li> <li>The expression list may be empty if the function's result type specifies names for its <a href="#Function_types">result parameters</a>. The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables. <pre> func complexF3() (re float64, im float64) { re = 7.0 im = 4.0 return } func (devnull) Write(p []byte) (n int, _ error) { n = len(p) return } </pre> </li> </ol> <p> Regardless of how they are declared, all the result values are initialized to the <a href="#The_zero_value">zero values</a> for their type upon entry to the function. A "return" statement that specifies results sets the result parameters before any deferred functions are executed. </p> <p> Implementation restriction: A compiler may disallow an empty expression list in a "return" statement if a different entity (constant, type, or variable) with the same name as a result parameter is in <a href="#Declarations_and_scope">scope</a> at the place of the return. </p> <pre> func f(n int) (res int, err error) { if _, err := f(n-1); err != nil { return // invalid return statement: err is shadowed } return } </pre> <h3 id="Break_statements">Break statements</h3> <p> A "break" statement terminates execution of the innermost <a href="#For_statements">"for"</a>, <a href="#Switch_statements">"switch"</a>, or <a href="#Select_statements">"select"</a> statement within the same function. </p> <pre class="ebnf"> BreakStmt = "break" [ Label ] . </pre> <p> If there is a label, it must be that of an enclosing "for", "switch", or "select" statement, and that is the one whose execution terminates. </p> <pre> OuterLoop: for i = 0; i < n; i++ { for j = 0; j < m; j++ { switch a[i][j] { case nil: state = Error break OuterLoop case item: state = Found break OuterLoop } } } </pre> <h3 id="Continue_statements">Continue statements</h3> <p> A "continue" statement begins the next iteration of the innermost <a href="#For_statements">"for" loop</a> at its post statement. The "for" loop must be within the same function. </p> <pre class="ebnf"> ContinueStmt = "continue" [ Label ] . </pre> <p> If there is a label, it must be that of an enclosing "for" statement, and that is the one whose execution advances. </p> <pre> RowLoop: for y, row := range rows { for x, data := range row { if data == endOfRow { continue RowLoop } row[x] = data + bias(x, y) } } </pre> <h3 id="Goto_statements">Goto statements</h3> <p> A "goto" statement transfers control to the statement with the corresponding label within the same function. </p> <pre class="ebnf"> GotoStmt = "goto" Label . </pre> <pre> goto Error </pre> <p> Executing the "goto" statement must not cause any variables to come into <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto. For instance, this example: </p> <pre> goto L // BAD v := 3 L: </pre> <p> is erroneous because the jump to label <code>L</code> skips the creation of <code>v</code>. </p> <p> A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block. For instance, this example: </p> <pre> if n%2 == 1 { goto L1 } for n > 0 { f() n-- L1: f() n-- } </pre> <p> is erroneous because the label <code>L1</code> is inside the "for" statement's block but the <code>goto</code> is not. </p> <h3 id="Fallthrough_statements">Fallthrough statements</h3> <p> A "fallthrough" statement transfers control to the first statement of the next case clause in an <a href="#Expression_switches">expression "switch" statement</a>. It may be used only as the final non-empty statement in such a clause. </p> <pre class="ebnf"> FallthroughStmt = "fallthrough" . </pre> <h3 id="Defer_statements">Defer statements</h3> <p> A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a <a href="#Return_statements">return statement</a>, reached the end of its <a href="#Function_declarations">function body</a>, or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>. </p> <pre class="ebnf"> DeferStmt = "defer" Expression . </pre> <p> The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for <a href="#Expression_statements">expression statements</a>. </p> <p> Each time a "defer" statement executes, the function value and parameters to the call are <a href="#Calls">evaluated as usual</a> and saved anew but the actual function is not invoked. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. That is, if the surrounding function returns through an explicit <a href="#Return_statements">return statement</a>, deferred functions are executed <i>after</i> any result parameters are set by that return statement but <i>before</i> the function returns to its caller. If a deferred function value evaluates to <code>nil</code>, execution <a href="#Handling_panics">panics</a> when the function is invoked, not when the "defer" statement is executed. </p> <p> For instance, if the deferred function is a <a href="#Function_literals">function literal</a> and the surrounding function has <a href="#Function_types">named result parameters</a> that are in scope within the literal, the deferred function may access and modify the result parameters before they are returned. If the deferred function has any return values, they are discarded when the function completes. (See also the section on <a href="#Handling_panics">handling panics</a>.) </p> <pre> lock(l) defer unlock(l) // unlocking happens before surrounding function returns // prints 3 2 1 0 before surrounding function returns for i := 0; i <= 3; i++ { defer fmt.Print(i) } // f returns 42 func f() (result int) { defer func() { // result is accessed after it was set to 6 by the return statement result = 7 }() return 6 } </pre> <h2 id="Built-in_functions">Built-in functions</h2> <p> Built-in functions are <a href="#Predeclared_identifiers">predeclared</a>. They are called like any other function but some of them accept a type instead of an expression as the first argument. </p> <p> The built-in functions do not have standard Go types, so they can only appear in <a href="#Calls">call expressions</a>; they cannot be used as function values. </p> <h3 id="Close">Close</h3> <p> For a channel <code>c</code>, the built-in function <code>close(c)</code> records that no more values will be sent on the channel. It is an error if <code>c</code> is a receive-only channel. Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>. Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>. After calling <code>close</code>, and after any previously sent values have been received, receive operations will return the zero value for the channel's type without blocking. The multi-valued <a href="#Receive_operator">receive operation</a> returns a received value along with an indication of whether the channel is closed. </p> <h3 id="Length_and_capacity">Length and capacity</h3> <p> The built-in functions <code>len</code> and <code>cap</code> take arguments of various types and return a result of type <code>int</code>. The implementation guarantees that the result always fits into an <code>int</code>. </p> <pre class="grammar"> Call Argument type Result len(s) string type string length in bytes [n]T, [n]T array length (== n) []T slice length map[K]T map length (number of defined keys) chan T number of elements queued in channel buffer cap(s) [n]T, [n]T array length (== n) []T slice capacity chan T channel buffer capacity </pre> <p> The capacity of a slice is the number of elements for which there is space allocated in the underlying array. At any time the following relationship holds: </p> <pre> 0 <= len(s) <= cap(s) </pre> <p> The length of a <code>nil</code> slice, map or channel is 0. The capacity of a <code>nil</code> slice or channel is 0. </p> <p> The expression <code>len(s)</code> is <a href="#Constants">constant</a> if <code>s</code> is a string constant. The expressions <code>len(s)</code> and <code>cap(s)</code> are constants if the type of <code>s</code> is an array or pointer to an array and the expression <code>s</code> does not contain <a href="#Receive_operator">channel receives</a> or (non-constant) <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated. Otherwise, invocations of <code>len</code> and <code>cap</code> are not constant and <code>s</code> is evaluated. </p> <pre> const ( c1 = imag(2i) // imag(2i) = 2.0 is a constant c2 = len([10]float64{2}) // [10]float64{2} contains no function calls c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call ) var z complex128 </pre> <h3 id="Allocation">Allocation</h3> <p> The built-in function <code>new</code> takes a type <code>T</code>, allocates storage for a <a href="#Variables">variable</a> of that type at run time, and returns a value of type <code>T</code> <a href="#Pointer_types">pointing</a> to it. The variable is initialized as described in the section on <a href="#The_zero_value">initial values</a>. </p> <pre class="grammar"> new(T) </pre> <p> For instance </p> <pre> type S struct { a int; b float64 } new(S) </pre> <p> allocates storage for a variable of type <code>S</code>, initializes it (<code>a=0</code>, <code>b=0.0</code>), and returns a value of type <code>S</code> containing the address of the location. </p> <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3> <p> The built-in function <code>make</code> takes a type <code>T</code>, which must be a slice, map or channel type, optionally followed by a type-specific list of expressions. It returns a value of type <code>T</code> (not <code>T</code>). The memory is initialized as described in the section on <a href="#The_zero_value">initial values</a>. </p> <pre class="grammar"> Call Type T Result make(T, n) slice slice of type T with length n and capacity n make(T, n, m) slice slice of type T with length n and capacity m make(T) map map of type T make(T, n) map map of type T with initial space for approximately n elements make(T) channel unbuffered channel of type T make(T, n) channel buffered channel of type T, buffer size n </pre> <p> Each of the size arguments <code>n</code> and <code>m</code> must be of integer type or an untyped <a href="#Constants">constant</a>. A constant size argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>. If both <code>n</code> and <code>m</code> are provided and are constant, then <code>n</code> must be no larger than <code>m</code>. If <code>n</code> is negative or larger than <code>m</code> at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <pre> s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int s := make([]int, 10, 0) // illegal: len(s) > cap(s) c := make(chan int, 10) // channel with a buffer size of 10 m := make(map[string]int, 100) // map with initial space for approximately 100 elements </pre> <p> Calling <code>make</code> with a map type and size hint <code>n</code> will create a map with initial space to hold <code>n</code> map elements. The precise behavior is implementation-dependent. </p> <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3> <p> The built-in functions <code>append</code> and <code>copy</code> assist in common slice operations. For both functions, the result is independent of whether the memory referenced by the arguments overlaps. </p> <p> The <a href="#Function_types">variadic</a> function <code>append</code> appends zero or more values <code>x</code> to <code>s</code> of type <code>S</code>, which must be a slice type, and returns the resulting slice, also of type <code>S</code>. The values <code>x</code> are passed to a parameter of type <code>...T</code> where <code>T</code> is the <a href="#Slice_types">element type</a> of <code>S</code> and the respective <a href="#Passing_arguments_to_..._parameters">parameter passing rules</a> apply. As a special case, <code>append</code> also accepts a first argument assignable to type <code>[]byte</code> with a second argument of string type followed by <code>...</code>. This form appends the bytes of the string. </p> <pre class="grammar"> append(s S, x ...T) S // T is the element type of S </pre> <p> If the capacity of <code>s</code> is not large enough to fit the additional values, <code>append</code> allocates a new, sufficiently large underlying array that fits both the existing slice elements and the additional values. Otherwise, <code>append</code> re-uses the underlying array. </p> <pre> s0 := []int{0, 0} s1 := append(s0, 2) // append a single element s1 == []int{0, 0, 2} s2 := append(s1, 3, 5, 7) // append multiple elements s2 == []int{0, 0, 2, 3, 5, 7} s3 := append(s2, s0...) // append a slice s3 == []int{0, 0, 2, 3, 5, 7, 0, 0} s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 == []int{3, 5, 7, 2, 3, 5, 7, 0, 0} var t []interface{} t = append(t, 42, 3.1415, "foo") // t == []interface{}{42, 3.1415, "foo"} var b []byte b = append(b, "bar"...) // append string contents b == []byte{'b', 'a', 'r' } </pre> <p> The function <code>copy</code> copies slice elements from a source <code>src</code> to a destination <code>dst</code> and returns the number of elements copied. Both arguments must have <a href="#Type_identity">identical</a> element type <code>T</code> and must be <a href="#Assignability">assignable</a> to a slice of type <code>[]T</code>. The number of elements copied is the minimum of <code>len(src)</code> and <code>len(dst)</code>. As a special case, <code>copy</code> also accepts a destination argument assignable to type <code>[]byte</code> with a source argument of a string type. This form copies the bytes from the string into the byte slice. </p> <pre class="grammar"> copy(dst, src []T) int copy(dst []byte, src string) int </pre> <p> Examples: </p> <pre> var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7} var s = make([]int, 6) var b = make([]byte, 5) n1 := copy(s, a[0:]) // n1 == 6, s == []int{0, 1, 2, 3, 4, 5} n2 := copy(s, s[2:]) // n2 == 4, s == []int{2, 3, 4, 5, 4, 5} n3 := copy(b, "Hello, World!") // n3 == 5, b == []byte("Hello") </pre> <h3 id="Deletion_of_map_elements">Deletion of map elements</h3> <p> The built-in function <code>delete</code> removes the element with key <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The type of <code>k</code> must be <a href="#Assignability">assignable</a> to the key type of <code>m</code>. </p> <pre class="grammar"> delete(m, k) // remove element m[k] from map m </pre> <p> If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code> does not exist, <code>delete</code> is a no-op. </p> <h3 id="Complex_numbers">Manipulating complex numbers</h3> <p> Three functions assemble and disassemble complex numbers. The built-in function <code>complex</code> constructs a complex value from a floating-point real and imaginary part, while <code>real</code> and <code>imag</code> extract the real and imaginary parts of a complex value. </p> <pre class="grammar"> complex(realPart, imaginaryPart floatT) complexT real(complexT) floatT imag(complexT) floatT </pre> <p> The type of the arguments and return value correspond. For <code>complex</code>, the two arguments must be of the same floating-point type and the return type is the complex type with the corresponding floating-point constituents: <code>complex64</code> for <code>float32</code> arguments, and <code>complex128</code> for <code>float64</code> arguments. If one of the arguments evaluates to an untyped constant, it is first implicitly <a href="#Conversions">converted</a> to the type of the other argument. If both arguments evaluate to untyped constants, they must be non-complex numbers or their imaginary parts must be zero, and the return value of the function is an untyped complex constant. </p> <p> For <code>real</code> and <code>imag</code>, the argument must be of complex type, and the return type is the corresponding floating-point type: <code>float32</code> for a <code>complex64</code> argument, and <code>float64</code> for a <code>complex128</code> argument. If the argument evaluates to an untyped constant, it must be a number, and the return value of the function is an untyped floating-point constant. </p> <p> The <code>real</code> and <code>imag</code> functions together form the inverse of <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>, <code>z == Z(complex(real(z), imag(z)))</code>. </p> <p> If the operands of these functions are all constants, the return value is a constant. </p> <pre> var a = complex(2, -2) // complex128 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i x := float32(math.Cos(math.Pi/2)) // float32 var c64 = complex(5, -x) // complex64 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift var rl = real(c64) // float32 var im = imag(a) // float64 const c = imag(b) // untyped constant -1.4 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift </pre> <h3 id="Handling_panics">Handling panics</h3> <p> Two built-in functions, <code>panic</code> and <code>recover</code>, assist in reporting and handling <a href="#Run_time_panics">run-time panics</a> and program-defined error conditions. </p> <pre class="grammar"> func panic(interface{}) func recover() interface{} </pre> <p> While executing a function <code>F</code>, an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a> terminates the execution of <code>F</code>. Any functions <a href="#Defer_statements">deferred</a> by <code>F</code> are then executed as usual. Next, any deferred functions run by <code>F's</code> caller are run, and so on up to any deferred by the top-level function in the executing goroutine. At that point, the program is terminated and the error condition is reported, including the value of the argument to <code>panic</code>. This termination sequence is called <i>panicking</i>. </p> <pre> panic(42) panic("unreachable") panic(Error("cannot parse")) </pre> <p> The <code>recover</code> function allows a program to manage behavior of a panicking goroutine. Suppose a function <code>G</code> defers a function <code>D</code> that calls <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code> is executing. When the running of deferred functions reaches <code>D</code>, the return value of <code>D</code>'s call to <code>recover</code> will be the value passed to the call of <code>panic</code>. If <code>D</code> returns normally, without starting a new <code>panic</code>, the panicking sequence stops. In that case, the state of functions called between <code>G</code> and the call to <code>panic</code> is discarded, and normal execution resumes. Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s execution terminates by returning to its caller. </p> <p> The return value of <code>recover</code> is <code>nil</code> if any of the following conditions holds: </p> <ul> <li> <code>panic</code>'s argument was <code>nil</code>; </li> <li> the goroutine is not panicking; </li> <li> <code>recover</code> was not called directly by a deferred function. </li> </ul> <p> The <code>protect</code> function in the example below invokes the function argument <code>g</code> and protects callers from run-time panics raised by <code>g</code>. </p> <pre> func protect(g func()) { defer func() { log.Println("done") // Println executes normally even if there is a panic if x := recover(); x != nil { log.Printf("run time panic: %v", x) } }() log.Println("start") g() } </pre> <h3 id="Bootstrapping">Bootstrapping</h3> <p> Current implementations provide several built-in functions useful during bootstrapping. These functions are documented for completeness but are not guaranteed to stay in the language. They do not return a result. </p> <pre class="grammar"> Function Behavior print prints all arguments; formatting of arguments is implementation-specific println like print but prints spaces between arguments and a newline at the end </pre> <p> Implementation restriction: <code>print</code> and <code>println</code> need not accept arbitrary argument types, but printing of boolean, numeric, and string <a href="#Types">types</a> must be supported. </p> <h2 id="Packages">Packages</h2> <p> Go programs are constructed by linking together <i>packages</i>. A package in turn is constructed from one or more source files that together declare constants, types, variables and functions belonging to the package and which are accessible in all files of the same package. Those elements may be <a href="#Exported_identifiers">exported</a> and used in another package. </p> <h3 id="Source_file_organization">Source file organization</h3> <p> Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants. </p> <pre class="ebnf"> SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } . </pre> <h3 id="Package_clause">Package clause</h3> <p> A package clause begins each source file and defines the package to which the file belongs. </p> <pre class="ebnf"> PackageClause = "package" PackageName . PackageName = identifier . </pre> <p> The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>. </p> <pre> package math </pre> <p> A set of files sharing the same PackageName form the implementation of a package. An implementation may require that all source files for a package inhabit the same directory. </p> <h3 id="Import_declarations">Import declarations</h3> <p> An import declaration states that the source file containing the declaration depends on functionality of the <i>imported</i> package (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>) and enables access to <a href="#Exported_identifiers">exported</a> identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported. </p> <pre class="ebnf"> ImportDecl = "import" ( ImportSpec \| "(" { ImportSpec ";" } ")" ) . ImportSpec = [ "." \| PackageName ] ImportPath . ImportPath = string_lit . </pre> <p> The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a> to access exported identifiers of the package within the importing source file. It is declared in the <a href="#Blocks">file block</a>. If the PackageName is omitted, it defaults to the identifier specified in the <a href="#Package_clause">package clause</a> of the imported package. If an explicit period (<code>.</code>) appears instead of a name, all the package's exported identifiers declared in that package's <a href="#Blocks">package block</a> will be declared in the importing source file's file block and must be accessed without a qualifier. </p> <p> The interpretation of the ImportPath is implementation-dependent but it is typically a substring of the full file name of the compiled package and may be relative to a repository of installed packages. </p> <p> Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging to <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a> L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters <code>!"#$%&'(),:;<=>?[\]^`{\|}</code> and the Unicode replacement character U+FFFD. </p> <p> Assume we have compiled a package containing the package clause <code>package math</code>, which exports function <code>Sin</code>, and installed the compiled package in the file identified by <code>"lib/math"</code>. This table illustrates how <code>Sin</code> is accessed in files that import the package after the various types of import declaration. </p> <pre class="grammar"> Import declaration Local name of Sin import "lib/math" math.Sin import m "lib/math" m.Sin import . "lib/math" Sin </pre> <p> An import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself, directly or indirectly, or to directly import a package without referring to any of its exported identifiers. To import a package solely for its side-effects (initialization), use the <a href="#Blank_identifier">blank</a> identifier as explicit package name: </p> <pre> import _ "lib/math" </pre> <h3 id="An_example_package">An example package</h3> <p> Here is a complete Go package that implements a concurrent prime sieve. </p> <pre> package main import "fmt" // Send the sequence 2, 3, 4, … to channel 'ch'. func generate(ch chan<- int) { for i := 2; ; i++ { ch <- i // Send 'i' to channel 'ch'. } } // Copy the values from channel 'src' to channel 'dst', // removing those divisible by 'prime'. func filter(src <-chan int, dst chan<- int, prime int) { for i := range src { // Loop over values received from 'src'. if i%prime != 0 { dst <- i // Send 'i' to channel 'dst'. } } } // The prime sieve: Daisy-chain filter processes together. func sieve() { ch := make(chan int) // Create a new channel. go generate(ch) // Start generate() as a subprocess. for { prime := <-ch fmt.Print(prime, "\n") ch1 := make(chan int) go filter(ch, ch1, prime) ch = ch1 } } func main() { sieve() } </pre> <h2 id="Program_initialization_and_execution">Program initialization and execution</h2> <h3 id="The_zero_value">The zero value</h3> <p> When storage is allocated for a <a href="#Variables">variable</a>, either through a declaration or a call of <code>new</code>, or when a new value is created, either through a composite literal or a call of <code>make</code>, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the <i>zero value</i> for its type: <code>false</code> for booleans, <code>0</code> for numeric types, <code>""</code> for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified. </p> <p> These two simple declarations are equivalent: </p> <pre> var i int var i int = 0 </pre> <p> After </p> <pre> type T struct { i int; f float64; next T } t := new(T) </pre> <p> the following holds: </p> <pre> t.i == 0 t.f == 0.0 t.next == nil </pre> <p> The same would also be true after </p> <pre> var t T </pre> <h3 id="Package_initialization">Package initialization</h3> <p> Within a package, package-level variable initialization proceeds stepwise, with each step selecting the variable earliest in <i>declaration order</i> which has no dependencies on uninitialized variables. </p> <p> More precisely, a package-level variable is considered <i>ready for initialization</i> if it is not yet initialized and either has no <a href="#Variable_declarations">initialization expression</a> or its initialization expression has no <i>dependencies</i> on uninitialized variables. Initialization proceeds by repeatedly initializing the next package-level variable that is earliest in declaration order and ready for initialization, until there are no variables ready for initialization. </p> <p> If any variables are still uninitialized when this process ends, those variables are part of one or more initialization cycles, and the program is not valid. </p> <p> Multiple variables on the left-hand side of a variable declaration initialized by single (multi-valued) expression on the right-hand side are initialized together: If any of the variables on the left-hand side is initialized, all those variables are initialized in the same step. </p> <pre> var x = a var a, b = f() // a and b are initialized together, before x is initialized </pre> <p> For the purpose of package initialization, <a href="#Blank_identifier">blank</a> variables are treated like any other variables in declarations. </p> <p> The declaration order of variables declared in multiple files is determined by the order in which the files are presented to the compiler: Variables declared in the first file are declared before any of the variables declared in the second file, and so on. </p> <p> Dependency analysis does not rely on the actual values of the variables, only on lexical <i>references</i> to them in the source, analyzed transitively. For instance, if a variable <code>x</code>'s initialization expression refers to a function whose body refers to variable <code>y</code> then <code>x</code> depends on <code>y</code>. Specifically: </p> <ul> <li> A reference to a variable or function is an identifier denoting that variable or function. </li> <li> A reference to a method <code>m</code> is a <a href="#Method_values">method value</a> or <a href="#Method_expressions">method expression</a> of the form <code>t.m</code>, where the (static) type of <code>t</code> is not an interface type, and the method <code>m</code> is in the <a href="#Method_sets">method set</a> of <code>t</code>. It is immaterial whether the resulting function value <code>t.m</code> is invoked. </li> <li> A variable, function, or method <code>x</code> depends on a variable <code>y</code> if <code>x</code>'s initialization expression or body (for functions and methods) contains a reference to <code>y</code> or to a function or method that depends on <code>y</code>. </li> </ul> <p> For example, given the declarations </p> <pre> var ( a = c + b // == 9 b = f() // == 4 c = f() // == 5 d = 3 // == 5 after initialization has finished ) func f() int { d++ return d } </pre> <p> the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>. Note that the order of subexpressions in initialization expressions is irrelevant: <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization order in this example. </p> <p> Dependency analysis is performed per package; only references referring to variables, functions, and (non-interface) methods declared in the current package are considered. If other, hidden, data dependencies exists between variables, the initialization order between those variables is unspecified. </p> <p> For instance, given the declarations </p> <pre> var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b var _ = sideEffect() // unrelated to x, a, or b var a = b var b = 42 type I interface { ab() []int } type T struct{} func (T) ab() []int { return []int{a, b} } </pre> <p> the variable <code>a</code> will be initialized after <code>b</code> but whether <code>x</code> is initialized before <code>b</code>, between <code>b</code> and <code>a</code>, or after <code>a</code>, and thus also the moment at which <code>sideEffect()</code> is called (before or after <code>x</code> is initialized) is not specified. </p> <p> Variables may also be initialized using functions named <code>init</code> declared in the package block, with no arguments and no result parameters. </p> <pre> func init() { … } </pre> <p> Multiple such functions may be defined per package, even within a single source file. In the package block, the <code>init</code> identifier can be used only to declare <code>init</code> functions, yet the identifier itself is not <a href="#Declarations_and_scope">declared</a>. Thus <code>init</code> functions cannot be referred to from anywhere in a program. </p> <p> A package with no imports is initialized by assigning initial values to all its package-level variables followed by calling all <code>init</code> functions in the order they appear in the source, possibly in multiple files, as presented to the compiler. If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package, the imported package will be initialized only once. The importing of packages, by construction, guarantees that there can be no cyclic initialization dependencies. </p> <p> Package initialization—variable initialization and the invocation of <code>init</code> functions—happens in a single goroutine, sequentially, one package at a time. An <code>init</code> function may launch other goroutines, which can run concurrently with the initialization code. However, initialization always sequences the <code>init</code> functions: it will not invoke the next one until the previous one has returned. </p> <p> To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler. </p> <h3 id="Program_execution">Program execution</h3> <p> A complete program is created by linking a single, unimported package called the <i>main package</i> with all the packages it imports, transitively. The main package must have package name <code>main</code> and declare a function <code>main</code> that takes no arguments and returns no value. </p> <pre> func main() { … } </pre> <p> Program execution begins by initializing the main package and then invoking the function <code>main</code>. When that function invocation returns, the program exits. It does not wait for other (non-<code>main</code>) goroutines to complete. </p> <h2 id="Errors">Errors</h2> <p> The predeclared type <code>error</code> is defined as </p> <pre> type error interface { Error() string } </pre> <p> It is the conventional interface for representing an error condition, with the nil value representing no error. For instance, a function to read data from a file might be defined: </p> <pre> func Read(f File, b []byte) (n int, err error) </pre> <h2 id="Run_time_panics">Run-time panics</h2> <p> Execution errors such as attempting to index an array out of bounds trigger a <i>run-time panic</i> equivalent to a call of the built-in function <a href="#Handling_panics"><code>panic</code></a> with a value of the implementation-defined interface type <code>runtime.Error</code>. That type satisfies the predeclared interface type <a href="#Errors"><code>error</code></a>. The exact error values that represent distinct run-time error conditions are unspecified. </p> <pre> package runtime type Error interface { error // and perhaps other methods } </pre> <h2 id="System_considerations">System considerations</h2> <h3 id="Package_unsafe">Package <code>unsafe</code></h3> <p> The built-in package <code>unsafe</code>, known to the compiler and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>, provides facilities for low-level programming including operations that violate the type system. A package using <code>unsafe</code> must be vetted manually for type safety and may not be portable. The package provides the following interface: </p> <pre class="grammar"> package unsafe type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type type Pointer ArbitraryType func Alignof(variable ArbitraryType) uintptr func Offsetof(selector ArbitraryType) uintptr func Sizeof(variable ArbitraryType) uintptr type IntegerType int // shorthand for an integer type; it is not a real type func Add(ptr Pointer, len IntegerType) Pointer func Slice(ptr ArbitraryType, len IntegerType) []ArbitraryType </pre> <p> A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code> value may not be <a href="#Address_operators">dereferenced</a>. Any pointer or value of <a href="#Types">underlying type</a> <code>uintptr</code> can be converted to a type of underlying type <code>Pointer</code> and vice versa. The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined. </p> <pre> var f float64 bits = (uint64)(unsafe.Pointer(&f)) type ptr unsafe.Pointer bits = (uint64)(ptr(&f)) var p ptr = nil </pre> <p> The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code> of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code> as if <code>v</code> was declared via <code>var v = x</code>. </p> <p> The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a> <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code> or <code>s</code>, and returns the field offset in bytes relative to the struct's address. If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable without pointer indirections through fields of the struct. For a struct <code>s</code> with field <code>f</code>: </p> <pre> uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f)) </pre> <p> Computer architectures may require memory addresses to be <i>aligned</i>; that is, for addresses of a variable to be a multiple of a factor, the variable's type's <i>alignment</i>. The function <code>Alignof</code> takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable <code>x</code>: </p> <pre> uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0 </pre> <p> Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code> are compile-time constant expressions of type <code>uintptr</code>. </p> <p> The function <code>Add</code> adds <code>len</code> to <code>ptr</code> and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code>. The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>. A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>. The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply. </p> <p> The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code> and whose length and capacity are <code>len</code>. <code>Slice(ptr, len)</code> is equivalent to </p> <pre> ([len]ArbitraryType)(unsafe.Pointer(ptr))[:] </pre> <p> except that, as a special case, if <code>ptr</code> is <code>nil</code> and <code>len</code> is zero, <code>Slice</code> returns <code>nil</code>. </p> <p> The <code>len</code> argument must be of integer type or an untyped <a href="#Constants">constant</a>. A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>. At run time, if <code>len</code> is negative, or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3> <p> For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed: </p> <pre class="grammar"> type size in bytes byte, uint8, int8 1 uint16, int16 2 uint32, int32, float32 4 uint64, int64, float64, complex64 8 complex128 16 </pre> <p> The following minimal alignment properties are guaranteed: </p> <ol> <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1. </li> <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1. </li> <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as the alignment of a variable of the array's element type. </li> </ol> <p> A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory. </p>	211.57 KB	2024-02-02 18:09
📄 go_mem.html <!--{ "Title": "The Go Memory Model", "Subtitle": "Version of June 6, 2022", "Path": "/ref/mem" }--> <style> p.rule { font-style: italic; } </style> <h2 id="introduction">Introduction</h2> <p> The Go memory model specifies the conditions under which reads of a variable in one goroutine can be guaranteed to observe values produced by writes to the same variable in a different goroutine. </p> <h3 id="advice">Advice</h3> <p> Programs that modify data being simultaneously accessed by multiple goroutines must serialize such access. </p> <p> To serialize access, protect the data with channel operations or other synchronization primitives such as those in the <a href="/pkg/sync/"><code>sync</code></a> and <a href="/pkg/sync/atomic/"><code>sync/atomic</code></a> packages. </p> <p> If you must read the rest of this document to understand the behavior of your program, you are being too clever. </p> <p> Don't be clever. </p> <h3 id="overview">Informal Overview</h3> <p> Go approaches its memory model in much the same way as the rest of the language, aiming to keep the semantics simple, understandable, and useful. This section gives a general overview of the approach and should suffice for most programmers. The memory model is specified more formally in the next section. </p> <p> A <em>data race</em> is defined as a write to a memory location happening concurrently with another read or write to that same location, unless all the accesses involved are atomic data accesses as provided by the <code>sync/atomic</code> package. As noted already, programmers are strongly encouraged to use appropriate synchronization to avoid data races. In the absence of data races, Go programs behave as if all the goroutines were multiplexed onto a single processor. This property is sometimes referred to as DRF-SC: data-race-free programs execute in a sequentially consistent manner. </p> <p> While programmers should write Go programs without data races, there are limitations to what a Go implementation can do in response to a data race. An implementation may always react to a data race by reporting the race and terminating the program. Otherwise, each read of a single-word-sized or sub-word-sized memory location must observe a value actually written to that location (perhaps by a concurrent executing goroutine) and not yet overwritten. These implementation constraints make Go more like Java or JavaScript, in that most races have a limited number of outcomes, and less like C and C++, where the meaning of any program with a race is entirely undefined, and the compiler may do anything at all. Go's approach aims to make errant programs more reliable and easier to debug, while still insisting that races are errors and that tools can diagnose and report them. </p> <h2 id="model">Memory Model</h2> <p> The following formal definition of Go's memory model closely follows the approach presented by Hans-J. Boehm and Sarita V. Adve in “<a href="https://www.hpl.hp.com/techreports/2008/HPL-2008-56.pdf">Foundations of the C++ Concurrency Memory Model</a>”, published in PLDI 2008. The definition of data-race-free programs and the guarantee of sequential consistency for race-free programs are equivalent to the ones in that work. </p> <p> The memory model describes the requirements on program executions, which are made up of goroutine executions, which in turn are made up of memory operations. </p> <p> A <i>memory operation</i> is modeled by four details: </p> <ul> <li>its kind, indicating whether it is an ordinary data read, an ordinary data write, or a <i>synchronizing operation</i> such as an atomic data access, a mutex operation, or a channel operation, <li>its location in the program, <li>the memory location or variable being accessed, and <li>the values read or written by the operation. </ul> <p> Some memory operations are <i>read-like</i>, including read, atomic read, mutex lock, and channel receive. Other memory operations are <i>write-like</i>, including write, atomic write, mutex unlock, channel send, and channel close. Some, such as atomic compare-and-swap, are both read-like and write-like. </p> <p> A <i>goroutine execution</i> is modeled as a set of memory operations executed by a single goroutine. </p> <p> <b>Requirement 1</b>: The memory operations in each goroutine must correspond to a correct sequential execution of that goroutine, given the values read from and written to memory. That execution must be consistent with the <i>sequenced before</i> relation, defined as the partial order requirements set out by the <a href="/ref/spec">Go language specification</a> for Go's control flow constructs as well as the <a href="/ref/spec#Order_of_evaluation">order of evaluation for expressions</a>. </p> <p> A Go <i>program execution</i> is modeled as a set of goroutine executions, together with a mapping <i>W</i> that specifies the write-like operation that each read-like operation reads from. (Multiple executions of the same program can have different program executions.) </p> <p> <b>Requirement 2</b>: For a given program execution, the mapping <i>W</i>, when limited to synchronizing operations, must be explainable by some implicit total order of the synchronizing operations that is consistent with sequencing and the values read and written by those operations. </p> <p> The <i>synchronized before</i> relation is a partial order on synchronizing memory operations, derived from <i>W</i>. If a synchronizing read-like memory operation <i>r</i> observes a synchronizing write-like memory operation <i>w</i> (that is, if <i>W</i>(<i>r</i>) = <i>w</i>), then <i>w</i> is synchronized before <i>r</i>. Informally, the synchronized before relation is a subset of the implied total order mentioned in the previous paragraph, limited to the information that <i>W</i> directly observes. </p> <p> The <i>happens before</i> relation is defined as the transitive closure of the union of the sequenced before and synchronized before relations. </p> <p> <b>Requirement 3</b>: For an ordinary (non-synchronizing) data read <i>r</i> on a memory location <i>x</i>, <i>W</i>(<i>r</i>) must be a write <i>w</i> that is <i>visible</i> to <i>r</i>, where visible means that both of the following hold: </p> <ol> <li><i>w</i> happens before <i>r</i>. <li><i>w</i> does not happen before any other write <i>w'</i> (to <i>x</i>) that happens before <i>r</i>. </ol> <p> A <i>read-write data race</i> on memory location <i>x</i> consists of a read-like memory operation <i>r</i> on <i>x</i> and a write-like memory operation <i>w</i> on <i>x</i>, at least one of which is non-synchronizing, which are unordered by happens before (that is, neither <i>r</i> happens before <i>w</i> nor <i>w</i> happens before <i>r</i>). </p> <p> A <i>write-write data race</i> on memory location <i>x</i> consists of two write-like memory operations <i>w</i> and <i>w'</i> on <i>x</i>, at least one of which is non-synchronizing, which are unordered by happens before. </p> <p> Note that if there are no read-write or write-write data races on memory location <i>x</i>, then any read <i>r</i> on <i>x</i> has only one possible <i>W</i>(<i>r</i>): the single <i>w</i> that immediately precedes it in the happens before order. </p> <p> More generally, it can be shown that any Go program that is data-race-free, meaning it has no program executions with read-write or write-write data races, can only have outcomes explained by some sequentially consistent interleaving of the goroutine executions. (The proof is the same as Section 7 of Boehm and Adve's paper cited above.) This property is called DRF-SC. </p> <p> The intent of the formal definition is to match the DRF-SC guarantee provided to race-free programs by other languages, including C, C++, Java, JavaScript, Rust, and Swift. </p> <p> Certain Go language operations such as goroutine creation and memory allocation act as synchronization operations. The effect of these operations on the synchronized-before partial order is documented in the “Synchronization” section below. Individual packages are responsible for providing similar documentation for their own operations. </p> <h2 id="restrictions">Implementation Restrictions for Programs Containing Data Races</h2> <p> The preceding section gave a formal definition of data-race-free program execution. This section informally describes the semantics that implementations must provide for programs that do contain races. </p> <p> Any implementation can, upon detecting a data race, report the race and halt execution of the program. Implementations using ThreadSanitizer (accessed with “<code>go</code> <code>build</code> <code>-race</code>”) do exactly this. </p> <p> A read of an array, struct, or complex number may by implemented as a read of each individual sub-value (array element, struct field, or real/imaginary component), in any order. Similarly, a write of an array, struct, or complex number may be implemented as a write of each individual sub-value, in any order. </p> <p> A read <i>r</i> of a memory location <i>x</i> holding a value that is not larger than a machine word must observe some write <i>w</i> such that <i>r</i> does not happen before <i>w</i> and there is no write <i>w'</i> such that <i>w</i> happens before <i>w'</i> and <i>w'</i> happens before <i>r</i>. That is, each read must observe a value written by a preceding or concurrent write. </p> <p> Additionally, observation of acausal and “out of thin air” writes is disallowed. </p> <p> Reads of memory locations larger than a single machine word are encouraged but not required to meet the same semantics as word-sized memory locations, observing a single allowed write <i>w</i>. For performance reasons, implementations may instead treat larger operations as a set of individual machine-word-sized operations in an unspecified order. This means that races on multiword data structures can lead to inconsistent values not corresponding to a single write. When the values depend on the consistency of internal (pointer, length) or (pointer, type) pairs, as can be the case for interface values, maps, slices, and strings in most Go implementations, such races can in turn lead to arbitrary memory corruption. </p> <p> Examples of incorrect synchronization are given in the “Incorrect synchronization” section below. </p> <p> Examples of the limitations on implementations are given in the “Incorrect compilation” section below. </p> <h2 id="synchronization">Synchronization</h2> <h3 id="init">Initialization</h3> <p> Program initialization runs in a single goroutine, but that goroutine may create other goroutines, which run concurrently. </p> <p class="rule"> If a package <code>p</code> imports package <code>q</code>, the completion of <code>q</code>'s <code>init</code> functions happens before the start of any of <code>p</code>'s. </p> <p class="rule"> The completion of all <code>init</code> functions is synchronized before the start of the function <code>main.main</code>. </p> <h3 id="go">Goroutine creation</h3> <p class="rule"> The <code>go</code> statement that starts a new goroutine is synchronized before the start of the goroutine's execution. </p> <p> For example, in this program: </p> <pre> var a string func f() { print(a) } func hello() { a = "hello, world" go f() } </pre> <p> calling <code>hello</code> will print <code>"hello, world"</code> at some point in the future (perhaps after <code>hello</code> has returned). </p> <h3 id="goexit">Goroutine destruction</h3> <p> The exit of a goroutine is not guaranteed to be synchronized before any event in the program. For example, in this program: </p> <pre> var a string func hello() { go func() { a = "hello" }() print(a) } </pre> <p> the assignment to <code>a</code> is not followed by any synchronization event, so it is not guaranteed to be observed by any other goroutine. In fact, an aggressive compiler might delete the entire <code>go</code> statement. </p> <p> If the effects of a goroutine must be observed by another goroutine, use a synchronization mechanism such as a lock or channel communication to establish a relative ordering. </p> <h3 id="chan">Channel communication</h3> <p> Channel communication is the main method of synchronization between goroutines. Each send on a particular channel is matched to a corresponding receive from that channel, usually in a different goroutine. </p> <p class="rule"> A send on a channel is synchronized before the completion of the corresponding receive from that channel. </p> <p> This program: </p> <pre> var c = make(chan int, 10) var a string func f() { a = "hello, world" c <- 0 } func main() { go f() <-c print(a) } </pre> <p> is guaranteed to print <code>"hello, world"</code>. The write to <code>a</code> is sequenced before the send on <code>c</code>, which is synchronized before the corresponding receive on <code>c</code> completes, which is sequenced before the <code>print</code>. </p> <p class="rule"> The closing of a channel is synchronized before a receive that returns a zero value because the channel is closed. </p> <p> In the previous example, replacing <code>c <- 0</code> with <code>close(c)</code> yields a program with the same guaranteed behavior. </p> <p class="rule"> A receive from an unbuffered channel is synchronized before the completion of the corresponding send on that channel. </p> <p> This program (as above, but with the send and receive statements swapped and using an unbuffered channel): </p> <pre> var c = make(chan int) var a string func f() { a = "hello, world" <-c } func main() { go f() c <- 0 print(a) } </pre> <p> is also guaranteed to print <code>"hello, world"</code>. The write to <code>a</code> is sequenced before the receive on <code>c</code>, which is synchronized before the corresponding send on <code>c</code> completes, which is sequenced before the <code>print</code>. </p> <p> If the channel were buffered (e.g., <code>c = make(chan int, 1)</code>) then the program would not be guaranteed to print <code>"hello, world"</code>. (It might print the empty string, crash, or do something else.) </p> <p class="rule"> The <i>k</i>th receive on a channel with capacity <i>C</i> is synchronized before the completion of the <i>k</i>+<i>C</i>th send from that channel completes. </p> <p> This rule generalizes the previous rule to buffered channels. It allows a counting semaphore to be modeled by a buffered channel: the number of items in the channel corresponds to the number of active uses, the capacity of the channel corresponds to the maximum number of simultaneous uses, sending an item acquires the semaphore, and receiving an item releases the semaphore. This is a common idiom for limiting concurrency. </p> <p> This program starts a goroutine for every entry in the work list, but the goroutines coordinate using the <code>limit</code> channel to ensure that at most three are running work functions at a time. </p> <pre> var limit = make(chan int, 3) func main() { for _, w := range work { go func(w func()) { limit <- 1 w() <-limit }(w) } select{} } </pre> <h3 id="locks">Locks</h3> <p> The <code>sync</code> package implements two lock data types, <code>sync.Mutex</code> and <code>sync.RWMutex</code>. </p> <p class="rule"> For any <code>sync.Mutex</code> or <code>sync.RWMutex</code> variable <code>l</code> and <i>n</i> < <i>m</i>, call <i>n</i> of <code>l.Unlock()</code> is synchronized before call <i>m</i> of <code>l.Lock()</code> returns. </p> <p> This program: </p> <pre> var l sync.Mutex var a string func f() { a = "hello, world" l.Unlock() } func main() { l.Lock() go f() l.Lock() print(a) } </pre> <p> is guaranteed to print <code>"hello, world"</code>. The first call to <code>l.Unlock()</code> (in <code>f</code>) is synchronized before the second call to <code>l.Lock()</code> (in <code>main</code>) returns, which is sequenced before the <code>print</code>. </p> <p class="rule"> For any call to <code>l.RLock</code> on a <code>sync.RWMutex</code> variable <code>l</code>, there is an <i>n</i> such that the <i>n</i>th call to <code>l.Unlock</code> is synchronized before the return from <code>l.RLock</code>, and the matching call to <code>l.RUnlock</code> is synchronized before the return from call <i>n</i>+1 to <code>l.Lock</code>. </p> <p class="rule"> A successful call to <code>l.TryLock</code> (or <code>l.TryRLock</code>) is equivalent to a call to <code>l.Lock</code> (or <code>l.RLock</code>). An unsuccessful call has no synchronizing effect at all. As far as the memory model is concerned, <code>l.TryLock</code> (or <code>l.TryRLock</code>) may be considered to be able to return false even when the mutex <i>l</i> is unlocked. </p> <h3 id="once">Once</h3> <p> The <code>sync</code> package provides a safe mechanism for initialization in the presence of multiple goroutines through the use of the <code>Once</code> type. Multiple threads can execute <code>once.Do(f)</code> for a particular <code>f</code>, but only one will run <code>f()</code>, and the other calls block until <code>f()</code> has returned. </p> <p class="rule"> The completion of a single call of <code>f()</code> from <code>once.Do(f)</code> is synchronized before the return of any call of <code>once.Do(f)</code>. </p> <p> In this program: </p> <pre> var a string var once sync.Once func setup() { a = "hello, world" } func doprint() { once.Do(setup) print(a) } func twoprint() { go doprint() go doprint() } </pre> <p> calling <code>twoprint</code> will call <code>setup</code> exactly once. The <code>setup</code> function will complete before either call of <code>print</code>. The result will be that <code>"hello, world"</code> will be printed twice. </p> <h3 id="atomic">Atomic Values</h3> <p> The APIs in the <a href="/pkg/sync/atomic/"><code>sync/atomic</code></a> package are collectively “atomic operations” that can be used to synchronize the execution of different goroutines. If the effect of an atomic operation <i>A</i> is observed by atomic operation <i>B</i>, then <i>A</i> is synchronized before <i>B</i>. All the atomic operations executed in a program behave as though executed in some sequentially consistent order. </p> <p> The preceding definition has the same semantics as C++’s sequentially consistent atomics and Java’s <code>volatile</code> variables. </p> <h3 id="finalizer">Finalizers</h3> <p> The <a href="/pkg/runtime/"><code>runtime</code></a> package provides a <code>SetFinalizer</code> function that adds a finalizer to be called when a particular object is no longer reachable by the program. A call to <code>SetFinalizer(x, f)</code> is synchronized before the finalization call <code>f(x)</code>. </p> <h3 id="more">Additional Mechanisms</h3> <p> The <code>sync</code> package provides additional synchronization abstractions, including <a href="/pkg/sync/#Cond">condition variables</a>, <a href="/pkg/sync/#Map">lock-free maps</a>, <a href="/pkg/sync/#Pool">allocation pools</a>, and <a href="/pkg/sync/#WaitGroup">wait groups</a>. The documentation for each of these specifies the guarantees it makes concerning synchronization. </p> <p> Other packages that provide synchronization abstractions should document the guarantees they make too. </p> <h2 id="badsync">Incorrect synchronization</h2> <p> Programs with races are incorrect and can exhibit non-sequentially consistent executions. In particular, note that a read <i>r</i> may observe the value written by any write <i>w</i> that executes concurrently with <i>r</i>. Even if this occurs, it does not imply that reads happening after <i>r</i> will observe writes that happened before <i>w</i>. </p> <p> In this program: </p> <pre> var a, b int func f() { a = 1 b = 2 } func g() { print(b) print(a) } func main() { go f() g() } </pre> <p> it can happen that <code>g</code> prints <code>2</code> and then <code>0</code>. </p> <p> This fact invalidates a few common idioms. </p> <p> Double-checked locking is an attempt to avoid the overhead of synchronization. For example, the <code>twoprint</code> program might be incorrectly written as: </p> <pre> var a string var done bool func setup() { a = "hello, world" done = true } func doprint() { if !done { once.Do(setup) } print(a) } func twoprint() { go doprint() go doprint() } </pre> <p> but there is no guarantee that, in <code>doprint</code>, observing the write to <code>done</code> implies observing the write to <code>a</code>. This version can (incorrectly) print an empty string instead of <code>"hello, world"</code>. </p> <p> Another incorrect idiom is busy waiting for a value, as in: </p> <pre> var a string var done bool func setup() { a = "hello, world" done = true } func main() { go setup() for !done { } print(a) } </pre> <p> As before, there is no guarantee that, in <code>main</code>, observing the write to <code>done</code> implies observing the write to <code>a</code>, so this program could print an empty string too. Worse, there is no guarantee that the write to <code>done</code> will ever be observed by <code>main</code>, since there are no synchronization events between the two threads. The loop in <code>main</code> is not guaranteed to finish. </p> <p> There are subtler variants on this theme, such as this program. </p> <pre> type T struct { msg string } var g T func setup() { t := new(T) t.msg = "hello, world" g = t } func main() { go setup() for g == nil { } print(g.msg) } </pre> <p> Even if <code>main</code> observes <code>g != nil</code> and exits its loop, there is no guarantee that it will observe the initialized value for <code>g.msg</code>. </p> <p> In all these examples, the solution is the same: use explicit synchronization. </p> <h2 id="badcompiler">Incorrect compilation</h2> <p> The Go memory model restricts compiler optimizations as much as it does Go programs. Some compiler optimizations that would be valid in single-threaded programs are not valid in all Go programs. In particular, a compiler must not introduce writes that do not exist in the original program, it must not allow a single read to observe multiple values, and it must not allow a single write to write multiple values. </p> <p> All the following examples assume that `p` and `q` refer to memory locations accessible to multiple goroutines. </p> <p> Not introducing data races into race-free programs means not moving writes out of conditional statements in which they appear. For example, a compiler must not invert the conditional in this program: </p> <pre> p = 1 if cond { p = 2 } </pre> <p> That is, the compiler must not rewrite the program into this one: </p> <pre> p = 2 if !cond { p = 1 } </pre> <p> If <code>cond</code> is false and another goroutine is reading <code>p</code>, then in the original program, the other goroutine can only observe any prior value of <code>p</code> and <code>1</code>. In the rewritten program, the other goroutine can observe <code>2</code>, which was previously impossible. </p> <p> Not introducing data races also means not assuming that loops terminate. For example, a compiler must in general not move the accesses to <code>p</code> or <code>q</code> ahead of the loop in this program: </p> <pre> n := 0 for e := list; e != nil; e = e.next { n++ } i := p q = 1 </pre> <p> If <code>list</code> pointed to a cyclic list, then the original program would never access <code>p</code> or <code>q</code>, but the rewritten program would. (Moving `p` ahead would be safe if the compiler can prove `p` will not panic; moving `q` ahead would also require the compiler proving that no other goroutine can access `q`.) </p> <p> Not introducing data races also means not assuming that called functions always return or are free of synchronization operations. For example, a compiler must not move the accesses to <code>p</code> or <code>q</code> ahead of the function call in this program (at least not without direct knowledge of the precise behavior of <code>f</code>): </p> <pre> f() i := p q = 1 </pre> <p> If the call never returned, then once again the original program would never access <code>p</code> or <code>q</code>, but the rewritten program would. And if the call contained synchronizing operations, then the original program could establish happens before edges preceding the accesses to <code>p</code> and <code>q</code>, but the rewritten program would not. </p> <p> Not allowing a single read to observe multiple values means not reloading local variables from shared memory. For example, a compiler must not discard <code>i</code> and reload it a second time from <code>p</code> in this program: </p> <pre> i := p if i < 0 \|\| i >= len(funcs) { panic("invalid function index") } ... complex code ... // compiler must NOT reload i = p here funcs[i]() </pre> <p> If the complex code needs many registers, a compiler for single-threaded programs could discard <code>i</code> without saving a copy and then reload <code>i = p</code> just before <code>funcs[i]()</code>. A Go compiler must not, because the value of <code>p</code> may have changed. (Instead, the compiler could spill <code>i</code> to the stack.) </p> <p> Not allowing a single write to write multiple values also means not using the memory where a local variable will be written as temporary storage before the write. For example, a compiler must not use <code>p</code> as temporary storage in this program: </p> <pre> p = i + p/2 </pre> <p> That is, it must not rewrite the program into this one: </p> <pre> p /= 2 p += i </pre> <p> If <code>i</code> and <code>p</code> start equal to 2, the original code does <code>p = 3</code>, so a racing thread can read only 2 or 3 from <code>p</code>. The rewritten code does <code>p = 1</code> and then <code>p = 3</code>, allowing a racing thread to read 1 as well. </p> <p> Note that all these optimizations are permitted in C/C++ compilers: a Go compiler sharing a back end with a C/C++ compiler must take care to disable optimizations that are invalid for Go. </p> <p> Note that the prohibition on introducing data races does not apply if the compiler can prove that the races do not affect correct execution on the target platform. For example, on essentially all CPUs, it is valid to rewrite </p> <pre> n := 0 for i := 0; i < m; i++ { n += shared } </pre> into: <pre> n := 0 local := shared for i := 0; i < m; i++ { n += local } </pre> <p> provided it can be proved that <code>*shared</code> will not fault on access, because the potential added read will not affect any existing concurrent reads or writes. On the other hand, the rewrite would not be valid in a source-to-source translator. </p> <h2 id="conclusion">Conclusion</h2> <p> Go programmers writing data-race-free programs can rely on sequentially consistent execution of those programs, just as in essentially all other modern programming languages. </p> <p> When it comes to programs with races, both programmers and compilers should remember the advice: don't be clever. </p>	26.61 KB	2024-02-02 18:09
📄 go_spec.html <!--{ "Title": "The Go Programming Language Specification", "Subtitle": "Language version go1.22 (Jan 30, 2023)", "Path": "/ref/spec" }--> <h2 id="Introduction">Introduction</h2> <p> This is the reference manual for the Go programming language. The pre-Go1.18 version, without generics, can be found <a href="/doc/go1.17_spec.html">here</a>. For more information and other documents, see <a href="/">go.dev</a>. </p> <p> Go is a general-purpose language designed with systems programming in mind. It is strongly typed and garbage-collected and has explicit support for concurrent programming. Programs are constructed from <i>packages</i>, whose properties allow efficient management of dependencies. </p> <p> The syntax is compact and simple to parse, allowing for easy analysis by automatic tools such as integrated development environments. </p> <h2 id="Notation">Notation</h2> <p> The syntax is specified using a <a href="https://en.wikipedia.org/wiki/Wirth_syntax_notation">variant</a> of Extended Backus-Naur Form (EBNF): </p> <pre class="grammar"> Syntax = { Production } . Production = production_name "=" [ Expression ] "." . Expression = Term { "\|" Term } . Term = Factor { Factor } . Factor = production_name \| token [ "…" token ] \| Group \| Option \| Repetition . Group = "(" Expression ")" . Option = "[" Expression "]" . Repetition = "{" Expression "}" . </pre> <p> Productions are expressions constructed from terms and the following operators, in increasing precedence: </p> <pre class="grammar"> \| alternation () grouping [] option (0 or 1 times) {} repetition (0 to n times) </pre> <p> Lowercase production names are used to identify lexical (terminal) tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes <code>""</code> or back quotes <code>``</code>. </p> <p> The form <code>a … b</code> represents the set of characters from <code>a</code> through <code>b</code> as alternatives. The horizontal ellipsis <code>…</code> is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character <code>…</code> (as opposed to the three characters <code>...</code>) is not a token of the Go language. </p> <p> A link of the form [<a href="#Language_versions">Go 1.xx</a>] indicates that a described language feature (or some aspect of it) was changed or added with language version 1.xx and thus requires at minimum that language version to build. For details, see the <a href="#Language_versions">linked section</a> in the <a href="#Appendix">appendix</a>. </p> <h2 id="Source_code_representation">Source code representation</h2> <p> Source code is Unicode text encoded in <a href="https://en.wikipedia.org/wiki/UTF-8">UTF-8</a>. The text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term <i>character</i> to refer to a Unicode code point in the source text. </p> <p> Each code point is distinct; for instance, uppercase and lowercase letters are different characters. </p> <p> Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text. </p> <p> Implementation restriction: For compatibility with other tools, a compiler may ignore a UTF-8-encoded byte order mark (U+FEFF) if it is the first Unicode code point in the source text. A byte order mark may be disallowed anywhere else in the source. </p> <h3 id="Characters">Characters</h3> <p> The following terms are used to denote specific Unicode character categories: </p> <pre class="ebnf"> newline = /* the Unicode code point U+000A / . unicode_char = / an arbitrary Unicode code point except newline / . unicode_letter = / a Unicode code point categorized as "Letter" / . unicode_digit = / a Unicode code point categorized as "Number, decimal digit" / . </pre> <p> In <a href="https://www.unicode.org/versions/Unicode8.0.0/">The Unicode Standard 8.0</a>, Section 4.5 "General Category" defines a set of character categories. Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo as Unicode letters, and those in the Number category Nd as Unicode digits. </p> <h3 id="Letters_and_digits">Letters and digits</h3> <p> The underscore character <code>_</code> (U+005F) is considered a lowercase letter. </p> <pre class="ebnf"> letter = unicode_letter \| "_" . decimal_digit = "0" … "9" . binary_digit = "0" \| "1" . octal_digit = "0" … "7" . hex_digit = "0" … "9" \| "A" … "F" \| "a" … "f" . </pre> <h2 id="Lexical_elements">Lexical elements</h2> <h3 id="Comments">Comments</h3> <p> Comments serve as program documentation. There are two forms: </p> <ol> <li> <i>Line comments</i> start with the character sequence <code>//</code> and stop at the end of the line. </li> <li> <i>General comments</i> start with the character sequence <code>/</code> and stop with the first subsequent character sequence <code>/</code>. </li> </ol> <p> A comment cannot start inside a <a href="#Rune_literals">rune</a> or <a href="#String_literals">string literal</a>, or inside a comment. A general comment containing no newlines acts like a space. Any other comment acts like a newline. </p> <h3 id="Tokens">Tokens</h3> <p> Tokens form the vocabulary of the Go language. There are four classes: <i>identifiers</i>, <i>keywords</i>, <i>operators and punctuation</i>, and <i>literals</i>. <i>White space</i>, formed from spaces (U+0020), horizontal tabs (U+0009), carriage returns (U+000D), and newlines (U+000A), is ignored except as it separates tokens that would otherwise combine into a single token. Also, a newline or end of file may trigger the insertion of a <a href="#Semicolons">semicolon</a>. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token. </p> <h3 id="Semicolons">Semicolons</h3> <p> The formal syntax uses semicolons <code>";"</code> as terminators in a number of productions. Go programs may omit most of these semicolons using the following two rules: </p> <ol> <li> When the input is broken into tokens, a semicolon is automatically inserted into the token stream immediately after a line's final token if that token is <ul> <li>an <a href="#Identifiers">identifier</a> </li> <li>an <a href="#Integer_literals">integer</a>, <a href="#Floating-point_literals">floating-point</a>, <a href="#Imaginary_literals">imaginary</a>, <a href="#Rune_literals">rune</a>, or <a href="#String_literals">string</a> literal </li> <li>one of the <a href="#Keywords">keywords</a> <code>break</code>, <code>continue</code>, <code>fallthrough</code>, or <code>return</code> </li> <li>one of the <a href="#Operators_and_punctuation">operators and punctuation</a> <code>++</code>, <code>--</code>, <code>)</code>, <code>]</code>, or <code>}</code> </li> </ul> </li> <li> To allow complex statements to occupy a single line, a semicolon may be omitted before a closing <code>")"</code> or <code>"}"</code>. </li> </ol> <p> To reflect idiomatic use, code examples in this document elide semicolons using these rules. </p> <h3 id="Identifiers">Identifiers</h3> <p> Identifiers name program entities such as variables and types. An identifier is a sequence of one or more letters and digits. The first character in an identifier must be a letter. </p> <pre class="ebnf"> identifier = letter { letter \| unicode_digit } . </pre> <pre> a _x9 ThisVariableIsExported αβ </pre> <p> Some identifiers are <a href="#Predeclared_identifiers">predeclared</a>. </p> <h3 id="Keywords">Keywords</h3> <p> The following keywords are reserved and may not be used as identifiers. </p> <pre class="grammar"> break default func interface select case defer go map struct chan else goto package switch const fallthrough if range type continue for import return var </pre> <h3 id="Operators_and_punctuation">Operators and punctuation</h3> <p> The following character sequences represent <a href="#Operators">operators</a> (including <a href="#Assignment_statements">assignment operators</a>) and punctuation [<a href="#Go_1.18">Go 1.18</a>]: </p> <pre class="grammar"> + & += &= && == != ( ) - \| -= \|= \|\| < <= [ ] ^ = ^= <- > >= { } / << /= <<= ++ = := , ; % >> %= >>= -- ! ... . : &^ &^= ~ </pre> <h3 id="Integer_literals">Integer literals</h3> <p> An integer literal is a sequence of digits representing an <a href="#Constants">integer constant</a>. An optional prefix sets a non-decimal base: <code>0b</code> or <code>0B</code> for binary, <code>0</code>, <code>0o</code>, or <code>0O</code> for octal, and <code>0x</code> or <code>0X</code> for hexadecimal [<a href="#Go_1.13">Go 1.13</a>]. A single <code>0</code> is considered a decimal zero. In hexadecimal literals, letters <code>a</code> through <code>f</code> and <code>A</code> through <code>F</code> represent values 10 through 15. </p> <p> For readability, an underscore character <code>_</code> may appear after a base prefix or between successive digits; such underscores do not change the literal's value. </p> <pre class="ebnf"> int_lit = decimal_lit \| binary_lit \| octal_lit \| hex_lit . decimal_lit = "0" \| ( "1" … "9" ) [ [ "_" ] decimal_digits ] . binary_lit = "0" ( "b" \| "B" ) [ "_" ] binary_digits . octal_lit = "0" [ "o" \| "O" ] [ "_" ] octal_digits . hex_lit = "0" ( "x" \| "X" ) [ "_" ] hex_digits . decimal_digits = decimal_digit { [ "_" ] decimal_digit } . binary_digits = binary_digit { [ "_" ] binary_digit } . octal_digits = octal_digit { [ "_" ] octal_digit } . hex_digits = hex_digit { [ "_" ] hex_digit } . </pre> <pre> 42 4_2 0600 0_600 0o600 0O600 // second character is capital letter 'O' 0xBadFace 0xBad_Face 0x_67_7a_2f_cc_40_c6 170141183460469231731687303715884105727 170_141183_460469_231731_687303_715884_105727 _42 // an identifier, not an integer literal 42_ // invalid: _ must separate successive digits 4__2 // invalid: only one _ at a time 0_xBadFace // invalid: _ must separate successive digits </pre> <h3 id="Floating-point_literals">Floating-point literals</h3> <p> A floating-point literal is a decimal or hexadecimal representation of a <a href="#Constants">floating-point constant</a>. </p> <p> A decimal floating-point literal consists of an integer part (decimal digits), a decimal point, a fractional part (decimal digits), and an exponent part (<code>e</code> or <code>E</code> followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; one of the decimal point or the exponent part may be elided. An exponent value exp scales the mantissa (integer and fractional part) by 10<sup>exp</sup>. </p> <p> A hexadecimal floating-point literal consists of a <code>0x</code> or <code>0X</code> prefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits), and an exponent part (<code>p</code> or <code>P</code> followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; the radix point may be elided as well, but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.) An exponent value exp scales the mantissa (integer and fractional part) by 2<sup>exp</sup> [<a href="#Go_1.13">Go 1.13</a>]. </p> <p> For readability, an underscore character <code>_</code> may appear after a base prefix or between successive digits; such underscores do not change the literal value. </p> <pre class="ebnf"> float_lit = decimal_float_lit \| hex_float_lit . decimal_float_lit = decimal_digits "." [ decimal_digits ] [ decimal_exponent ] \| decimal_digits decimal_exponent \| "." decimal_digits [ decimal_exponent ] . decimal_exponent = ( "e" \| "E" ) [ "+" \| "-" ] decimal_digits . hex_float_lit = "0" ( "x" \| "X" ) hex_mantissa hex_exponent . hex_mantissa = [ "_" ] hex_digits "." [ hex_digits ] \| [ "_" ] hex_digits \| "." hex_digits . hex_exponent = ( "p" \| "P" ) [ "+" \| "-" ] decimal_digits . </pre> <pre> 0. 72.40 072.40 // == 72.40 2.71828 1.e+0 6.67428e-11 1E6 .25 .12345E+5 1_5. // == 15.0 0.15e+0_2 // == 15.0 0x1p-2 // == 0.25 0x2.p10 // == 2048.0 0x1.Fp+0 // == 1.9375 0X.8p-0 // == 0.5 0X_1FFFP-16 // == 0.1249847412109375 0x15e-2 // == 0x15e - 2 (integer subtraction) 0x.p1 // invalid: mantissa has no digits 1p-2 // invalid: p exponent requires hexadecimal mantissa 0x1.5e-2 // invalid: hexadecimal mantissa requires p exponent 1_.5 // invalid: _ must separate successive digits 1._5 // invalid: _ must separate successive digits 1.5_e1 // invalid: _ must separate successive digits 1.5e_1 // invalid: _ must separate successive digits 1.5e1_ // invalid: _ must separate successive digits </pre> <h3 id="Imaginary_literals">Imaginary literals</h3> <p> An imaginary literal represents the imaginary part of a <a href="#Constants">complex constant</a>. It consists of an <a href="#Integer_literals">integer</a> or <a href="#Floating-point_literals">floating-point</a> literal followed by the lowercase letter <code>i</code>. The value of an imaginary literal is the value of the respective integer or floating-point literal multiplied by the imaginary unit <i>i</i> [<a href="#Go_1.13">Go 1.13</a>] </p> <pre class="ebnf"> imaginary_lit = (decimal_digits \| int_lit \| float_lit) "i" . </pre> <p> For backward compatibility, an imaginary literal's integer part consisting entirely of decimal digits (and possibly underscores) is considered a decimal integer, even if it starts with a leading <code>0</code>. </p> <pre> 0i 0123i // == 123i for backward-compatibility 0o123i // == 0o123 1i == 83i 0xabci // == 0xabc * 1i == 2748i 0.i 2.71828i 1.e+0i 6.67428e-11i 1E6i .25i .12345E+5i 0x1p-2i // == 0x1p-2 * 1i == 0.25i </pre> <h3 id="Rune_literals">Rune literals</h3> <p> A rune literal represents a <a href="#Constants">rune constant</a>, an integer value identifying a Unicode code point. A rune literal is expressed as one or more characters enclosed in single quotes, as in <code>'x'</code> or <code>'\n'</code>. Within the quotes, any character may appear except newline and unescaped single quote. A single quoted character represents the Unicode value of the character itself, while multi-character sequences beginning with a backslash encode values in various formats. </p> <p> The simplest form represents the single character within the quotes; since Go source text is Unicode characters encoded in UTF-8, multiple UTF-8-encoded bytes may represent a single integer value. For instance, the literal <code>'a'</code> holds a single byte representing a literal <code>a</code>, Unicode U+0061, value <code>0x61</code>, while <code>'ä'</code> holds two bytes (<code>0xc3</code> <code>0xa4</code>) representing a literal <code>a</code>-dieresis, U+00E4, value <code>0xe4</code>. </p> <p> Several backslash escapes allow arbitrary values to be encoded as ASCII text. There are four ways to represent the integer value as a numeric constant: <code>\x</code> followed by exactly two hexadecimal digits; <code>\u</code> followed by exactly four hexadecimal digits; <code>\U</code> followed by exactly eight hexadecimal digits, and a plain backslash <code>\</code> followed by exactly three octal digits. In each case the value of the literal is the value represented by the digits in the corresponding base. </p> <p> Although these representations all result in an integer, they have different valid ranges. Octal escapes must represent a value between 0 and 255 inclusive. Hexadecimal escapes satisfy this condition by construction. The escapes <code>\u</code> and <code>\U</code> represent Unicode code points so within them some values are illegal, in particular those above <code>0x10FFFF</code> and surrogate halves. </p> <p> After a backslash, certain single-character escapes represent special values: </p> <pre class="grammar"> \a U+0007 alert or bell \b U+0008 backspace \f U+000C form feed \n U+000A line feed or newline \r U+000D carriage return \t U+0009 horizontal tab \v U+000B vertical tab \\ U+005C backslash \' U+0027 single quote (valid escape only within rune literals) \" U+0022 double quote (valid escape only within string literals) </pre> <p> An unrecognized character following a backslash in a rune literal is illegal. </p> <pre class="ebnf"> rune_lit = "'" ( unicode_value \| byte_value ) "'" . unicode_value = unicode_char \| little_u_value \| big_u_value \| escaped_char . byte_value = octal_byte_value \| hex_byte_value . octal_byte_value = `\` octal_digit octal_digit octal_digit . hex_byte_value = `\` "x" hex_digit hex_digit . little_u_value = `\` "u" hex_digit hex_digit hex_digit hex_digit . big_u_value = `\` "U" hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit hex_digit . escaped_char = `\` ( "a" \| "b" \| "f" \| "n" \| "r" \| "t" \| "v" \| `\` \| "'" \| `"` ) . </pre> <pre> 'a' 'ä' '本' '\t' '\000' '\007' '\377' '\x07' '\xff' '\u12e4' '\U00101234' '\'' // rune literal containing single quote character 'aa' // illegal: too many characters '\k' // illegal: k is not recognized after a backslash '\xa' // illegal: too few hexadecimal digits '\0' // illegal: too few octal digits '\400' // illegal: octal value over 255 '\uDFFF' // illegal: surrogate half '\U00110000' // illegal: invalid Unicode code point </pre> <h3 id="String_literals">String literals</h3> <p> A string literal represents a <a href="#Constants">string constant</a> obtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals. </p> <p> Raw string literals are character sequences between back quotes, as in <code>`foo`</code>. Within the quotes, any character may appear except back quote. The value of a raw string literal is the string composed of the uninterpreted (implicitly UTF-8-encoded) characters between the quotes; in particular, backslashes have no special meaning and the string may contain newlines. Carriage return characters ('\r') inside raw string literals are discarded from the raw string value. </p> <p> Interpreted string literals are character sequences between double quotes, as in <code>"bar"</code>. Within the quotes, any character may appear except newline and unescaped double quote. The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in <a href="#Rune_literals">rune literals</a> (except that <code>\'</code> is illegal and <code>\"</code> is legal), with the same restrictions. The three-digit octal (<code>\</code><i>nnn</i>) and two-digit hexadecimal (<code>\x</code><i>nn</i>) escapes represent individual <i>bytes</i> of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual <i>characters</i>. Thus inside a string literal <code>\377</code> and <code>\xFF</code> represent a single byte of value <code>0xFF</code>=255, while <code>ÿ</code>, <code>\u00FF</code>, <code>\U000000FF</code> and <code>\xc3\xbf</code> represent the two bytes <code>0xc3</code> <code>0xbf</code> of the UTF-8 encoding of character U+00FF. </p> <pre class="ebnf"> string_lit = raw_string_lit \| interpreted_string_lit . raw_string_lit = "`" { unicode_char \| newline } "`" . interpreted_string_lit = `"` { unicode_value \| byte_value } `"` . </pre> <pre> `abc` // same as "abc" `\n \n` // same as "\\n\n\\n" "\n" "\"" // same as `"` "Hello, world!\n" "日本語" "\u65e5本\U00008a9e" "\xff\u00FF" "\uD800" // illegal: surrogate half "\U00110000" // illegal: invalid Unicode code point </pre> <p> These examples all represent the same string: </p> <pre> "日本語" // UTF-8 input text `日本語` // UTF-8 input text as a raw literal "\u65e5\u672c\u8a9e" // the explicit Unicode code points "\U000065e5\U0000672c\U00008a9e" // the explicit Unicode code points "\xe6\x97\xa5\xe6\x9c\xac\xe8\xaa\x9e" // the explicit UTF-8 bytes </pre> <p> If the source code represents a character as two code points, such as a combining form involving an accent and a letter, the result will be an error if placed in a rune literal (it is not a single code point), and will appear as two code points if placed in a string literal. </p> <h2 id="Constants">Constants</h2> <p>There are <i>boolean constants</i>, <i>rune constants</i>, <i>integer constants</i>, <i>floating-point constants</i>, <i>complex constants</i>, and <i>string constants</i>. Rune, integer, floating-point, and complex constants are collectively called <i>numeric constants</i>. </p> <p> A constant value is represented by a <a href="#Rune_literals">rune</a>, <a href="#Integer_literals">integer</a>, <a href="#Floating-point_literals">floating-point</a>, <a href="#Imaginary_literals">imaginary</a>, or <a href="#String_literals">string</a> literal, an identifier denoting a constant, a <a href="#Constant_expressions">constant expression</a>, a <a href="#Conversions">conversion</a> with a result that is a constant, or the result value of some built-in functions such as <code>min</code> or <code>max</code> applied to constant arguments, <code>unsafe.Sizeof</code> applied to <a href="#Package_unsafe">certain values</a>, <code>cap</code> or <code>len</code> applied to <a href="#Length_and_capacity">some expressions</a>, <code>real</code> and <code>imag</code> applied to a complex constant and <code>complex</code> applied to numeric constants. The boolean truth values are represented by the predeclared constants <code>true</code> and <code>false</code>. The predeclared identifier <a href="#Iota">iota</a> denotes an integer constant. </p> <p> In general, complex constants are a form of <a href="#Constant_expressions">constant expression</a> and are discussed in that section. </p> <p> Numeric constants represent exact values of arbitrary precision and do not overflow. Consequently, there are no constants denoting the IEEE-754 negative zero, infinity, and not-a-number values. </p> <p> Constants may be <a href="#Types">typed</a> or <i>untyped</i>. Literal constants, <code>true</code>, <code>false</code>, <code>iota</code>, and certain <a href="#Constant_expressions">constant expressions</a> containing only untyped constant operands are untyped. </p> <p> A constant may be given a type explicitly by a <a href="#Constant_declarations">constant declaration</a> or <a href="#Conversions">conversion</a>, or implicitly when used in a <a href="#Variable_declarations">variable declaration</a> or an <a href="#Assignment_statements">assignment statement</a> or as an operand in an <a href="#Expressions">expression</a>. It is an error if the constant value cannot be <a href="#Representability">represented</a> as a value of the respective type. If the type is a type parameter, the constant is converted into a non-constant value of the type parameter. </p> <p> An untyped constant has a <i>default type</i> which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a <a href="#Short_variable_declarations">short variable declaration</a> such as <code>i := 0</code> where there is no explicit type. The default type of an untyped constant is <code>bool</code>, <code>rune</code>, <code>int</code>, <code>float64</code>, <code>complex128</code>, or <code>string</code> respectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant. </p> <p> Implementation restriction: Although numeric constants have arbitrary precision in the language, a compiler may implement them using an internal representation with limited precision. That said, every implementation must: </p> <ul> <li>Represent integer constants with at least 256 bits.</li> <li>Represent floating-point constants, including the parts of a complex constant, with a mantissa of at least 256 bits and a signed binary exponent of at least 16 bits.</li> <li>Give an error if unable to represent an integer constant precisely.</li> <li>Give an error if unable to represent a floating-point or complex constant due to overflow.</li> <li>Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.</li> </ul> <p> These requirements apply both to literal constants and to the result of evaluating <a href="#Constant_expressions">constant expressions</a>. </p> <h2 id="Variables">Variables</h2> <p> A variable is a storage location for holding a <i>value</i>. The set of permissible values is determined by the variable's <i><a href="#Types">type</a></i>. </p> <p> A <a href="#Variable_declarations">variable declaration</a> or, for function parameters and results, the signature of a <a href="#Function_declarations">function declaration</a> or <a href="#Function_literals">function literal</a> reserves storage for a named variable. Calling the built-in function <a href="#Allocation"><code>new</code></a> or taking the address of a <a href="#Composite_literals">composite literal</a> allocates storage for a variable at run time. Such an anonymous variable is referred to via a (possibly implicit) <a href="#Address_operators">pointer indirection</a>. </p> <p> <i>Structured</i> variables of <a href="#Array_types">array</a>, <a href="#Slice_types">slice</a>, and <a href="#Struct_types">struct</a> types have elements and fields that may be <a href="#Address_operators">addressed</a> individually. Each such element acts like a variable. </p> <p> The <i>static type</i> (or just <i>type</i>) of a variable is the type given in its declaration, the type provided in the <code>new</code> call or composite literal, or the type of an element of a structured variable. Variables of interface type also have a distinct <i>dynamic type</i>, which is the (non-interface) type of the value assigned to the variable at run time (unless the value is the predeclared identifier <code>nil</code>, which has no type). The dynamic type may vary during execution but values stored in interface variables are always <a href="#Assignability">assignable</a> to the static type of the variable. </p> <pre> var x interface{} // x is nil and has static type interface{} var v T // v has value nil, static type T x = 42 // x has value 42 and dynamic type int x = v // x has value (T)(nil) and dynamic type T </pre> <p> A variable's value is retrieved by referring to the variable in an <a href="#Expressions">expression</a>; it is the most recent value <a href="#Assignment_statements">assigned</a> to the variable. If a variable has not yet been assigned a value, its value is the <a href="#The_zero_value">zero value</a> for its type. </p> <h2 id="Types">Types</h2> <p> A type determines a set of values together with operations and methods specific to those values. A type may be denoted by a <i>type name</i>, if it has one, which must be followed by <a href="#Instantiations">type arguments</a> if the type is generic. A type may also be specified using a <i>type literal</i>, which composes a type from existing types. </p> <pre class="ebnf"> Type = TypeName [ TypeArgs ] \| TypeLit \| "(" Type ")" . TypeName = identifier \| QualifiedIdent . TypeArgs = "[" TypeList [ "," ] "]" . TypeList = Type { "," Type } . TypeLit = ArrayType \| StructType \| PointerType \| FunctionType \| InterfaceType \| SliceType \| MapType \| ChannelType . </pre> <p> The language <a href="#Predeclared_identifiers">predeclares</a> certain type names. Others are introduced with <a href="#Type_declarations">type declarations</a> or <a href="#Type_parameter_declarations">type parameter lists</a>. <i>Composite types</i>—array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals. </p> <p> Predeclared types, defined types, and type parameters are called <i>named types</i>. An alias denotes a named type if the type given in the alias declaration is a named type. </p> <h3 id="Boolean_types">Boolean types</h3> <p> A <i>boolean type</i> represents the set of Boolean truth values denoted by the predeclared constants <code>true</code> and <code>false</code>. The predeclared boolean type is <code>bool</code>; it is a <a href="#Type_definitions">defined type</a>. </p> <h3 id="Numeric_types">Numeric types</h3> <p> An <i>integer</i>, <i>floating-point</i>, or <i>complex</i> type represents the set of integer, floating-point, or complex values, respectively. They are collectively called <i>numeric types</i>. The predeclared architecture-independent numeric types are: </p> <pre class="grammar"> uint8 the set of all unsigned 8-bit integers (0 to 255) uint16 the set of all unsigned 16-bit integers (0 to 65535) uint32 the set of all unsigned 32-bit integers (0 to 4294967295) uint64 the set of all unsigned 64-bit integers (0 to 18446744073709551615) int8 the set of all signed 8-bit integers (-128 to 127) int16 the set of all signed 16-bit integers (-32768 to 32767) int32 the set of all signed 32-bit integers (-2147483648 to 2147483647) int64 the set of all signed 64-bit integers (-9223372036854775808 to 9223372036854775807) float32 the set of all IEEE-754 32-bit floating-point numbers float64 the set of all IEEE-754 64-bit floating-point numbers complex64 the set of all complex numbers with float32 real and imaginary parts complex128 the set of all complex numbers with float64 real and imaginary parts byte alias for uint8 rune alias for int32 </pre> <p> The value of an <i>n</i>-bit integer is <i>n</i> bits wide and represented using <a href="https://en.wikipedia.org/wiki/Two's_complement">two's complement arithmetic</a>. </p> <p> There is also a set of predeclared integer types with implementation-specific sizes: </p> <pre class="grammar"> uint either 32 or 64 bits int same size as uint uintptr an unsigned integer large enough to store the uninterpreted bits of a pointer value </pre> <p> To avoid portability issues all numeric types are <a href="#Type_definitions">defined types</a> and thus distinct except <code>byte</code>, which is an <a href="#Alias_declarations">alias</a> for <code>uint8</code>, and <code>rune</code>, which is an alias for <code>int32</code>. Explicit conversions are required when different numeric types are mixed in an expression or assignment. For instance, <code>int32</code> and <code>int</code> are not the same type even though they may have the same size on a particular architecture. </p> <h3 id="String_types">String types</h3> <p> A <i>string type</i> represents the set of string values. A string value is a (possibly empty) sequence of bytes. The number of bytes is called the length of the string and is never negative. Strings are immutable: once created, it is impossible to change the contents of a string. The predeclared string type is <code>string</code>; it is a <a href="#Type_definitions">defined type</a>. </p> <p> The length of a string <code>s</code> can be discovered using the built-in function <a href="#Length_and_capacity"><code>len</code></a>. The length is a compile-time constant if the string is a constant. A string's bytes can be accessed by integer <a href="#Index_expressions">indices</a> 0 through <code>len(s)-1</code>. It is illegal to take the address of such an element; if <code>s[i]</code> is the <code>i</code>'th byte of a string, <code>&s[i]</code> is invalid. </p> <h3 id="Array_types">Array types</h3> <p> An array is a numbered sequence of elements of a single type, called the element type. The number of elements is called the length of the array and is never negative. </p> <pre class="ebnf"> ArrayType = "[" ArrayLength "]" ElementType . ArrayLength = Expression . ElementType = Type . </pre> <p> The length is part of the array's type; it must evaluate to a non-negative <a href="#Constants">constant</a> <a href="#Representability">representable</a> by a value of type <code>int</code>. The length of array <code>a</code> can be discovered using the built-in function <a href="#Length_and_capacity"><code>len</code></a>. The elements can be addressed by integer <a href="#Index_expressions">indices</a> 0 through <code>len(a)-1</code>. Array types are always one-dimensional but may be composed to form multi-dimensional types. </p> <pre> [32]byte [2N] struct { x, y int32 } [1000]float64 [3][5]int [2][2][2]float64 // same as [2]([2]([2]float64)) </pre> <p> An array type <code>T</code> may not have an element of type <code>T</code>, or of a type containing <code>T</code> as a component, directly or indirectly, if those containing types are only array or struct types. </p> <pre> // invalid array types type ( T1 [10]T1 // element type of T1 is T1 T2 [10]struct{ f T2 } // T2 contains T2 as component of a struct T3 [10]T4 // T3 contains T3 as component of a struct in T4 T4 struct{ f T3 } // T4 contains T4 as component of array T3 in a struct ) // valid array types type ( T5 [10]T5 // T5 contains T5 as component of a pointer T6 [10]func() T6 // T6 contains T6 as component of a function type T7 [10]struct{ f []T7 } // T7 contains T7 as component of a slice in a struct ) </pre> <h3 id="Slice_types">Slice types</h3> <p> A slice is a descriptor for a contiguous segment of an <i>underlying array</i> and provides access to a numbered sequence of elements from that array. A slice type denotes the set of all slices of arrays of its element type. The number of elements is called the length of the slice and is never negative. The value of an uninitialized slice is <code>nil</code>. </p> <pre class="ebnf"> SliceType = "[" "]" ElementType . </pre> <p> The length of a slice <code>s</code> can be discovered by the built-in function <a href="#Length_and_capacity"><code>len</code></a>; unlike with arrays it may change during execution. The elements can be addressed by integer <a href="#Index_expressions">indices</a> 0 through <code>len(s)-1</code>. The slice index of a given element may be less than the index of the same element in the underlying array. </p> <p> A slice, once initialized, is always associated with an underlying array that holds its elements. A slice therefore shares storage with its array and with other slices of the same array; by contrast, distinct arrays always represent distinct storage. </p> <p> The array underlying a slice may extend past the end of the slice. The <i>capacity</i> is a measure of that extent: it is the sum of the length of the slice and the length of the array beyond the slice; a slice of length up to that capacity can be created by <a href="#Slice_expressions"><i>slicing</i></a> a new one from the original slice. The capacity of a slice <code>a</code> can be discovered using the built-in function <a href="#Length_and_capacity"><code>cap(a)</code></a>. </p> <p> A new, initialized slice value for a given element type <code>T</code> may be made using the built-in function <a href="#Making_slices_maps_and_channels"><code>make</code></a>, which takes a slice type and parameters specifying the length and optionally the capacity. A slice created with <code>make</code> always allocates a new, hidden array to which the returned slice value refers. That is, executing </p> <pre> make([]T, length, capacity) </pre> <p> produces the same slice as allocating an array and <a href="#Slice_expressions">slicing</a> it, so these two expressions are equivalent: </p> <pre> make([]int, 50, 100) new([100]int)[0:50] </pre> <p> Like arrays, slices are always one-dimensional but may be composed to construct higher-dimensional objects. With arrays of arrays, the inner arrays are, by construction, always the same length; however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. Moreover, the inner slices must be initialized individually. </p> <h3 id="Struct_types">Struct types</h3> <p> A struct is a sequence of named elements, called fields, each of which has a name and a type. Field names may be specified explicitly (IdentifierList) or implicitly (EmbeddedField). Within a struct, non-<a href="#Blank_identifier">blank</a> field names must be <a href="#Uniqueness_of_identifiers">unique</a>. </p> <pre class="ebnf"> StructType = "struct" "{" { FieldDecl ";" } "}" . FieldDecl = (IdentifierList Type \| EmbeddedField) [ Tag ] . EmbeddedField = [ "" ] TypeName [ TypeArgs ] . Tag = string_lit . </pre> <pre> // An empty struct. struct {} // A struct with 6 fields. struct { x, y int u float32 _ float32 // padding A []int F func() } </pre> <p> A field declared with a type but no explicit field name is called an <i>embedded field</i>. An embedded field must be specified as a type name <code>T</code> or as a pointer to a non-interface type name <code>T</code>, and <code>T</code> itself may not be a pointer type. The unqualified type name acts as the field name. </p> <pre> // A struct with four embedded fields of types T1, T2, P.T3 and P.T4 struct { T1 // field name is T1 T2 // field name is T2 P.T3 // field name is T3 P.T4 // field name is T4 x, y int // field names are x and y } </pre> <p> The following declaration is illegal because field names must be unique in a struct type: </p> <pre> struct { T // conflicts with embedded field T and P.T T // conflicts with embedded field T and P.T P.T // conflicts with embedded field T and T } </pre> <p> A field or <a href="#Method_declarations">method</a> <code>f</code> of an embedded field in a struct <code>x</code> is called <i>promoted</i> if <code>x.f</code> is a legal <a href="#Selectors">selector</a> that denotes that field or method <code>f</code>. </p> <p> Promoted fields act like ordinary fields of a struct except that they cannot be used as field names in <a href="#Composite_literals">composite literals</a> of the struct. </p> <p> Given a struct type <code>S</code> and a <a href="#Types">named type</a> <code>T</code>, promoted methods are included in the method set of the struct as follows: </p> <ul> <li> If <code>S</code> contains an embedded field <code>T</code>, the <a href="#Method_sets">method sets</a> of <code>S</code> and <code>S</code> both include promoted methods with receiver <code>T</code>. The method set of <code>S</code> also includes promoted methods with receiver <code>T</code>. </li> <li> If <code>S</code> contains an embedded field <code>T</code>, the method sets of <code>S</code> and <code>S</code> both include promoted methods with receiver <code>T</code> or <code>T</code>. </li> </ul> <p> A field declaration may be followed by an optional string literal <i>tag</i>, which becomes an attribute for all the fields in the corresponding field declaration. An empty tag string is equivalent to an absent tag. The tags are made visible through a <a href="/pkg/reflect/#StructTag">reflection interface</a> and take part in <a href="#Type_identity">type identity</a> for structs but are otherwise ignored. </p> <pre> struct { x, y float64 "" // an empty tag string is like an absent tag name string "any string is permitted as a tag" _ [4]byte "ceci n'est pas un champ de structure" } // A struct corresponding to a TimeStamp protocol buffer. // The tag strings define the protocol buffer field numbers; // they follow the convention outlined by the reflect package. struct { microsec uint64 `protobuf:"1"` serverIP6 uint64 `protobuf:"2"` } </pre> <p> A struct type <code>T</code> may not contain a field of type <code>T</code>, or of a type containing <code>T</code> as a component, directly or indirectly, if those containing types are only array or struct types. </p> <pre> // invalid struct types type ( T1 struct{ T1 } // T1 contains a field of T1 T2 struct{ f [10]T2 } // T2 contains T2 as component of an array T3 struct{ T4 } // T3 contains T3 as component of an array in struct T4 T4 struct{ f [10]T3 } // T4 contains T4 as component of struct T3 in an array ) // valid struct types type ( T5 struct{ f T5 } // T5 contains T5 as component of a pointer T6 struct{ f func() T6 } // T6 contains T6 as component of a function type T7 struct{ f [10][]T7 } // T7 contains T7 as component of a slice in an array ) </pre> <h3 id="Pointer_types">Pointer types</h3> <p> A pointer type denotes the set of all pointers to <a href="#Variables">variables</a> of a given type, called the <i>base type</i> of the pointer. The value of an uninitialized pointer is <code>nil</code>. </p> <pre class="ebnf"> PointerType = "" BaseType . BaseType = Type . </pre> <pre> Point [4]int </pre> <h3 id="Function_types">Function types</h3> <p> A function type denotes the set of all functions with the same parameter and result types. The value of an uninitialized variable of function type is <code>nil</code>. </p> <pre class="ebnf"> FunctionType = "func" Signature . Signature = Parameters [ Result ] . Result = Parameters \| Type . Parameters = "(" [ ParameterList [ "," ] ] ")" . ParameterList = ParameterDecl { "," ParameterDecl } . ParameterDecl = [ IdentifierList ] [ "..." ] Type . </pre> <p> Within a list of parameters or results, the names (IdentifierList) must either all be present or all be absent. If present, each name stands for one item (parameter or result) of the specified type and all non-<a href="#Blank_identifier">blank</a> names in the signature must be <a href="#Uniqueness_of_identifiers">unique</a>. If absent, each type stands for one item of that type. Parameter and result lists are always parenthesized except that if there is exactly one unnamed result it may be written as an unparenthesized type. </p> <p> The final incoming parameter in a function signature may have a type prefixed with <code>...</code>. A function with such a parameter is called <i>variadic</i> and may be invoked with zero or more arguments for that parameter. </p> <pre> func() func(x int) int func(a, _ int, z float32) bool func(a, b int, z float32) (bool) func(prefix string, values ...int) func(a, b int, z float64, opt ...interface{}) (success bool) func(int, int, float64) (float64, []int) func(n int) func(p T) </pre> <h3 id="Interface_types">Interface types</h3> <p> An interface type defines a <i>type set</i>. A variable of interface type can store a value of any type that is in the type set of the interface. Such a type is said to <a href="#Implementing_an_interface">implement the interface</a>. The value of an uninitialized variable of interface type is <code>nil</code>. </p> <pre class="ebnf"> InterfaceType = "interface" "{" { InterfaceElem ";" } "}" . InterfaceElem = MethodElem \| TypeElem . MethodElem = MethodName Signature . MethodName = identifier . TypeElem = TypeTerm { "\|" TypeTerm } . TypeTerm = Type \| UnderlyingType . UnderlyingType = "~" Type . </pre> <p> An interface type is specified by a list of <i>interface elements</i>. An interface element is either a <i>method</i> or a <i>type element</i>, where a type element is a union of one or more <i>type terms</i>. A type term is either a single type or a single underlying type. </p> <h4 id="Basic_interfaces">Basic interfaces</h4> <p> In its most basic form an interface specifies a (possibly empty) list of methods. The type set defined by such an interface is the set of types which implement all of those methods, and the corresponding <a href="#Method_sets">method set</a> consists exactly of the methods specified by the interface. Interfaces whose type sets can be defined entirely by a list of methods are called <i>basic interfaces.</i> </p> <pre> // A simple File interface. interface { Read([]byte) (int, error) Write([]byte) (int, error) Close() error } </pre> <p> The name of each explicitly specified method must be <a href="#Uniqueness_of_identifiers">unique</a> and not <a href="#Blank_identifier">blank</a>. </p> <pre> interface { String() string String() string // illegal: String not unique _(x int) // illegal: method must have non-blank name } </pre> <p> More than one type may implement an interface. For instance, if two types <code>S1</code> and <code>S2</code> have the method set </p> <pre> func (p T) Read(p []byte) (n int, err error) func (p T) Write(p []byte) (n int, err error) func (p T) Close() error </pre> <p> (where <code>T</code> stands for either <code>S1</code> or <code>S2</code>) then the <code>File</code> interface is implemented by both <code>S1</code> and <code>S2</code>, regardless of what other methods <code>S1</code> and <code>S2</code> may have or share. </p> <p> Every type that is a member of the type set of an interface implements that interface. Any given type may implement several distinct interfaces. For instance, all types implement the <i>empty interface</i> which stands for the set of all (non-interface) types: </p> <pre> interface{} </pre> <p> For convenience, the predeclared type <code>any</code> is an alias for the empty interface. [<a href="#Go_1.18">Go 1.18</a>] </p> <p> Similarly, consider this interface specification, which appears within a <a href="#Type_declarations">type declaration</a> to define an interface called <code>Locker</code>: </p> <pre> type Locker interface { Lock() Unlock() } </pre> <p> If <code>S1</code> and <code>S2</code> also implement </p> <pre> func (p T) Lock() { … } func (p T) Unlock() { … } </pre> <p> they implement the <code>Locker</code> interface as well as the <code>File</code> interface. </p> <h4 id="Embedded_interfaces">Embedded interfaces</h4> <p> In a slightly more general form an interface <code>T</code> may use a (possibly qualified) interface type name <code>E</code> as an interface element. This is called <i>embedding</i> interface <code>E</code> in <code>T</code> [<a href="#Go_1.14">Go 1.14</a>]. The type set of <code>T</code> is the <i>intersection</i> of the type sets defined by <code>T</code>'s explicitly declared methods and the type sets of <code>T</code>’s embedded interfaces. In other words, the type set of <code>T</code> is the set of all types that implement all the explicitly declared methods of <code>T</code> and also all the methods of <code>E</code> [<a href="#Go_1.18">Go 1.18</a>]. </p> <pre> type Reader interface { Read(p []byte) (n int, err error) Close() error } type Writer interface { Write(p []byte) (n int, err error) Close() error } // ReadWriter's methods are Read, Write, and Close. type ReadWriter interface { Reader // includes methods of Reader in ReadWriter's method set Writer // includes methods of Writer in ReadWriter's method set } </pre> <p> When embedding interfaces, methods with the <a href="#Uniqueness_of_identifiers">same</a> names must have <a href="#Type_identity">identical</a> signatures. </p> <pre> type ReadCloser interface { Reader // includes methods of Reader in ReadCloser's method set Close() // illegal: signatures of Reader.Close and Close are different } </pre> <h4 id="General_interfaces">General interfaces</h4> <p> In their most general form, an interface element may also be an arbitrary type term <code>T</code>, or a term of the form <code>~T</code> specifying the underlying type <code>T</code>, or a union of terms <code>t<sub>1</sub>\|t<sub>2</sub>\|…\|t<sub>n</sub></code> [<a href="#Go_1.18">Go 1.18</a>]. Together with method specifications, these elements enable the precise definition of an interface's type set as follows: </p> <ul> <li>The type set of the empty interface is the set of all non-interface types. </li> <li>The type set of a non-empty interface is the intersection of the type sets of its interface elements. </li> <li>The type set of a method specification is the set of all non-interface types whose method sets include that method. </li> <li>The type set of a non-interface type term is the set consisting of just that type. </li> <li>The type set of a term of the form <code>~T</code> is the set of all types whose underlying type is <code>T</code>. </li> <li>The type set of a <i>union</i> of terms <code>t<sub>1</sub>\|t<sub>2</sub>\|…\|t<sub>n</sub></code> is the union of the type sets of the terms. </li> </ul> <p> The quantification "the set of all non-interface types" refers not just to all (non-interface) types declared in the program at hand, but all possible types in all possible programs, and hence is infinite. Similarly, given the set of all non-interface types that implement a particular method, the intersection of the method sets of those types will contain exactly that method, even if all types in the program at hand always pair that method with another method. </p> <p> By construction, an interface's type set never contains an interface type. </p> <pre> // An interface representing only the type int. interface { int } // An interface representing all types with underlying type int. interface { ~int } // An interface representing all types with underlying type int that implement the String method. interface { ~int String() string } // An interface representing an empty type set: there is no type that is both an int and a string. interface { int string } </pre> <p> In a term of the form <code>~T</code>, the underlying type of <code>T</code> must be itself, and <code>T</code> cannot be an interface. </p> <pre> type MyInt int interface { ~[]byte // the underlying type of []byte is itself ~MyInt // illegal: the underlying type of MyInt is not MyInt ~error // illegal: error is an interface } </pre> <p> Union elements denote unions of type sets: </p> <pre> // The Float interface represents all floating-point types // (including any named types whose underlying types are // either float32 or float64). type Float interface { ~float32 \| ~float64 } </pre> <p> The type <code>T</code> in a term of the form <code>T</code> or <code>~T</code> cannot be a <a href="#Type_parameter_declarations">type parameter</a>, and the type sets of all non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty). Given a type parameter <code>P</code>: </p> <pre> interface { P // illegal: P is a type parameter int \| ~P // illegal: P is a type parameter ~int \| MyInt // illegal: the type sets for ~int and MyInt are not disjoint (~int includes MyInt) float32 \| Float // overlapping type sets but Float is an interface } </pre> <p> Implementation restriction: A union (with more than one term) cannot contain the <a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code> or interfaces that specify methods, or embed <code>comparable</code> or interfaces that specify methods. </p> <p> Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type constraints, or as elements of other interfaces used as constraints. They cannot be the types of values or variables, or components of other, non-interface types. </p> <pre> var x Float // illegal: Float is not a basic interface var x interface{} = Float(nil) // illegal type Floatish struct { f Float // illegal } </pre> <p> An interface type <code>T</code> may not embed a type element that is, contains, or embeds <code>T</code>, directly or indirectly. </p> <pre> // illegal: Bad may not embed itself type Bad interface { Bad } // illegal: Bad1 may not embed itself using Bad2 type Bad1 interface { Bad2 } type Bad2 interface { Bad1 } // illegal: Bad3 may not embed a union containing Bad3 type Bad3 interface { ~int \| ~string \| Bad3 } // illegal: Bad4 may not embed an array containing Bad4 as element type type Bad4 interface { [10]Bad4 } </pre> <h4 id="Implementing_an_interface">Implementing an interface</h4> <p> A type <code>T</code> implements an interface <code>I</code> if </p> <ul> <li> <code>T</code> is not an interface and is an element of the type set of <code>I</code>; or </li> <li> <code>T</code> is an interface and the type set of <code>T</code> is a subset of the type set of <code>I</code>. </li> </ul> <p> A value of type <code>T</code> implements an interface if <code>T</code> implements the interface. </p> <h3 id="Map_types">Map types</h3> <p> A map is an unordered group of elements of one type, called the element type, indexed by a set of unique <i>keys</i> of another type, called the key type. The value of an uninitialized map is <code>nil</code>. </p> <pre class="ebnf"> MapType = "map" "[" KeyType "]" ElementType . KeyType = Type . </pre> <p> The <a href="#Comparison_operators">comparison operators</a> <code>==</code> and <code>!=</code> must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a <a href="#Run_time_panics">run-time panic</a>. </p> <pre> map[string]int map[T]struct{ x, y float64 } map[string]interface{} </pre> <p> The number of map elements is called its length. For a map <code>m</code>, it can be discovered using the built-in function <a href="#Length_and_capacity"><code>len</code></a> and may change during execution. Elements may be added during execution using <a href="#Assignment_statements">assignments</a> and retrieved with <a href="#Index_expressions">index expressions</a>; they may be removed with the <a href="#Deletion_of_map_elements"><code>delete</code></a> and <a href="#Clear"><code>clear</code></a> built-in function. </p> <p> A new, empty map value is made using the built-in function <a href="#Making_slices_maps_and_channels"><code>make</code></a>, which takes the map type and an optional capacity hint as arguments: </p> <pre> make(map[string]int) make(map[string]int, 100) </pre> <p> The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of <code>nil</code> maps. A <code>nil</code> map is equivalent to an empty map except that no elements may be added. <h3 id="Channel_types">Channel types</h3> <p> A channel provides a mechanism for <a href="#Go_statements">concurrently executing functions</a> to communicate by <a href="#Send_statements">sending</a> and <a href="#Receive_operator">receiving</a> values of a specified element type. The value of an uninitialized channel is <code>nil</code>. </p> <pre class="ebnf"> ChannelType = ( "chan" \| "chan" "<-" \| "<-" "chan" ) ElementType . </pre> <p> The optional <code><-</code> operator specifies the channel <i>direction</i>, <i>send</i> or <i>receive</i>. If a direction is given, the channel is <i>directional</i>, otherwise it is <i>bidirectional</i>. A channel may be constrained only to send or only to receive by <a href="#Assignment_statements">assignment</a> or explicit <a href="#Conversions">conversion</a>. </p> <pre> chan T // can be used to send and receive values of type T chan<- float64 // can only be used to send float64s <-chan int // can only be used to receive ints </pre> <p> The <code><-</code> operator associates with the leftmost <code>chan</code> possible: </p> <pre> chan<- chan int // same as chan<- (chan int) chan<- <-chan int // same as chan<- (<-chan int) <-chan <-chan int // same as <-chan (<-chan int) chan (<-chan int) </pre> <p> A new, initialized channel value can be made using the built-in function <a href="#Making_slices_maps_and_channels"><code>make</code></a>, which takes the channel type and an optional <i>capacity</i> as arguments: </p> <pre> make(chan int, 100) </pre> <p> The capacity, in number of elements, sets the size of the buffer in the channel. If the capacity is zero or absent, the channel is unbuffered and communication succeeds only when both a sender and receiver are ready. Otherwise, the channel is buffered and communication succeeds without blocking if the buffer is not full (sends) or not empty (receives). A <code>nil</code> channel is never ready for communication. </p> <p> A channel may be closed with the built-in function <a href="#Close"><code>close</code></a>. The multi-valued assignment form of the <a href="#Receive_operator">receive operator</a> reports whether a received value was sent before the channel was closed. </p> <p> A single channel may be used in <a href="#Send_statements">send statements</a>, <a href="#Receive_operator">receive operations</a>, and calls to the built-in functions <a href="#Length_and_capacity"><code>cap</code></a> and <a href="#Length_and_capacity"><code>len</code></a> by any number of goroutines without further synchronization. Channels act as first-in-first-out queues. For example, if one goroutine sends values on a channel and a second goroutine receives them, the values are received in the order sent. </p> <h2 id="Properties_of_types_and_values">Properties of types and values</h2> <h3 id="Underlying_types">Underlying types</h3> <p> Each type <code>T</code> has an <i>underlying type</i>: If <code>T</code> is one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is <code>T</code> itself. Otherwise, <code>T</code>'s underlying type is the underlying type of the type to which <code>T</code> refers in its declaration. For a type parameter that is the underlying type of its <a href="#Type_constraints">type constraint</a>, which is always an interface. </p> <pre> type ( A1 = string A2 = A1 ) type ( B1 string B2 B1 B3 []B1 B4 B3 ) func f[P any](x P) { … } </pre> <p> The underlying type of <code>string</code>, <code>A1</code>, <code>A2</code>, <code>B1</code>, and <code>B2</code> is <code>string</code>. The underlying type of <code>[]B1</code>, <code>B3</code>, and <code>B4</code> is <code>[]B1</code>. The underlying type of <code>P</code> is <code>interface{}</code>. </p> <h3 id="Core_types">Core types</h3> <p> Each non-interface type <code>T</code> has a <i>core type</i>, which is the same as the <a href="#Underlying_types">underlying type</a> of <code>T</code>. </p> <p> An interface <code>T</code> has a core type if one of the following conditions is satisfied: </p> <ol> <li> There is a single type <code>U</code> which is the <a href="#Underlying_types">underlying type</a> of all types in the <a href="#Interface_types">type set</a> of <code>T</code>; or </li> <li> the type set of <code>T</code> contains only <a href="#Channel_types">channel types</a> with identical element type <code>E</code>, and all directional channels have the same direction. </li> </ol> <p> No other interfaces have a core type. </p> <p> The core type of an interface is, depending on the condition that is satisfied, either: </p> <ol> <li> the type <code>U</code>; or </li> <li> the type <code>chan E</code> if <code>T</code> contains only bidirectional channels, or the type <code>chan<- E</code> or <code><-chan E</code> depending on the direction of the directional channels present. </li> </ol> <p> By definition, a core type is never a <a href="#Type_definitions">defined type</a>, <a href="#Type_parameter_declarations">type parameter</a>, or <a href="#Interface_types">interface type</a>. </p> <p> Examples of interfaces with core types: </p> <pre> type Celsius float32 type Kelvin float32 interface{ int } // int interface{ Celsius\|Kelvin } // float32 interface{ ~chan int } // chan int interface{ ~chan int\|~chan<- int } // chan<- int interface{ ~[]data; String() string } // []data </pre> <p> Examples of interfaces without core types: </p> <pre> interface{} // no single underlying type interface{ Celsius\|float64 } // no single underlying type interface{ chan int \| chan<- string } // channels have different element types interface{ <-chan int \| chan<- int } // directional channels have different directions </pre> <p> Some operations (<a href="#Slice_expressions">slice expressions</a>, <a href="#Appending_and_copying_slices"><code>append</code> and <code>copy</code></a>) rely on a slightly more loose form of core types which accept byte slices and strings. Specifically, if there are exactly two types, <code>[]byte</code> and <code>string</code>, which are the underlying types of all types in the type set of interface <code>T</code>, the core type of <code>T</code> is called <code>bytestring</code>. </p> <p> Examples of interfaces with <code>bytestring</code> core types: </p> <pre> interface{ int } // int (same as ordinary core type) interface{ []byte \| string } // bytestring interface{ ~[]byte \| myString } // bytestring </pre> <p> Note that <code>bytestring</code> is not a real type; it cannot be used to declare variables or compose other types. It exists solely to describe the behavior of some operations that read from a sequence of bytes, which may be a byte slice or a string. </p> <h3 id="Type_identity">Type identity</h3> <p> Two types are either <i>identical</i> or <i>different</i>. </p> <p> A <a href="#Types">named type</a> is always different from any other type. Otherwise, two types are identical if their <a href="#Types">underlying</a> type literals are structurally equivalent; that is, they have the same literal structure and corresponding components have identical types. In detail: </p> <ul> <li>Two array types are identical if they have identical element types and the same array length.</li> <li>Two slice types are identical if they have identical element types.</li> <li>Two struct types are identical if they have the same sequence of fields, and if corresponding fields have the same names, and identical types, and identical tags. <a href="#Exported_identifiers">Non-exported</a> field names from different packages are always different.</li> <li>Two pointer types are identical if they have identical base types.</li> <li>Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is. Parameter and result names are not required to match.</li> <li>Two interface types are identical if they define the same type set. </li> <li>Two map types are identical if they have identical key and element types.</li> <li>Two channel types are identical if they have identical element types and the same direction.</li> <li>Two <a href="#Instantiations">instantiated</a> types are identical if their defined types and all type arguments are identical. </li> </ul> <p> Given the declarations </p> <pre> type ( A0 = []string A1 = A0 A2 = struct{ a, b int } A3 = int A4 = func(A3, float64) A0 A5 = func(x int, _ float64) []string B0 A0 B1 []string B2 struct{ a, b int } B3 struct{ a, c int } B4 func(int, float64) B0 B5 func(x int, y float64) A1 C0 = B0 D0[P1, P2 any] struct{ x P1; y P2 } E0 = D0[int, string] ) </pre> <p> these types are identical: </p> <pre> A0, A1, and []string A2 and struct{ a, b int } A3 and int A4, func(int, float64) []string, and A5 B0 and C0 D0[int, string] and E0 []int and []int struct{ a, b B5 } and struct{ a, b B5 } func(x int, y float64) []string, func(int, float64) (result []string), and A5 </pre> <p> <code>B0</code> and <code>B1</code> are different because they are new types created by distinct <a href="#Type_definitions">type definitions</a>; <code>func(int, float64) B0</code> and <code>func(x int, y float64) []string</code> are different because <code>B0</code> is different from <code>[]string</code>; and <code>P1</code> and <code>P2</code> are different because they are different type parameters. <code>D0[int, string]</code> and <code>struct{ x int; y string }</code> are different because the former is an <a href="#Instantiations">instantiated</a> defined type while the latter is a type literal (but they are still <a href="#Assignability">assignable</a>). </p> <h3 id="Assignability">Assignability</h3> <p> A value <code>x</code> of type <code>V</code> is <i>assignable</i> to a <a href="#Variables">variable</a> of type <code>T</code> ("<code>x</code> is assignable to <code>T</code>") if one of the following conditions applies: </p> <ul> <li> <code>V</code> and <code>T</code> are identical. </li> <li> <code>V</code> and <code>T</code> have identical <a href="#Underlying_types">underlying types</a> but are not type parameters and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>. </li> <li> <code>V</code> and <code>T</code> are channel types with identical element types, <code>V</code> is a bidirectional channel, and at least one of <code>V</code> or <code>T</code> is not a <a href="#Types">named type</a>. </li> <li> <code>T</code> is an interface type, but not a type parameter, and <code>x</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>. </li> <li> <code>x</code> is the predeclared identifier <code>nil</code> and <code>T</code> is a pointer, function, slice, map, channel, or interface type, but not a type parameter. </li> <li> <code>x</code> is an untyped <a href="#Constants">constant</a> <a href="#Representability">representable</a> by a value of type <code>T</code>. </li> </ul> <p> Additionally, if <code>x</code>'s type <code>V</code> or <code>T</code> are type parameters, <code>x</code> is assignable to a variable of type <code>T</code> if one of the following conditions applies: </p> <ul> <li> <code>x</code> is the predeclared identifier <code>nil</code>, <code>T</code> is a type parameter, and <code>x</code> is assignable to each type in <code>T</code>'s type set. </li> <li> <code>V</code> is not a <a href="#Types">named type</a>, <code>T</code> is a type parameter, and <code>x</code> is assignable to each type in <code>T</code>'s type set. </li> <li> <code>V</code> is a type parameter and <code>T</code> is not a named type, and values of each type in <code>V</code>'s type set are assignable to <code>T</code>. </li> </ul> <h3 id="Representability">Representability</h3> <p> A <a href="#Constants">constant</a> <code>x</code> is <i>representable</i> by a value of type <code>T</code>, where <code>T</code> is not a <a href="#Type_parameter_declarations">type parameter</a>, if one of the following conditions applies: </p> <ul> <li> <code>x</code> is in the set of values <a href="#Types">determined</a> by <code>T</code>. </li> <li> <code>T</code> is a <a href="#Numeric_types">floating-point type</a> and <code>x</code> can be rounded to <code>T</code>'s precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE negative zero further simplified to an unsigned zero. Note that constant values never result in an IEEE negative zero, NaN, or infinity. </li> <li> <code>T</code> is a complex type, and <code>x</code>'s <a href="#Complex_numbers">components</a> <code>real(x)</code> and <code>imag(x)</code> are representable by values of <code>T</code>'s component type (<code>float32</code> or <code>float64</code>). </li> </ul> <p> If <code>T</code> is a type parameter, <code>x</code> is representable by a value of type <code>T</code> if <code>x</code> is representable by a value of each type in <code>T</code>'s type set. </p> <pre> x T x is representable by a value of T because 'a' byte 97 is in the set of byte values 97 rune rune is an alias for int32, and 97 is in the set of 32-bit integers "foo" string "foo" is in the set of string values 1024 int16 1024 is in the set of 16-bit integers 42.0 byte 42 is in the set of unsigned 8-bit integers 1e10 uint64 10000000000 is in the set of unsigned 64-bit integers 2.718281828459045 float32 2.718281828459045 rounds to 2.7182817 which is in the set of float32 values -1e-1000 float64 -1e-1000 rounds to IEEE -0.0 which is further simplified to 0.0 0i int 0 is an integer value (42 + 0i) float32 42.0 (with zero imaginary part) is in the set of float32 values </pre> <pre> x T x is not representable by a value of T because 0 bool 0 is not in the set of boolean values 'a' string 'a' is a rune, it is not in the set of string values 1024 byte 1024 is not in the set of unsigned 8-bit integers -1 uint16 -1 is not in the set of unsigned 16-bit integers 1.1 int 1.1 is not an integer value 42i float32 (0 + 42i) is not in the set of float32 values 1e1000 float64 1e1000 overflows to IEEE +Inf after rounding </pre> <h3 id="Method_sets">Method sets</h3> <p> The <i>method set</i> of a type determines the methods that can be <a href="#Calls">called</a> on an <a href="#Operands">operand</a> of that type. Every type has a (possibly empty) method set associated with it: </p> <ul> <li>The method set of a <a href="#Type_definitions">defined type</a> <code>T</code> consists of all <a href="#Method_declarations">methods</a> declared with receiver type <code>T</code>. </li> <li> The method set of a pointer to a defined type <code>T</code> (where <code>T</code> is neither a pointer nor an interface) is the set of all methods declared with receiver <code>T</code> or <code>T</code>. </li> <li>The method set of an <a href="#Interface_types">interface type</a> is the intersection of the method sets of each type in the interface's <a href="#Interface_types">type set</a> (the resulting method set is usually just the set of declared methods in the interface). </li> </ul> <p> Further rules apply to structs (and pointer to structs) containing embedded fields, as described in the section on <a href="#Struct_types">struct types</a>. Any other type has an empty method set. </p> <p> In a method set, each method must have a <a href="#Uniqueness_of_identifiers">unique</a> non-<a href="#Blank_identifier">blank</a> <a href="#MethodName">method name</a>. </p> <h2 id="Blocks">Blocks</h2> <p> A <i>block</i> is a possibly empty sequence of declarations and statements within matching brace brackets. </p> <pre class="ebnf"> Block = "{" StatementList "}" . StatementList = { Statement ";" } . </pre> <p> In addition to explicit blocks in the source code, there are implicit blocks: </p> <ol> <li>The <i>universe block</i> encompasses all Go source text.</li> <li>Each <a href="#Packages">package</a> has a <i>package block</i> containing all Go source text for that package.</li> <li>Each file has a <i>file block</i> containing all Go source text in that file.</li> <li>Each <a href="#If_statements">"if"</a>, <a href="#For_statements">"for"</a>, and <a href="#Switch_statements">"switch"</a> statement is considered to be in its own implicit block.</li> <li>Each clause in a <a href="#Switch_statements">"switch"</a> or <a href="#Select_statements">"select"</a> statement acts as an implicit block.</li> </ol> <p> Blocks nest and influence <a href="#Declarations_and_scope">scoping</a>. </p> <h2 id="Declarations_and_scope">Declarations and scope</h2> <p> A <i>declaration</i> binds a non-<a href="#Blank_identifier">blank</a> identifier to a <a href="#Constant_declarations">constant</a>, <a href="#Type_declarations">type</a>, <a href="#Type_parameter_declarations">type parameter</a>, <a href="#Variable_declarations">variable</a>, <a href="#Function_declarations">function</a>, <a href="#Labeled_statements">label</a>, or <a href="#Import_declarations">package</a>. Every identifier in a program must be declared. No identifier may be declared twice in the same block, and no identifier may be declared in both the file and package block. </p> <p> The <a href="#Blank_identifier">blank identifier</a> may be used like any other identifier in a declaration, but it does not introduce a binding and thus is not declared. In the package block, the identifier <code>init</code> may only be used for <a href="#Package_initialization"><code>init</code> function</a> declarations, and like the blank identifier it does not introduce a new binding. </p> <pre class="ebnf"> Declaration = ConstDecl \| TypeDecl \| VarDecl . TopLevelDecl = Declaration \| FunctionDecl \| MethodDecl . </pre> <p> The <i>scope</i> of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package. </p> <p> Go is lexically scoped using <a href="#Blocks">blocks</a>: </p> <ol> <li>The scope of a <a href="#Predeclared_identifiers">predeclared identifier</a> is the universe block.</li> <li>The scope of an identifier denoting a constant, type, variable, or function (but not method) declared at top level (outside any function) is the package block.</li> <li>The scope of the package name of an imported package is the file block of the file containing the import declaration.</li> <li>The scope of an identifier denoting a method receiver, function parameter, or result variable is the function body.</li> <li>The scope of an identifier denoting a type parameter of a function or declared by a method receiver begins after the name of the function and ends at the end of the function body.</li> <li>The scope of an identifier denoting a type parameter of a type begins after the name of the type and ends at the end of the TypeSpec.</li> <li>The scope of a constant or variable identifier declared inside a function begins at the end of the ConstSpec or VarSpec (ShortVarDecl for short variable declarations) and ends at the end of the innermost containing block.</li> <li>The scope of a type identifier declared inside a function begins at the identifier in the TypeSpec and ends at the end of the innermost containing block.</li> </ol> <p> An identifier declared in a block may be redeclared in an inner block. While the identifier of the inner declaration is in scope, it denotes the entity declared by the inner declaration. </p> <p> The <a href="#Package_clause">package clause</a> is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same <a href="#Packages">package</a> and to specify the default package name for import declarations. </p> <h3 id="Label_scopes">Label scopes</h3> <p> Labels are declared by <a href="#Labeled_statements">labeled statements</a> and are used in the <a href="#Break_statements">"break"</a>, <a href="#Continue_statements">"continue"</a>, and <a href="#Goto_statements">"goto"</a> statements. It is illegal to define a label that is never used. In contrast to other identifiers, labels are not block scoped and do not conflict with identifiers that are not labels. The scope of a label is the body of the function in which it is declared and excludes the body of any nested function. </p> <h3 id="Blank_identifier">Blank identifier</h3> <p> The <i>blank identifier</i> is represented by the underscore character <code>_</code>. It serves as an anonymous placeholder instead of a regular (non-blank) identifier and has special meaning in <a href="#Declarations_and_scope">declarations</a>, as an <a href="#Operands">operand</a>, and in <a href="#Assignment_statements">assignment statements</a>. </p> <h3 id="Predeclared_identifiers">Predeclared identifiers</h3> <p> The following identifiers are implicitly declared in the <a href="#Blocks">universe block</a> [<a href="#Go_1.18">Go 1.18</a>] [<a href="#Go_1.21">Go 1.21</a>]: </p> <pre class="grammar"> Types: any bool byte comparable complex64 complex128 error float32 float64 int int8 int16 int32 int64 rune string uint uint8 uint16 uint32 uint64 uintptr Constants: true false iota Zero value: nil Functions: append cap clear close complex copy delete imag len make max min new panic print println real recover </pre> <h3 id="Exported_identifiers">Exported identifiers</h3> <p> An identifier may be <i>exported</i> to permit access to it from another package. An identifier is exported if both: </p> <ol> <li>the first character of the identifier's name is a Unicode uppercase letter (Unicode character category Lu); and</li> <li>the identifier is declared in the <a href="#Blocks">package block</a> or it is a <a href="#Struct_types">field name</a> or <a href="#MethodName">method name</a>.</li> </ol> <p> All other identifiers are not exported. </p> <h3 id="Uniqueness_of_identifiers">Uniqueness of identifiers</h3> <p> Given a set of identifiers, an identifier is called <i>unique</i> if it is <i>different</i> from every other in the set. Two identifiers are different if they are spelled differently, or if they appear in different <a href="#Packages">packages</a> and are not <a href="#Exported_identifiers">exported</a>. Otherwise, they are the same. </p> <h3 id="Constant_declarations">Constant declarations</h3> <p> A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of <a href="#Constant_expressions">constant expressions</a>. The number of identifiers must be equal to the number of expressions, and the <i>n</i>th identifier on the left is bound to the value of the <i>n</i>th expression on the right. </p> <pre class="ebnf"> ConstDecl = "const" ( ConstSpec \| "(" { ConstSpec ";" } ")" ) . ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] . IdentifierList = identifier { "," identifier } . ExpressionList = Expression { "," Expression } . </pre> <p> If the type is present, all constants take the type specified, and the expressions must be <a href="#Assignability">assignable</a> to that type, which must not be a type parameter. If the type is omitted, the constants take the individual types of the corresponding expressions. If the expression values are untyped <a href="#Constants">constants</a>, the declared constants remain untyped and the constant identifiers denote the constant values. For instance, if the expression is a floating-point literal, the constant identifier denotes a floating-point constant, even if the literal's fractional part is zero. </p> <pre> const Pi float64 = 3.14159265358979323846 const zero = 0.0 // untyped floating-point constant const ( size int64 = 1024 eof = -1 // untyped integer constant ) const a, b, c = 3, 4, "foo" // a = 3, b = 4, c = "foo", untyped integer and string constants const u, v float32 = 0, 3 // u = 0.0, v = 3.0 </pre> <p> Within a parenthesized <code>const</code> declaration list the expression list may be omitted from any but the first ConstSpec. Such an empty list is equivalent to the textual substitution of the first preceding non-empty expression list and its type if any. Omitting the list of expressions is therefore equivalent to repeating the previous list. The number of identifiers must be equal to the number of expressions in the previous list. Together with the <a href="#Iota"><code>iota</code> constant generator</a> this mechanism permits light-weight declaration of sequential values: </p> <pre> const ( Sunday = iota Monday Tuesday Wednesday Thursday Friday Partyday numberOfDays // this constant is not exported ) </pre> <h3 id="Iota">Iota</h3> <p> Within a <a href="#Constant_declarations">constant declaration</a>, the predeclared identifier <code>iota</code> represents successive untyped integer <a href="#Constants"> constants</a>. Its value is the index of the respective <a href="#ConstSpec">ConstSpec</a> in that constant declaration, starting at zero. It can be used to construct a set of related constants: </p> <pre> const ( c0 = iota // c0 == 0 c1 = iota // c1 == 1 c2 = iota // c2 == 2 ) const ( a = 1 << iota // a == 1 (iota == 0) b = 1 << iota // b == 2 (iota == 1) c = 3 // c == 3 (iota == 2, unused) d = 1 << iota // d == 8 (iota == 3) ) const ( u = iota 42 // u == 0 (untyped integer constant) v float64 = iota * 42 // v == 42.0 (float64 constant) w = iota * 42 // w == 84 (untyped integer constant) ) const x = iota // x == 0 const y = iota // y == 0 </pre> <p> By definition, multiple uses of <code>iota</code> in the same ConstSpec all have the same value: </p> <pre> const ( bit0, mask0 = 1 << iota, 1<<iota - 1 // bit0 == 1, mask0 == 0 (iota == 0) bit1, mask1 // bit1 == 2, mask1 == 1 (iota == 1) _, _ // (iota == 2, unused) bit3, mask3 // bit3 == 8, mask3 == 7 (iota == 3) ) </pre> <p> This last example exploits the <a href="#Constant_declarations">implicit repetition</a> of the last non-empty expression list. </p> <h3 id="Type_declarations">Type declarations</h3> <p> A type declaration binds an identifier, the <i>type name</i>, to a <a href="#Types">type</a>. Type declarations come in two forms: alias declarations and type definitions. </p> <pre class="ebnf"> TypeDecl = "type" ( TypeSpec \| "(" { TypeSpec ";" } ")" ) . TypeSpec = AliasDecl \| TypeDef . </pre> <h4 id="Alias_declarations">Alias declarations</h4> <p> An alias declaration binds an identifier to the given type [<a href="#Go_1.9">Go 1.9</a>]. </p> <pre class="ebnf"> AliasDecl = identifier "=" Type . </pre> <p> Within the <a href="#Declarations_and_scope">scope</a> of the identifier, it serves as an <i>alias</i> for the type. </p> <pre> type ( nodeList = []Node // nodeList and []Node are identical types Polar = polar // Polar and polar denote identical types ) </pre> <h4 id="Type_definitions">Type definitions</h4> <p> A type definition creates a new, distinct type with the same <a href="#Underlying_types">underlying type</a> and operations as the given type and binds an identifier, the <i>type name</i>, to it. </p> <pre class="ebnf"> TypeDef = identifier [ TypeParameters ] Type . </pre> <p> The new type is called a <i>defined type</i>. It is <a href="#Type_identity">different</a> from any other type, including the type it is created from. </p> <pre> type ( Point struct{ x, y float64 } // Point and struct{ x, y float64 } are different types polar Point // polar and Point denote different types ) type TreeNode struct { left, right TreeNode value any } type Block interface { BlockSize() int Encrypt(src, dst []byte) Decrypt(src, dst []byte) } </pre> <p> A defined type may have <a href="#Method_declarations">methods</a> associated with it. It does not inherit any methods bound to the given type, but the <a href="#Method_sets">method set</a> of an interface type or of elements of a composite type remains unchanged: </p> <pre> // A Mutex is a data type with two methods, Lock and Unlock. type Mutex struct { / Mutex fields / } func (m Mutex) Lock() { /* Lock implementation / } func (m Mutex) Unlock() { /* Unlock implementation / } // NewMutex has the same composition as Mutex but its method set is empty. type NewMutex Mutex // The method set of PtrMutex's underlying type Mutex remains unchanged, // but the method set of PtrMutex is empty. type PtrMutex Mutex // The method set of PrintableMutex contains the methods // Lock and Unlock bound to its embedded field Mutex. type PrintableMutex struct { Mutex } // MyBlock is an interface type that has the same method set as Block. type MyBlock Block </pre> <p> Type definitions may be used to define different boolean, numeric, or string types and associate methods with them: </p> <pre> type TimeZone int const ( EST TimeZone = -(5 + iota) CST MST PST ) func (tz TimeZone) String() string { return fmt.Sprintf("GMT%+dh", tz) } </pre> <p> If the type definition specifies <a href="#Type_parameter_declarations">type parameters</a>, the type name denotes a <i>generic type</i>. Generic types must be <a href="#Instantiations">instantiated</a> when they are used. </p> <pre> type List[T any] struct { next List[T] value T } </pre> <p> In a type definition the given type cannot be a type parameter. </p> <pre> type T[P any] P // illegal: P is a type parameter func f[T any]() { type L T // illegal: T is a type parameter declared by the enclosing function } </pre> <p> A generic type may also have <a href="#Method_declarations">methods</a> associated with it. In this case, the method receivers must declare the same number of type parameters as present in the generic type definition. </p> <pre> // The method Len returns the number of elements in the linked list l. func (l List[T]) Len() int { … } </pre> <h3 id="Type_parameter_declarations">Type parameter declarations</h3> <p> A type parameter list declares the <i>type parameters</i> of a generic function or type declaration. The type parameter list looks like an ordinary <a href="#Function_types">function parameter list</a> except that the type parameter names must all be present and the list is enclosed in square brackets rather than parentheses [<a href="#Go_1.18">Go 1.18</a>]. </p> <pre class="ebnf"> TypeParameters = "[" TypeParamList [ "," ] "]" . TypeParamList = TypeParamDecl { "," TypeParamDecl } . TypeParamDecl = IdentifierList TypeConstraint . </pre> <p> All non-blank names in the list must be unique. Each name declares a type parameter, which is a new and different <a href="#Types">named type</a> that acts as a placeholder for an (as of yet) unknown type in the declaration. The type parameter is replaced with a <i>type argument</i> upon <a href="#Instantiations">instantiation</a> of the generic function or type. </p> <pre> [P any] [S interface{ ~[]byte\|string }] [S ~[]E, E any] [P Constraint[int]] [_ any] </pre> <p> Just as each ordinary function parameter has a parameter type, each type parameter has a corresponding (meta-)type which is called its <a href="#Type_constraints"><i>type constraint</i></a>. </p> <p> A parsing ambiguity arises when the type parameter list for a generic type declares a single type parameter <code>P</code> with a constraint <code>C</code> such that the text <code>P C</code> forms a valid expression: </p> <pre> type T[P C] … type T[P (C)] … type T[P C\|Q] … … </pre> <p> In these rare cases, the type parameter list is indistinguishable from an expression and the type declaration is parsed as an array type declaration. To resolve the ambiguity, embed the constraint in an <a href="#Interface_types">interface</a> or use a trailing comma: </p> <pre> type T[P interface{C}] … type T[P C,] … </pre> <p> Type parameters may also be declared by the receiver specification of a <a href="#Method_declarations">method declaration</a> associated with a generic type. </p> <p> Within a type parameter list of a generic type <code>T</code>, a type constraint may not (directly, or indirectly through the type parameter list of another generic type) refer to <code>T</code>. </p> <pre> type T1[P T1[P]] … // illegal: T1 refers to itself type T2[P interface{ T2[int] }] … // illegal: T2 refers to itself type T3[P interface{ m(T3[int])}] … // illegal: T3 refers to itself type T4[P T5[P]] … // illegal: T4 refers to T5 and type T5[P T4[P]] … // T5 refers to T4 type T6[P int] struct{ f T6[P] } // ok: reference to T6 is not in type parameter list </pre> <h4 id="Type_constraints">Type constraints</h4> <p> A <i>type constraint</i> is an <a href="#Interface_types">interface</a> that defines the set of permissible type arguments for the respective type parameter and controls the operations supported by values of that type parameter [<a href="#Go_1.18">Go 1.18</a>]. </p> <pre class="ebnf"> TypeConstraint = TypeElem . </pre> <p> If the constraint is an interface literal of the form <code>interface{E}</code> where <code>E</code> is an embedded <a href="#Interface_types">type element</a> (not a method), in a type parameter list the enclosing <code>interface{ … }</code> may be omitted for convenience: </p> <pre> [T []P] // = [T interface{[]P}] [T ~int] // = [T interface{~int}] [T int\|string] // = [T interface{int\|string}] type Constraint ~int // illegal: ~int is not in a type parameter list </pre> <!-- We should be able to simplify the rules for comparable or delegate some of them elsewhere since we have a section that clearly defines how interfaces implement other interfaces based on their type sets. But this should get us going for now. --> <p> The <a href="#Predeclared_identifiers">predeclared</a> <a href="#Interface_types">interface type</a> <code>comparable</code> denotes the set of all non-interface types that are <a href="#Comparison_operators">strictly comparable</a> [<a href="#Go_1.18">Go 1.18</a>]. </p> <p> Even though interfaces that are not type parameters are <a href="#Comparison_operators">comparable</a>, they are not strictly comparable and therefore they do not implement <code>comparable</code>. However, they <a href="#Satisfying_a_type_constraint">satisfy</a> <code>comparable</code>. </p> <pre> int // implements comparable (int is strictly comparable) []byte // does not implement comparable (slices cannot be compared) interface{} // does not implement comparable (see above) interface{ ~int \| ~string } // type parameter only: implements comparable (int, string types are strictly comparable) interface{ comparable } // type parameter only: implements comparable (comparable implements itself) interface{ ~int \| ~[]byte } // type parameter only: does not implement comparable (slices are not comparable) interface{ ~struct{ any } } // type parameter only: does not implement comparable (field any is not strictly comparable) </pre> <p> The <code>comparable</code> interface and interfaces that (directly or indirectly) embed <code>comparable</code> may only be used as type constraints. They cannot be the types of values or variables, or components of other, non-interface types. </p> <h4 id="Satisfying_a_type_constraint">Satisfying a type constraint</h4> <p> A type argument <code>T</code><i> satisfies</i> a type constraint <code>C</code> if <code>T</code> is an element of the type set defined by <code>C</code>; i.e., if <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>. As an exception, a <a href="#Comparison_operators">strictly comparable</a> type constraint may also be satisfied by a <a href="#Comparison_operators">comparable</a> (not necessarily strictly comparable) type argument [<a href="#Go_1.20">Go 1.20</a>]. More precisely: </p> <p> A type T <i>satisfies</i> a constraint <code>C</code> if </p> <ul> <li> <code>T</code> <a href="#Implementing_an_interface">implements</a> <code>C</code>; or </li> <li> <code>C</code> can be written in the form <code>interface{ comparable; E }</code>, where <code>E</code> is a <a href="#Basic_interfaces">basic interface</a> and <code>T</code> is <a href="#Comparison_operators">comparable</a> and implements <code>E</code>. </li> </ul> <pre> type argument type constraint // constraint satisfaction int interface{ ~int } // satisfied: int implements interface{ ~int } string comparable // satisfied: string implements comparable (string is strictly comparable) []byte comparable // not satisfied: slices are not comparable any interface{ comparable; int } // not satisfied: any does not implement interface{ int } any comparable // satisfied: any is comparable and implements the basic interface any struct{f any} comparable // satisfied: struct{f any} is comparable and implements the basic interface any any interface{ comparable; m() } // not satisfied: any does not implement the basic interface interface{ m() } interface{ m() } interface{ comparable; m() } // satisfied: interface{ m() } is comparable and implements the basic interface interface{ m() } </pre> <p> Because of the exception in the constraint satisfaction rule, comparing operands of type parameter type may panic at run-time (even though comparable type parameters are always strictly comparable). </p> <h3 id="Variable_declarations">Variable declarations</h3> <p> A variable declaration creates one or more <a href="#Variables">variables</a>, binds corresponding identifiers to them, and gives each a type and an initial value. </p> <pre class="ebnf"> VarDecl = "var" ( VarSpec \| "(" { VarSpec ";" } ")" ) . VarSpec = IdentifierList ( Type [ "=" ExpressionList ] \| "=" ExpressionList ) . </pre> <pre> var i int var U, V, W float64 var k = 0 var x, y float32 = -1, -2 var ( i int u, v, s = 2.0, 3.0, "bar" ) var re, im = complexSqrt(-1) var _, found = entries[name] // map lookup; only interested in "found" </pre> <p> If a list of expressions is given, the variables are initialized with the expressions following the rules for <a href="#Assignment_statements">assignment statements</a>. Otherwise, each variable is initialized to its <a href="#The_zero_value">zero value</a>. </p> <p> If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment. If that value is an untyped constant, it is first implicitly <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>; if it is an untyped boolean value, it is first implicitly converted to type <code>bool</code>. The predeclared value <code>nil</code> cannot be used to initialize a variable with no explicit type. </p> <pre> var d = math.Sin(0.5) // d is float64 var i = 42 // i is int var t, ok = x.(T) // t is T, ok is bool var n = nil // illegal </pre> <p> Implementation restriction: A compiler may make it illegal to declare a variable inside a <a href="#Function_declarations">function body</a> if the variable is never used. </p> <h3 id="Short_variable_declarations">Short variable declarations</h3> <p> A <i>short variable declaration</i> uses the syntax: </p> <pre class="ebnf"> ShortVarDecl = IdentifierList ":=" ExpressionList . </pre> <p> It is shorthand for a regular <a href="#Variable_declarations">variable declaration</a> with initializer expressions but no types: </p> <pre class="grammar"> "var" IdentifierList "=" ExpressionList . </pre> <pre> i, j := 0, 10 f := func() int { return 7 } ch := make(chan int) r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any _, y, _ := coord(p) // coord() returns three values; only interested in y coordinate </pre> <p> Unlike regular variable declarations, a short variable declaration may <i>redeclare</i> variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-<a href="#Blank_identifier">blank</a> variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original. The non-blank variable names on the left side of <code>:=</code> must be <a href="#Uniqueness_of_identifiers">unique</a>. </p> <pre> field1, offset := nextField(str, 0) field2, offset := nextField(str, offset) // redeclares offset x, y, x := 1, 2, 3 // illegal: x repeated on left side of := </pre> <p> Short variable declarations may appear only inside functions. In some contexts such as the initializers for <a href="#If_statements">"if"</a>, <a href="#For_statements">"for"</a>, or <a href="#Switch_statements">"switch"</a> statements, they can be used to declare local temporary variables. </p> <h3 id="Function_declarations">Function declarations</h3> <!-- Given the importance of functions, this section has always been woefully underdeveloped. Would be nice to expand this a bit. --> <p> A function declaration binds an identifier, the <i>function name</i>, to a function. </p> <pre class="ebnf"> FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] . FunctionName = identifier . FunctionBody = Block . </pre> <p> If the function's <a href="#Function_types">signature</a> declares result parameters, the function body's statement list must end in a <a href="#Terminating_statements">terminating statement</a>. </p> <pre> func IndexRune(s string, r rune) int { for i, c := range s { if c == r { return i } } // invalid: missing return statement } </pre> <p> If the function declaration specifies <a href="#Type_parameter_declarations">type parameters</a>, the function name denotes a <i>generic function</i>. A generic function must be <a href="#Instantiations">instantiated</a> before it can be called or used as a value. </p> <pre> func min[T ~int\|~float64](x, y T) T { if x < y { return x } return y } </pre> <p> A function declaration without type parameters may omit the body. Such a declaration provides the signature for a function implemented outside Go, such as an assembly routine. </p> <pre> func flushICache(begin, end uintptr) // implemented externally </pre> <h3 id="Method_declarations">Method declarations</h3> <p> A method is a <a href="#Function_declarations">function</a> with a <i>receiver</i>. A method declaration binds an identifier, the <i>method name</i>, to a method, and associates the method with the receiver's <i>base type</i>. </p> <pre class="ebnf"> MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] . Receiver = Parameters . </pre> <p> The receiver is specified via an extra parameter section preceding the method name. That parameter section must declare a single non-variadic parameter, the receiver. Its type must be a <a href="#Type_definitions">defined</a> type <code>T</code> or a pointer to a defined type <code>T</code>, possibly followed by a list of type parameter names <code>[P1, P2, …]</code> enclosed in square brackets. <code>T</code> is called the receiver <i>base type</i>. A receiver base type cannot be a pointer or interface type and it must be defined in the same package as the method. The method is said to be <i>bound</i> to its receiver base type and the method name is visible only within <a href="#Selectors">selectors</a> for type <code>T</code> or <code>T</code>. </p> <p> A non-<a href="#Blank_identifier">blank</a> receiver identifier must be <a href="#Uniqueness_of_identifiers">unique</a> in the method signature. If the receiver's value is not referenced inside the body of the method, its identifier may be omitted in the declaration. The same applies in general to parameters of functions and methods. </p> <p> For a base type, the non-blank names of methods bound to it must be unique. If the base type is a <a href="#Struct_types">struct type</a>, the non-blank method and field names must be distinct. </p> <p> Given defined type <code>Point</code> the declarations </p> <pre> func (p Point) Length() float64 { return math.Sqrt(p.x p.x + p.y * p.y) } func (p Point) Scale(factor float64) { p.x = factor p.y = factor } </pre> <p> bind the methods <code>Length</code> and <code>Scale</code>, with receiver type <code>Point</code>, to the base type <code>Point</code>. </p> <p> If the receiver base type is a <a href="#Type_declarations">generic type</a>, the receiver specification must declare corresponding type parameters for the method to use. This makes the receiver type parameters available to the method. Syntactically, this type parameter declaration looks like an <a href="#Instantiations">instantiation</a> of the receiver base type: the type arguments must be identifiers denoting the type parameters being declared, one for each type parameter of the receiver base type. The type parameter names do not need to match their corresponding parameter names in the receiver base type definition, and all non-blank parameter names must be unique in the receiver parameter section and the method signature. The receiver type parameter constraints are implied by the receiver base type definition: corresponding type parameters have corresponding constraints. </p> <pre> type Pair[A, B any] struct { a A b B } func (p Pair[A, B]) Swap() Pair[B, A] { … } // receiver declares A, B func (p Pair[First, _]) First() First { … } // receiver declares First, corresponds to A in Pair </pre> <h2 id="Expressions">Expressions</h2> <p> An expression specifies the computation of a value by applying operators and functions to operands. </p> <h3 id="Operands">Operands</h3> <p> Operands denote the elementary values in an expression. An operand may be a literal, a (possibly <a href="#Qualified_identifiers">qualified</a>) non-<a href="#Blank_identifier">blank</a> identifier denoting a <a href="#Constant_declarations">constant</a>, <a href="#Variable_declarations">variable</a>, or <a href="#Function_declarations">function</a>, or a parenthesized expression. </p> <pre class="ebnf"> Operand = Literal \| OperandName [ TypeArgs ] \| "(" Expression ")" . Literal = BasicLit \| CompositeLit \| FunctionLit . BasicLit = int_lit \| float_lit \| imaginary_lit \| rune_lit \| string_lit . OperandName = identifier \| QualifiedIdent . </pre> <p> An operand name denoting a <a href="#Function_declarations">generic function</a> may be followed by a list of <a href="#Instantiations">type arguments</a>; the resulting operand is an <a href="#Instantiations">instantiated</a> function. </p> <p> The <a href="#Blank_identifier">blank identifier</a> may appear as an operand only on the left-hand side of an <a href="#Assignment_statements">assignment statement</a>. </p> <p> Implementation restriction: A compiler need not report an error if an operand's type is a <a href="#Type_parameter_declarations">type parameter</a> with an empty <a href="#Interface_types">type set</a>. Functions with such type parameters cannot be <a href="#Instantiations">instantiated</a>; any attempt will lead to an error at the instantiation site. </p> <h3 id="Qualified_identifiers">Qualified identifiers</h3> <p> A <i>qualified identifier</i> is an identifier qualified with a package name prefix. Both the package name and the identifier must not be <a href="#Blank_identifier">blank</a>. </p> <pre class="ebnf"> QualifiedIdent = PackageName "." identifier . </pre> <p> A qualified identifier accesses an identifier in a different package, which must be <a href="#Import_declarations">imported</a>. The identifier must be <a href="#Exported_identifiers">exported</a> and declared in the <a href="#Blocks">package block</a> of that package. </p> <pre> math.Sin // denotes the Sin function in package math </pre> <h3 id="Composite_literals">Composite literals</h3> <p> Composite literals construct new composite values each time they are evaluated. They consist of the type of the literal followed by a brace-bound list of elements. Each element may optionally be preceded by a corresponding key. </p> <pre class="ebnf"> CompositeLit = LiteralType LiteralValue . LiteralType = StructType \| ArrayType \| "[" "..." "]" ElementType \| SliceType \| MapType \| TypeName [ TypeArgs ] . LiteralValue = "{" [ ElementList [ "," ] ] "}" . ElementList = KeyedElement { "," KeyedElement } . KeyedElement = [ Key ":" ] Element . Key = FieldName \| Expression \| LiteralValue . FieldName = identifier . Element = Expression \| LiteralValue . </pre> <p> The LiteralType's <a href="#Core_types">core type</a> <code>T</code> must be a struct, array, slice, or map type (the syntax enforces this constraint except when the type is given as a TypeName). The types of the elements and keys must be <a href="#Assignability">assignable</a> to the respective field, element, and key types of type <code>T</code>; there is no additional conversion. The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key. It is an error to specify multiple elements with the same field name or constant key value. For non-constant map keys, see the section on <a href="#Order_of_evaluation">evaluation order</a>. </p> <p> For struct literals the following rules apply: </p> <ul> <li>A key must be a field name declared in the struct type. </li> <li>An element list that does not contain any keys must list an element for each struct field in the order in which the fields are declared. </li> <li>If any element has a key, every element must have a key. </li> <li>An element list that contains keys does not need to have an element for each struct field. Omitted fields get the zero value for that field. </li> <li>A literal may omit the element list; such a literal evaluates to the zero value for its type. </li> <li>It is an error to specify an element for a non-exported field of a struct belonging to a different package. </li> </ul> <p> Given the declarations </p> <pre> type Point3D struct { x, y, z float64 } type Line struct { p, q Point3D } </pre> <p> one may write </p> <pre> origin := Point3D{} // zero value for Point3D line := Line{origin, Point3D{y: -4, z: 12.3}} // zero value for line.q.x </pre> <p> For array and slice literals the following rules apply: </p> <ul> <li>Each element has an associated integer index marking its position in the array. </li> <li>An element with a key uses the key as its index. The key must be a non-negative constant <a href="#Representability">representable</a> by a value of type <code>int</code>; and if it is typed it must be of <a href="#Numeric_types">integer type</a>. </li> <li>An element without a key uses the previous element's index plus one. If the first element has no key, its index is zero. </li> </ul> <p> <a href="#Address_operators">Taking the address</a> of a composite literal generates a pointer to a unique <a href="#Variables">variable</a> initialized with the literal's value. </p> <pre> var pointer Point3D = &Point3D{y: 1000} </pre> <p> Note that the <a href="#The_zero_value">zero value</a> for a slice or map type is not the same as an initialized but empty value of the same type. Consequently, taking the address of an empty slice or map composite literal does not have the same effect as allocating a new slice or map value with <a href="#Allocation">new</a>. </p> <pre> p1 := &[]int{} // p1 points to an initialized, empty slice with value []int{} and length 0 p2 := new([]int) // p2 points to an uninitialized slice with value nil and length 0 </pre> <p> The length of an array literal is the length specified in the literal type. If fewer elements than the length are provided in the literal, the missing elements are set to the zero value for the array element type. It is an error to provide elements with index values outside the index range of the array. The notation <code>...</code> specifies an array length equal to the maximum element index plus one. </p> <pre> buffer := [10]string{} // len(buffer) == 10 intSet := [6]int{1, 2, 3, 5} // len(intSet) == 6 days := [...]string{"Sat", "Sun"} // len(days) == 2 </pre> <p> A slice literal describes the entire underlying array literal. Thus the length and capacity of a slice literal are the maximum element index plus one. A slice literal has the form </p> <pre> []T{x1, x2, … xn} </pre> <p> and is shorthand for a slice operation applied to an array: </p> <pre> tmp := [n]T{x1, x2, … xn} tmp[0 : n] </pre> <p> Within a composite literal of array, slice, or map type <code>T</code>, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of <code>T</code>. Similarly, elements or keys that are addresses of composite literals may elide the <code>&T</code> when the element or key type is <code>T</code>. </p> <pre> [...]Point{{1.5, -3.5}, {0, 0}} // same as [...]Point{Point{1.5, -3.5}, Point{0, 0}} [][]int{{1, 2, 3}, {4, 5}} // same as [][]int{[]int{1, 2, 3}, []int{4, 5}} [][]Point{{{0, 1}, {1, 2}}} // same as [][]Point{[]Point{Point{0, 1}, Point{1, 2}}} map[string]Point{"orig": {0, 0}} // same as map[string]Point{"orig": Point{0, 0}} map[Point]string{{0, 0}: "orig"} // same as map[Point]string{Point{0, 0}: "orig"} type PPoint Point [2]Point{{1.5, -3.5}, {}} // same as [2]Point{&Point{1.5, -3.5}, &Point{}} [2]PPoint{{1.5, -3.5}, {}} // same as [2]PPoint{PPoint(&Point{1.5, -3.5}), PPoint(&Point{})} </pre> <p> A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the <a href="#Keywords">keyword</a> and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses. </p> <pre> if x == (T{a,b,c}[i]) { … } if (x == T{a,b,c}[i]) { … } </pre> <p> Examples of valid array, slice, and map literals: </p> <pre> // list of prime numbers primes := []int{2, 3, 5, 7, 9, 2147483647} // vowels[ch] is true if ch is a vowel vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true} // the array [10]float32{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1} filter := [10]float32{-1, 4: -0.1, -0.1, 9: -1} // frequencies in Hz for equal-tempered scale (A4 = 440Hz) noteFrequency := map[string]float32{ "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83, "G0": 24.50, "A0": 27.50, "B0": 30.87, } </pre> <h3 id="Function_literals">Function literals</h3> <p> A function literal represents an anonymous <a href="#Function_declarations">function</a>. Function literals cannot declare type parameters. </p> <pre class="ebnf"> FunctionLit = "func" Signature FunctionBody . </pre> <pre> func(a, b int, z float64) bool { return ab < int(z) } </pre> <p> A function literal can be assigned to a variable or invoked directly. </p> <pre> f := func(x, y int) int { return x + y } func(ch chan int) { ch <- ACK }(replyChan) </pre> <p> Function literals are <i>closures</i>: they may refer to variables defined in a surrounding function. Those variables are then shared between the surrounding function and the function literal, and they survive as long as they are accessible. </p> <h3 id="Primary_expressions">Primary expressions</h3> <p> Primary expressions are the operands for unary and binary expressions. </p> <pre class="ebnf"> PrimaryExpr = Operand \| Conversion \| MethodExpr \| PrimaryExpr Selector \| PrimaryExpr Index \| PrimaryExpr Slice \| PrimaryExpr TypeAssertion \| PrimaryExpr Arguments . Selector = "." identifier . Index = "[" Expression [ "," ] "]" . Slice = "[" [ Expression ] ":" [ Expression ] "]" \| "[" [ Expression ] ":" Expression ":" Expression "]" . TypeAssertion = "." "(" Type ")" . Arguments = "(" [ ( ExpressionList \| Type [ "," ExpressionList ] ) [ "..." ] [ "," ] ] ")" . </pre> <pre> x 2 (s + ".txt") f(3.1415, true) Point{1, 2} m["foo"] s[i : j + 1] obj.color f.p[i].x() </pre> <h3 id="Selectors">Selectors</h3> <p> For a <a href="#Primary_expressions">primary expression</a> <code>x</code> that is not a <a href="#Package_clause">package name</a>, the <i>selector expression</i> </p> <pre> x.f </pre> <p> denotes the field or method <code>f</code> of the value <code>x</code> (or sometimes <code>x</code>; see below). The identifier <code>f</code> is called the (field or method) <i>selector</i>; it must not be the <a href="#Blank_identifier">blank identifier</a>. The type of the selector expression is the type of <code>f</code>. If <code>x</code> is a package name, see the section on <a href="#Qualified_identifiers">qualified identifiers</a>. </p> <p> A selector <code>f</code> may denote a field or method <code>f</code> of a type <code>T</code>, or it may refer to a field or method <code>f</code> of a nested <a href="#Struct_types">embedded field</a> of <code>T</code>. The number of embedded fields traversed to reach <code>f</code> is called its <i>depth</i> in <code>T</code>. The depth of a field or method <code>f</code> declared in <code>T</code> is zero. The depth of a field or method <code>f</code> declared in an embedded field <code>A</code> in <code>T</code> is the depth of <code>f</code> in <code>A</code> plus one. </p> <p> The following rules apply to selectors: </p> <ol> <li> For a value <code>x</code> of type <code>T</code> or <code>T</code> where <code>T</code> is not a pointer or interface type, <code>x.f</code> denotes the field or method at the shallowest depth in <code>T</code> where there is such an <code>f</code>. If there is not exactly <a href="#Uniqueness_of_identifiers">one <code>f</code></a> with shallowest depth, the selector expression is illegal. </li> <li> For a value <code>x</code> of type <code>I</code> where <code>I</code> is an interface type, <code>x.f</code> denotes the actual method with name <code>f</code> of the dynamic value of <code>x</code>. If there is no method with name <code>f</code> in the <a href="#Method_sets">method set</a> of <code>I</code>, the selector expression is illegal. </li> <li> As an exception, if the type of <code>x</code> is a <a href="#Type_definitions">defined</a> pointer type and <code>(x).f</code> is a valid selector expression denoting a field (but not a method), <code>x.f</code> is shorthand for <code>(x).f</code>. </li> <li> In all other cases, <code>x.f</code> is illegal. </li> <li> If <code>x</code> is of pointer type and has the value <code>nil</code> and <code>x.f</code> denotes a struct field, assigning to or evaluating <code>x.f</code> causes a <a href="#Run_time_panics">run-time panic</a>. </li> <li> If <code>x</code> is of interface type and has the value <code>nil</code>, <a href="#Calls">calling</a> or <a href="#Method_values">evaluating</a> the method <code>x.f</code> causes a <a href="#Run_time_panics">run-time panic</a>. </li> </ol> <p> For example, given the declarations: </p> <pre> type T0 struct { x int } func (T0) M0() type T1 struct { y int } func (T1) M1() type T2 struct { z int T1 T0 } func (T2) M2() type Q T2 var t T2 // with t.T0 != nil var p T2 // with p != nil and (p).T0 != nil var q Q = p </pre> <p> one may write: </p> <pre> t.z // t.z t.y // t.T1.y t.x // (t.T0).x p.z // (p).z p.y // (p).T1.y p.x // ((p).T0).x q.x // ((q).T0).x (q).x is a valid field selector p.M0() // ((p).T0).M0() M0 expects T0 receiver p.M1() // ((p).T1).M1() M1 expects T1 receiver p.M2() // p.M2() M2 expects T2 receiver t.M2() // (&t).M2() M2 expects T2 receiver, see section on Calls </pre> <p> but the following is invalid: </p> <pre> q.M0() // (q).M0 is valid but not a field selector </pre> <h3 id="Method_expressions">Method expressions</h3> <p> If <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>, <code>T.M</code> is a function that is callable as a regular function with the same arguments as <code>M</code> prefixed by an additional argument that is the receiver of the method. </p> <pre class="ebnf"> MethodExpr = ReceiverType "." MethodName . ReceiverType = Type . </pre> <p> Consider a struct type <code>T</code> with two methods, <code>Mv</code>, whose receiver is of type <code>T</code>, and <code>Mp</code>, whose receiver is of type <code>T</code>. </p> <pre> type T struct { a int } func (tv T) Mv(a int) int { return 0 } // value receiver func (tp T) Mp(f float32) float32 { return 1 } // pointer receiver var t T </pre> <p> The expression </p> <pre> T.Mv </pre> <p> yields a function equivalent to <code>Mv</code> but with an explicit receiver as its first argument; it has signature </p> <pre> func(tv T, a int) int </pre> <p> That function may be called normally with an explicit receiver, so these five invocations are equivalent: </p> <pre> t.Mv(7) T.Mv(t, 7) (T).Mv(t, 7) f1 := T.Mv; f1(t, 7) f2 := (T).Mv; f2(t, 7) </pre> <p> Similarly, the expression </p> <pre> (T).Mp </pre> <p> yields a function value representing <code>Mp</code> with signature </p> <pre> func(tp T, f float32) float32 </pre> <p> For a method with a value receiver, one can derive a function with an explicit pointer receiver, so </p> <pre> (T).Mv </pre> <p> yields a function value representing <code>Mv</code> with signature </p> <pre> func(tv T, a int) int </pre> <p> Such a function indirects through the receiver to create a value to pass as the receiver to the underlying method; the method does not overwrite the value whose address is passed in the function call. </p> <p> The final case, a value-receiver function for a pointer-receiver method, is illegal because pointer-receiver methods are not in the method set of the value type. </p> <p> Function values derived from methods are called with function call syntax; the receiver is provided as the first argument to the call. That is, given <code>f := T.Mv</code>, <code>f</code> is invoked as <code>f(t, 7)</code> not <code>t.f(7)</code>. To construct a function that binds the receiver, use a <a href="#Function_literals">function literal</a> or <a href="#Method_values">method value</a>. </p> <p> It is legal to derive a function value from a method of an interface type. The resulting function takes an explicit receiver of that interface type. </p> <h3 id="Method_values">Method values</h3> <p> If the expression <code>x</code> has static type <code>T</code> and <code>M</code> is in the <a href="#Method_sets">method set</a> of type <code>T</code>, <code>x.M</code> is called a <i>method value</i>. The method value <code>x.M</code> is a function value that is callable with the same arguments as a method call of <code>x.M</code>. The expression <code>x</code> is evaluated and saved during the evaluation of the method value; the saved copy is then used as the receiver in any calls, which may be executed later. </p> <pre> type S struct { T } type T int func (t T) M() { print(t) } t := new(T) s := S{T: t} f := t.M // receiver t is evaluated and stored in f g := s.M // receiver (s.T) is evaluated and stored in g t = 42 // does not affect stored receivers in f and g </pre> <p> The type <code>T</code> may be an interface or non-interface type. </p> <p> As in the discussion of <a href="#Method_expressions">method expressions</a> above, consider a struct type <code>T</code> with two methods, <code>Mv</code>, whose receiver is of type <code>T</code>, and <code>Mp</code>, whose receiver is of type <code>T</code>. </p> <pre> type T struct { a int } func (tv T) Mv(a int) int { return 0 } // value receiver func (tp T) Mp(f float32) float32 { return 1 } // pointer receiver var t T var pt T func makeT() T </pre> <p> The expression </p> <pre> t.Mv </pre> <p> yields a function value of type </p> <pre> func(int) int </pre> <p> These two invocations are equivalent: </p> <pre> t.Mv(7) f := t.Mv; f(7) </pre> <p> Similarly, the expression </p> <pre> pt.Mp </pre> <p> yields a function value of type </p> <pre> func(float32) float32 </pre> <p> As with <a href="#Selectors">selectors</a>, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: <code>pt.Mv</code> is equivalent to <code>(pt).Mv</code>. </p> <p> As with <a href="#Calls">method calls</a>, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: <code>t.Mp</code> is equivalent to <code>(&t).Mp</code>. </p> <pre> f := t.Mv; f(7) // like t.Mv(7) f := pt.Mp; f(7) // like pt.Mp(7) f := pt.Mv; f(7) // like (pt).Mv(7) f := t.Mp; f(7) // like (&t).Mp(7) f := makeT().Mp // invalid: result of makeT() is not addressable </pre> <p> Although the examples above use non-interface types, it is also legal to create a method value from a value of interface type. </p> <pre> var i interface { M(int) } = myVal f := i.M; f(7) // like i.M(7) </pre> <h3 id="Index_expressions">Index expressions</h3> <p> A primary expression of the form </p> <pre> a[x] </pre> <p> denotes the element of the array, pointer to array, slice, string or map <code>a</code> indexed by <code>x</code>. The value <code>x</code> is called the <i>index</i> or <i>map key</i>, respectively. The following rules apply: </p> <p> If <code>a</code> is neither a map nor a type parameter: </p> <ul> <li>the index <code>x</code> must be an untyped constant or its <a href="#Core_types">core type</a> must be an <a href="#Numeric_types">integer</a></li> <li>a constant index must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code></li> <li>a constant index that is untyped is given type <code>int</code></li> <li>the index <code>x</code> is <i>in range</i> if <code>0 <= x < len(a)</code>, otherwise it is <i>out of range</i></li> </ul> <p> For <code>a</code> of <a href="#Array_types">array type</a> <code>A</code>: </p> <ul> <li>a <a href="#Constants">constant</a> index must be in range</li> <li>if <code>x</code> is out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs</li> <li><code>a[x]</code> is the array element at index <code>x</code> and the type of <code>a[x]</code> is the element type of <code>A</code></li> </ul> <p> For <code>a</code> of <a href="#Pointer_types">pointer</a> to array type: </p> <ul> <li><code>a[x]</code> is shorthand for <code>(a)[x]</code></li> </ul> <p> For <code>a</code> of <a href="#Slice_types">slice type</a> <code>S</code>: </p> <ul> <li>if <code>x</code> is out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs</li> <li><code>a[x]</code> is the slice element at index <code>x</code> and the type of <code>a[x]</code> is the element type of <code>S</code></li> </ul> <p> For <code>a</code> of <a href="#String_types">string type</a>: </p> <ul> <li>a <a href="#Constants">constant</a> index must be in range if the string <code>a</code> is also constant</li> <li>if <code>x</code> is out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs</li> <li><code>a[x]</code> is the non-constant byte value at index <code>x</code> and the type of <code>a[x]</code> is <code>byte</code></li> <li><code>a[x]</code> may not be assigned to</li> </ul> <p> For <code>a</code> of <a href="#Map_types">map type</a> <code>M</code>: </p> <ul> <li><code>x</code>'s type must be <a href="#Assignability">assignable</a> to the key type of <code>M</code></li> <li>if the map contains an entry with key <code>x</code>, <code>a[x]</code> is the map element with key <code>x</code> and the type of <code>a[x]</code> is the element type of <code>M</code></li> <li>if the map is <code>nil</code> or does not contain such an entry, <code>a[x]</code> is the <a href="#The_zero_value">zero value</a> for the element type of <code>M</code></li> </ul> <p> For <code>a</code> of <a href="#Type_parameter_declarations">type parameter type</a> <code>P</code>: </p> <ul> <li>The index expression <code>a[x]</code> must be valid for values of all types in <code>P</code>'s type set.</li> <li>The element types of all types in <code>P</code>'s type set must be identical. In this context, the element type of a string type is <code>byte</code>.</li> <li>If there is a map type in the type set of <code>P</code>, all types in that type set must be map types, and the respective key types must be all identical.</li> <li><code>a[x]</code> is the array, slice, or string element at index <code>x</code>, or the map element with key <code>x</code> of the type argument that <code>P</code> is instantiated with, and the type of <code>a[x]</code> is the type of the (identical) element types.</li> <li><code>a[x]</code> may not be assigned to if <code>P</code>'s type set includes string types.</li> </ul> <p> Otherwise <code>a[x]</code> is illegal. </p> <p> An index expression on a map <code>a</code> of type <code>map[K]V</code> used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form </p> <pre> v, ok = a[x] v, ok := a[x] var v, ok = a[x] </pre> <p> yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code> if the key <code>x</code> is present in the map, and <code>false</code> otherwise. </p> <p> Assigning to an element of a <code>nil</code> map causes a <a href="#Run_time_panics">run-time panic</a>. </p> <h3 id="Slice_expressions">Slice expressions</h3> <p> Slice expressions construct a substring or slice from a string, array, pointer to array, or slice. There are two variants: a simple form that specifies a low and high bound, and a full form that also specifies a bound on the capacity. </p> <h4>Simple slice expressions</h4> <p> The primary expression </p> <pre> a[low : high] </pre> <p> constructs a substring or slice. The <a href="#Core_types">core type</a> of <code>a</code> must be a string, array, pointer to array, slice, or a <a href="#Core_types"><code>bytestring</code></a>. The <i>indices</i> <code>low</code> and <code>high</code> select which elements of operand <code>a</code> appear in the result. The result has indices starting at 0 and length equal to <code>high</code> - <code>low</code>. After slicing the array <code>a</code> </p> <pre> a := [5]int{1, 2, 3, 4, 5} s := a[1:4] </pre> <p> the slice <code>s</code> has type <code>[]int</code>, length 3, capacity 4, and elements </p> <pre> s[0] == 2 s[1] == 3 s[2] == 4 </pre> <p> For convenience, any of the indices may be omitted. A missing <code>low</code> index defaults to zero; a missing <code>high</code> index defaults to the length of the sliced operand: </p> <pre> a[2:] // same as a[2 : len(a)] a[:3] // same as a[0 : 3] a[:] // same as a[0 : len(a)] </pre> <p> If <code>a</code> is a pointer to an array, <code>a[low : high]</code> is shorthand for <code>(a)[low : high]</code>. </p> <p> For arrays or strings, the indices are <i>in range</i> if <code>0</code> <= <code>low</code> <= <code>high</code> <= <code>len(a)</code>, otherwise they are <i>out of range</i>. For slices, the upper index bound is the slice capacity <code>cap(a)</code> rather than the length. A <a href="#Constants">constant</a> index must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; for arrays or constant strings, constant indices must also be in range. If both indices are constant, they must satisfy <code>low <= high</code>. If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <p> Except for <a href="#Constants">untyped strings</a>, if the sliced operand is a string or slice, the result of the slice operation is a non-constant value of the same type as the operand. For untyped string operands the result is a non-constant value of type <code>string</code>. If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a> and the result of the slice operation is a slice with the same element type as the array. </p> <p> If the sliced operand of a valid slice expression is a <code>nil</code> slice, the result is a <code>nil</code> slice. Otherwise, if the result is a slice, it shares its underlying array with the operand. </p> <pre> var a [10]int s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5] s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5] s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element var s []int s3 := s[:0] // s3 == nil </pre> <h4>Full slice expressions</h4> <p> The primary expression </p> <pre> a[low : high : max] </pre> <p> constructs a slice of the same type, and with the same length and elements as the simple slice expression <code>a[low : high]</code>. Additionally, it controls the resulting slice's capacity by setting it to <code>max - low</code>. Only the first index may be omitted; it defaults to 0. The <a href="#Core_types">core type</a> of <code>a</code> must be an array, pointer to array, or slice (but not a string). After slicing the array <code>a</code> </p> <pre> a := [5]int{1, 2, 3, 4, 5} t := a[1:3:5] </pre> <p> the slice <code>t</code> has type <code>[]int</code>, length 2, capacity 4, and elements </p> <pre> t[0] == 2 t[1] == 3 </pre> <p> As for simple slice expressions, if <code>a</code> is a pointer to an array, <code>a[low : high : max]</code> is shorthand for <code>(a)[low : high : max]</code>. If the sliced operand is an array, it must be <a href="#Address_operators">addressable</a>. </p> <p> The indices are <i>in range</i> if <code>0 <= low <= high <= max <= cap(a)</code>, otherwise they are <i>out of range</i>. A <a href="#Constants">constant</a> index must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; for arrays, constant indices must also be in range. If multiple indices are constant, the constants that are present must be in range relative to each other. If the indices are out of range at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <h3 id="Type_assertions">Type assertions</h3> <p> For an expression <code>x</code> of <a href="#Interface_types">interface type</a>, but not a <a href="#Type_parameter_declarations">type parameter</a>, and a type <code>T</code>, the primary expression </p> <pre> x.(T) </pre> <p> asserts that <code>x</code> is not <code>nil</code> and that the value stored in <code>x</code> is of type <code>T</code>. The notation <code>x.(T)</code> is called a <i>type assertion</i>. </p> <p> More precisely, if <code>T</code> is not an interface type, <code>x.(T)</code> asserts that the dynamic type of <code>x</code> is <a href="#Type_identity">identical</a> to the type <code>T</code>. In this case, <code>T</code> must <a href="#Method_sets">implement</a> the (interface) type of <code>x</code>; otherwise the type assertion is invalid since it is not possible for <code>x</code> to store a value of type <code>T</code>. If <code>T</code> is an interface type, <code>x.(T)</code> asserts that the dynamic type of <code>x</code> <a href="#Implementing_an_interface">implements</a> the interface <code>T</code>. </p> <p> If the type assertion holds, the value of the expression is the value stored in <code>x</code> and its type is <code>T</code>. If the type assertion is false, a <a href="#Run_time_panics">run-time panic</a> occurs. In other words, even though the dynamic type of <code>x</code> is known only at run time, the type of <code>x.(T)</code> is known to be <code>T</code> in a correct program. </p> <pre> var x interface{} = 7 // x has dynamic type int and value 7 i := x.(int) // i has type int and value 7 type I interface { m() } func f(y I) { s := y.(string) // illegal: string does not implement I (missing method m) r := y.(io.Reader) // r has type io.Reader and the dynamic type of y must implement both I and io.Reader … } </pre> <p> A type assertion used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form </p> <pre> v, ok = x.(T) v, ok := x.(T) var v, ok = x.(T) var v, ok interface{} = x.(T) // dynamic types of v and ok are T and bool </pre> <p> yields an additional untyped boolean value. The value of <code>ok</code> is <code>true</code> if the assertion holds. Otherwise it is <code>false</code> and the value of <code>v</code> is the <a href="#The_zero_value">zero value</a> for type <code>T</code>. No <a href="#Run_time_panics">run-time panic</a> occurs in this case. </p> <h3 id="Calls">Calls</h3> <p> Given an expression <code>f</code> with a <a href="#Core_types">core type</a> <code>F</code> of <a href="#Function_types">function type</a>, </p> <pre> f(a1, a2, … an) </pre> <p> calls <code>f</code> with arguments <code>a1, a2, … an</code>. Except for one special case, arguments must be single-valued expressions <a href="#Assignability">assignable</a> to the parameter types of <code>F</code> and are evaluated before the function is called. The type of the expression is the result type of <code>F</code>. A method invocation is similar but the method itself is specified as a selector upon a value of the receiver type for the method. </p> <pre> math.Atan2(x, y) // function call var pt Point pt.Scale(3.5) // method call with receiver pt </pre> <p> If <code>f</code> denotes a generic function, it must be <a href="#Instantiations">instantiated</a> before it can be called or used as a function value. </p> <p> In a function call, the function value and arguments are evaluated in <a href="#Order_of_evaluation">the usual order</a>. After they are evaluated, the parameters of the call are passed by value to the function and the called function begins execution. The return parameters of the function are passed by value back to the caller when the function returns. </p> <p> Calling a <code>nil</code> function value causes a <a href="#Run_time_panics">run-time panic</a>. </p> <p> As a special case, if the return values of a function or method <code>g</code> are equal in number and individually assignable to the parameters of another function or method <code>f</code>, then the call <code>f(g(<i>parameters_of_g</i>))</code> will invoke <code>f</code> after binding the return values of <code>g</code> to the parameters of <code>f</code> in order. The call of <code>f</code> must contain no parameters other than the call of <code>g</code>, and <code>g</code> must have at least one return value. If <code>f</code> has a final <code>...</code> parameter, it is assigned the return values of <code>g</code> that remain after assignment of regular parameters. </p> <pre> func Split(s string, pos int) (string, string) { return s[0:pos], s[pos:] } func Join(s, t string) string { return s + t } if Join(Split(value, len(value)/2)) != value { log.Panic("test fails") } </pre> <p> A method call <code>x.m()</code> is valid if the <a href="#Method_sets">method set</a> of (the type of) <code>x</code> contains <code>m</code> and the argument list can be assigned to the parameter list of <code>m</code>. If <code>x</code> is <a href="#Address_operators">addressable</a> and <code>&x</code>'s method set contains <code>m</code>, <code>x.m()</code> is shorthand for <code>(&x).m()</code>: </p> <pre> var p Point p.Scale(3.5) </pre> <p> There is no distinct method type and there are no method literals. </p> <h3 id="Passing_arguments_to_..._parameters">Passing arguments to <code>...</code> parameters</h3> <p> If <code>f</code> is <a href="#Function_types">variadic</a> with a final parameter <code>p</code> of type <code>...T</code>, then within <code>f</code> the type of <code>p</code> is equivalent to type <code>[]T</code>. If <code>f</code> is invoked with no actual arguments for <code>p</code>, the value passed to <code>p</code> is <code>nil</code>. Otherwise, the value passed is a new slice of type <code>[]T</code> with a new underlying array whose successive elements are the actual arguments, which all must be <a href="#Assignability">assignable</a> to <code>T</code>. The length and capacity of the slice is therefore the number of arguments bound to <code>p</code> and may differ for each call site. </p> <p> Given the function and calls </p> <pre> func Greeting(prefix string, who ...string) Greeting("nobody") Greeting("hello:", "Joe", "Anna", "Eileen") </pre> <p> within <code>Greeting</code>, <code>who</code> will have the value <code>nil</code> in the first call, and <code>[]string{"Joe", "Anna", "Eileen"}</code> in the second. </p> <p> If the final argument is assignable to a slice type <code>[]T</code> and is followed by <code>...</code>, it is passed unchanged as the value for a <code>...T</code> parameter. In this case no new slice is created. </p> <p> Given the slice <code>s</code> and call </p> <pre> s := []string{"James", "Jasmine"} Greeting("goodbye:", s...) </pre> <p> within <code>Greeting</code>, <code>who</code> will have the same value as <code>s</code> with the same underlying array. </p> <h3 id="Instantiations">Instantiations</h3> <p> A generic function or type is <i>instantiated</i> by substituting <i>type arguments</i> for the type parameters [<a href="#Go_1.18">Go 1.18</a>]. Instantiation proceeds in two steps: </p> <ol> <li> Each type argument is substituted for its corresponding type parameter in the generic declaration. This substitution happens across the entire function or type declaration, including the type parameter list itself and any types in that list. </li> <li> After substitution, each type argument must <a href="#Satisfying_a_type_constraint">satisfy</a> the <a href="#Type_parameter_declarations">constraint</a> (instantiated, if necessary) of the corresponding type parameter. Otherwise instantiation fails. </li> </ol> <p> Instantiating a type results in a new non-generic <a href="#Types">named type</a>; instantiating a function produces a new non-generic function. </p> <pre> type parameter list type arguments after substitution [P any] int int satisfies any [S ~[]E, E any] []int, int []int satisfies ~[]int, int satisfies any [P io.Writer] string illegal: string doesn't satisfy io.Writer [P comparable] any any satisfies (but does not implement) comparable </pre> <p> When using a generic function, type arguments may be provided explicitly, or they may be partially or completely <a href="#Type_inference">inferred</a> from the context in which the function is used. Provided that they can be inferred, type argument lists may be omitted entirely if the function is: </p> <ul> <li> <a href="#Calls">called</a> with ordinary arguments, </li> <li> <a href="#Assignment_statements">assigned</a> to a variable with a known type </li> <li> <a href="#Calls">passed as an argument</a> to another function, or </li> <li> <a href="#Return_statements">returned as a result</a>. </li> </ul> <p> In all other cases, a (possibly partial) type argument list must be present. If a type argument list is absent or partial, all missing type arguments must be inferrable from the context in which the function is used. </p> <pre> // sum returns the sum (concatenation, for strings) of its arguments. func sum[T ~int \| ~float64 \| ~string](x... T) T { … } x := sum // illegal: the type of x is unknown intSum := sum[int] // intSum has type func(x... int) int a := intSum(2, 3) // a has value 5 of type int b := sum[float64](2.0, 3) // b has value 5.0 of type float64 c := sum(b, -1) // c has value 4.0 of type float64 type sumFunc func(x... string) string var f sumFunc = sum // same as var f sumFunc = sum[string] f = sum // same as f = sum[string] </pre> <p> A partial type argument list cannot be empty; at least the first argument must be present. The list is a prefix of the full list of type arguments, leaving the remaining arguments to be inferred. Loosely speaking, type arguments may be omitted from "right to left". </p> <pre> func apply[S ~[]E, E any](s S, f func(E) E) S { … } f0 := apply[] // illegal: type argument list cannot be empty f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred f2 := apply[[]string, string] // both type arguments explicitly provided var bytes []byte r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments </pre> <p> For a generic type, all type arguments must always be provided explicitly. </p> <h3 id="Type_inference">Type inference</h3> <p> A use of a generic function may omit some or all type arguments if they can be <i>inferred</i> from the context within which the function is used, including the constraints of the function's type parameters. Type inference succeeds if it can infer the missing type arguments and <a href="#Instantiations">instantiation</a> succeeds with the inferred type arguments. Otherwise, type inference fails and the program is invalid. </p> <p> Type inference uses the type relationships between pairs of types for inference: For instance, a function argument must be <a href="#Assignability">assignable</a> to its respective function parameter; this establishes a relationship between the type of the argument and the type of the parameter. If either of these two types contains type parameters, type inference looks for the type arguments to substitute the type parameters with such that the assignability relationship is satisfied. Similarly, type inference uses the fact that a type argument must <a href="#Satisfying_a_type_constraint">satisfy</a> the constraint of its respective type parameter. </p> <p> Each such pair of matched types corresponds to a <i>type equation</i> containing one or multiple type parameters, from one or possibly multiple generic functions. Inferring the missing type arguments means solving the resulting set of type equations for the respective type parameters. </p> <p> For example, given </p> <pre> // dedup returns a copy of the argument slice with any duplicate entries removed. func dedup[S ~[]E, E comparable](S) S { … } type Slice []int var s Slice s = dedup(s) // same as s = dedup[Slice, int](s) </pre> <p> the variable <code>s</code> of type <code>Slice</code> must be assignable to the function parameter type <code>S</code> for the program to be valid. To reduce complexity, type inference ignores the directionality of assignments, so the type relationship between <code>Slice</code> and <code>S</code> can be expressed via the (symmetric) type equation <code>Slice ≡<sub>A</sub> S</code> (or <code>S ≡<sub>A</sub> Slice</code> for that matter), where the <code><sub>A</sub></code> in <code>≡<sub>A</sub></code> indicates that the LHS and RHS types must match per assignability rules (see the section on <a href="#Type_unification">type unification</a> for details). Similarly, the type parameter <code>S</code> must satisfy its constraint <code>~[]E</code>. This can be expressed as <code>S ≡<sub>C</sub> ~[]E</code> where <code>X ≡<sub>C</sub> Y</code> stands for "<code>X</code> satisfies constraint <code>Y</code>". These observations lead to a set of two equations </p> <pre> Slice ≡<sub>A</sub> S (1) S ≡<sub>C</sub> ~[]E (2) </pre> <p> which now can be solved for the type parameters <code>S</code> and <code>E</code>. From (1) a compiler can infer that the type argument for <code>S</code> is <code>Slice</code>. Similarly, because the underlying type of <code>Slice</code> is <code>[]int</code> and <code>[]int</code> must match <code>[]E</code> of the constraint, a compiler can infer that <code>E</code> must be <code>int</code>. Thus, for these two equations, type inference infers </p> <pre> S ➞ Slice E ➞ int </pre> <p> Given a set of type equations, the type parameters to solve for are the type parameters of the functions that need to be instantiated and for which no explicit type arguments is provided. These type parameters are called <i>bound</i> type parameters. For instance, in the <code>dedup</code> example above, the type parameters <code>S</code> and <code>E</code> are bound to <code>dedup</code>. An argument to a generic function call may be a generic function itself. The type parameters of that function are included in the set of bound type parameters. The types of function arguments may contain type parameters from other functions (such as a generic function enclosing a function call). Those type parameters may also appear in type equations but they are not bound in that context. Type equations are always solved for the bound type parameters only. </p> <p> Type inference supports calls of generic functions and assignments of generic functions to (explicitly function-typed) variables. This includes passing generic functions as arguments to other (possibly also generic) functions, and returning generic functions as results. Type inference operates on a set of equations specific to each of these cases. The equations are as follows (type argument lists are omitted for clarity): </p> <ul> <li> <p> For a function call <code>f(a<sub>0</sub>, a<sub>1</sub>, …)</code> where <code>f</code> or a function argument <code>a<sub>i</sub></code> is a generic function: <br> Each pair <code>(a<sub>i</sub>, p<sub>i</sub>)</code> of corresponding function arguments and parameters where <code>a<sub>i</sub></code> is not an <a href="#Constants">untyped constant</a> yields an equation <code>typeof(p<sub>i</sub>) ≡<sub>A</sub> typeof(a<sub>i</sub>)</code>. <br> If <code>a<sub>i</sub></code> is an untyped constant <code>c<sub>j</sub></code>, and <code>typeof(p<sub>i</sub>)</code> is a bound type parameter <code>P<sub>k</sub></code>, the pair <code>(c<sub>j</sub>, P<sub>k</sub>)</code> is collected separately from the type equations. </p> </li> <li> <p> For an assignment <code>v = f</code> of a generic function <code>f</code> to a (non-generic) variable <code>v</code> of function type: <br> <code>typeof(v) ≡<sub>A</sub> typeof(f)</code>. </p> </li> <li> <p> For a return statement <code>return …, f, … </code> where <code>f</code> is a generic function returned as a result to a (non-generic) result variable <code>r</code> of function type: <br> <code>typeof(r) ≡<sub>A</sub> typeof(f)</code>. </p> </li> </ul> <p> Additionally, each type parameter <code>P<sub>k</sub></code> and corresponding type constraint <code>C<sub>k</sub></code> yields the type equation <code>P<sub>k</sub> ≡<sub>C</sub> C<sub>k</sub></code>. </p> <p> Type inference gives precedence to type information obtained from typed operands before considering untyped constants. Therefore, inference proceeds in two phases: </p> <ol> <li> <p> The type equations are solved for the bound type parameters using <a href="#Type_unification">type unification</a>. If unification fails, type inference fails. </p> </li> <li> <p> For each bound type parameter <code>P<sub>k</sub></code> for which no type argument has been inferred yet and for which one or more pairs <code>(c<sub>j</sub>, P<sub>k</sub>)</code> with that same type parameter were collected, determine the <a href="#Constant_expressions">constant kind</a> of the constants <code>c<sub>j</sub></code> in all those pairs the same way as for <a href="#Constant_expressions">constant expressions</a>. The type argument for <code>P<sub>k</sub></code> is the <a href="#Constants">default type</a> for the determined constant kind. If a constant kind cannot be determined due to conflicting constant kinds, type inference fails. </p> </li> </ol> <p> If not all type arguments have been found after these two phases, type inference fails. </p> <p> If the two phases are successful, type inference determined a type argument for each bound type parameter: </p> <pre> P<sub>k</sub> ➞ A<sub>k</sub> </pre> <p> A type argument <code>A<sub>k</sub></code> may be a composite type, containing other bound type parameters <code>P<sub>k</sub></code> as element types (or even be just another bound type parameter). In a process of repeated simplification, the bound type parameters in each type argument are substituted with the respective type arguments for those type parameters until each type argument is free of bound type parameters. </p> <p> If type arguments contain cyclic references to themselves through bound type parameters, simplification and thus type inference fails. Otherwise, type inference succeeds. </p> <h4 id="Type_unification">Type unification</h4> <p> Type inference solves type equations through <i>type unification</i>. Type unification recursively compares the LHS and RHS types of an equation, where either or both types may be or contain bound type parameters, and looks for type arguments for those type parameters such that the LHS and RHS match (become identical or assignment-compatible, depending on context). To that effect, type inference maintains a map of bound type parameters to inferred type arguments; this map is consulted and updated during type unification. Initially, the bound type parameters are known but the map is empty. During type unification, if a new type argument <code>A</code> is inferred, the respective mapping <code>P ➞ A</code> from type parameter to argument is added to the map. Conversely, when comparing types, a known type argument (a type argument for which a map entry already exists) takes the place of its corresponding type parameter. As type inference progresses, the map is populated more and more until all equations have been considered, or until unification fails. Type inference succeeds if no unification step fails and the map has an entry for each type parameter. </p> <p> For example, given the type equation with the bound type parameter <code>P</code> </p> <pre> [10]struct{ elem P, list []P } ≡<sub>A</sub> [10]struct{ elem string; list []string } </pre> <p> type inference starts with an empty map. Unification first compares the top-level structure of the LHS and RHS types. Both are arrays of the same length; they unify if the element types unify. Both element types are structs; they unify if they have the same number of fields with the same names and if the field types unify. The type argument for <code>P</code> is not known yet (there is no map entry), so unifying <code>P</code> with <code>string</code> adds the mapping <code>P ➞ string</code> to the map. Unifying the types of the <code>list</code> field requires unifying <code>[]P</code> and <code>[]string</code> and thus <code>P</code> and <code>string</code>. Since the type argument for <code>P</code> is known at this point (there is a map entry for <code>P</code>), its type argument <code>string</code> takes the place of <code>P</code>. And since <code>string</code> is identical to <code>string</code>, this unification step succeeds as well. Unification of the LHS and RHS of the equation is now finished. Type inference succeeds because there is only one type equation, no unification step failed, and the map is fully populated. </p> <p> Unification uses a combination of <i>exact</i> and <i>loose</i> unification depending on whether two types have to be <a href="#Type_identity">identical</a>, <a href="#Assignability">assignment-compatible</a>, or only structurally equal. The respective <a href="#Type_unification_rules">type unification rules</a> are spelled out in detail in the <a href="#Appendix">Appendix</a>. </p> <p> For an equation of the form <code>X ≡<sub>A</sub> Y</code>, where <code>X</code> and <code>Y</code> are types involved in an assignment (including parameter passing and return statements), the top-level type structures may unify loosely but element types must unify exactly, matching the rules for assignments. </p> <p> For an equation of the form <code>P ≡<sub>C</sub> C</code>, where <code>P</code> is a type parameter and <code>C</code> its corresponding constraint, the unification rules are bit more complicated: </p> <ul> <li> If <code>C</code> has a <a href="#Core_types">core type</a> <code>core(C)</code> and <code>P</code> has a known type argument <code>A</code>, <code>core(C)</code> and <code>A</code> must unify loosely. If <code>P</code> does not have a known type argument and <code>C</code> contains exactly one type term <code>T</code> that is not an underlying (tilde) type, unification adds the mapping <code>P ➞ T</code> to the map. </li> <li> If <code>C</code> does not have a core type and <code>P</code> has a known type argument <code>A</code>, <code>A</code> must have all methods of <code>C</code>, if any, and corresponding method types must unify exactly. </li> </ul> <p> When solving type equations from type constraints, solving one equation may infer additional type arguments, which in turn may enable solving other equations that depend on those type arguments. Type inference repeats type unification as long as new type arguments are inferred. </p> <h3 id="Operators">Operators</h3> <p> Operators combine operands into expressions. </p> <pre class="ebnf"> Expression = UnaryExpr \| Expression binary_op Expression . UnaryExpr = PrimaryExpr \| unary_op UnaryExpr . binary_op = "\|\|" \| "&&" \| rel_op \| add_op \| mul_op . rel_op = "==" \| "!=" \| "<" \| "<=" \| ">" \| ">=" . add_op = "+" \| "-" \| "\|" \| "^" . mul_op = "" \| "/" \| "%" \| "<<" \| ">>" \| "&" \| "&^" . unary_op = "+" \| "-" \| "!" \| "^" \| "" \| "&" \| "<-" . </pre> <p> Comparisons are discussed <a href="#Comparison_operators">elsewhere</a>. For other binary operators, the operand types must be <a href="#Type_identity">identical</a> unless the operation involves shifts or untyped <a href="#Constants">constants</a>. For operations involving constants only, see the section on <a href="#Constant_expressions">constant expressions</a>. </p> <p> Except for shift operations, if one operand is an untyped <a href="#Constants">constant</a> and the other operand is not, the constant is implicitly <a href="#Conversions">converted</a> to the type of the other operand. </p> <p> The right operand in a shift expression must have <a href="#Numeric_types">integer type</a> [<a href="#Go_1.13">Go 1.13</a>] or be an untyped constant <a href="#Representability">representable</a> by a value of type <code>uint</code>. If the left operand of a non-constant shift expression is an untyped constant, it is first implicitly converted to the type it would assume if the shift expression were replaced by its left operand alone. </p> <pre> var a [1024]byte var s uint = 33 // The results of the following examples are given for 64-bit ints. var i = 1<<s // 1 has type int var j int32 = 1<<s // 1 has type int32; j == 0 var k = uint64(1<<s) // 1 has type uint64; k == 1<<33 var m int = 1.0<<s // 1.0 has type int; m == 1<<33 var n = 1.0<<s == j // 1.0 has type int32; n == true var o = 1<<s == 2<<s // 1 and 2 have type int; o == false var p = 1<<s == 1<<33 // 1 has type int; p == true var u = 1.0<<s // illegal: 1.0 has type float64, cannot shift var u1 = 1.0<<s != 0 // illegal: 1.0 has type float64, cannot shift var u2 = 1<<s != 1.0 // illegal: 1 has type float64, cannot shift var v1 float32 = 1<<s // illegal: 1 has type float32, cannot shift var v2 = string(1<<s) // illegal: 1 is converted to a string, cannot shift var w int64 = 1.0<<33 // 1.0<<33 is a constant shift expression; w == 1<<33 var x = a[1.0<<s] // panics: 1.0 has type int, but 1<<33 overflows array bounds var b = make([]byte, 1.0<<s) // 1.0 has type int; len(b) == 1<<33 // The results of the following examples are given for 32-bit ints, // which means the shifts will overflow. var mm int = 1.0<<s // 1.0 has type int; mm == 0 var oo = 1<<s == 2<<s // 1 and 2 have type int; oo == true var pp = 1<<s == 1<<33 // illegal: 1 has type int, but 1<<33 overflows int var xx = a[1.0<<s] // 1.0 has type int; xx == a[0] var bb = make([]byte, 1.0<<s) // 1.0 has type int; len(bb) == 0 </pre> <h4 id="Operator_precedence">Operator precedence</h4> <p> Unary operators have the highest precedence. As the <code>++</code> and <code>--</code> operators form statements, not expressions, they fall outside the operator hierarchy. As a consequence, statement <code>p++</code> is the same as <code>(p)++</code>. </p> <p> There are five precedence levels for binary operators. Multiplication operators bind strongest, followed by addition operators, comparison operators, <code>&&</code> (logical AND), and finally <code>\|\|</code> (logical OR): </p> <pre class="grammar"> Precedence Operator 5 / % << >> & &^ 4 + - \| ^ 3 == != < <= > >= 2 && 1 \|\| </pre> <p> Binary operators of the same precedence associate from left to right. For instance, <code>x / y * z</code> is the same as <code>(x / y) * z</code>. </p> <pre> +x // x 42 + a - b // (42 + a) - b 23 + 3x[i] // 23 + (3 x[i]) x <= f() // x <= f() ^a >> b // (^a) >> b f() \|\| g() // f() \|\| g() x == y+1 && <-chanInt > 0 // (x == (y+1)) && ((<-chanInt) > 0) </pre> <h3 id="Arithmetic_operators">Arithmetic operators</h3> <p> Arithmetic operators apply to numeric values and yield a result of the same type as the first operand. The four standard arithmetic operators (<code>+</code>, <code>-</code>, <code></code>, <code>/</code>) apply to <a href="#Numeric_types">integer</a>, <a href="#Numeric_types">floating-point</a>, and <a href="#Numeric_types">complex</a> types; <code>+</code> also applies to <a href="#String_types">strings</a>. The bitwise logical and shift operators apply to integers only. </p> <pre class="grammar"> + sum integers, floats, complex values, strings - difference integers, floats, complex values product integers, floats, complex values / quotient integers, floats, complex values % remainder integers & bitwise AND integers \| bitwise OR integers ^ bitwise XOR integers &^ bit clear (AND NOT) integers << left shift integer << integer >= 0 >> right shift integer >> integer >= 0 </pre> <p> If the operand type is a <a href="#Type_parameter_declarations">type parameter</a>, the operator must apply to each type in that type set. The operands are represented as values of the type argument that the type parameter is <a href="#Instantiations">instantiated</a> with, and the operation is computed with the precision of that type argument. For example, given the function: </p> <pre> func dotProduct[F ~float32\|~float64](v1, v2 []F) F { var s F for i, x := range v1 { y := v2[i] s += x * y } return s } </pre> <p> the product <code>x * y</code> and the addition <code>s += x * y</code> are computed with <code>float32</code> or <code>float64</code> precision, respectively, depending on the type argument for <code>F</code>. </p> <h4 id="Integer_operators">Integer operators</h4> <p> For two integer values <code>x</code> and <code>y</code>, the integer quotient <code>q = x / y</code> and remainder <code>r = x % y</code> satisfy the following relationships: </p> <pre> x = qy + r and \|r\| < \|y\| </pre> <p> with <code>x / y</code> truncated towards zero (<a href="https://en.wikipedia.org/wiki/Modulo_operation">"truncated division"</a>). </p> <pre> x y x / y x % y 5 3 1 2 -5 3 -1 -2 5 -3 -1 2 -5 -3 1 -2 </pre> <p> The one exception to this rule is that if the dividend <code>x</code> is the most negative value for the int type of <code>x</code>, the quotient <code>q = x / -1</code> is equal to <code>x</code> (and <code>r = 0</code>) due to two's-complement <a href="#Integer_overflow">integer overflow</a>: </p> <pre> x, q int8 -128 int16 -32768 int32 -2147483648 int64 -9223372036854775808 </pre> <p> If the divisor is a <a href="#Constants">constant</a>, it must not be zero. If the divisor is zero at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. If the dividend is non-negative and the divisor is a constant power of 2, the division may be replaced by a right shift, and computing the remainder may be replaced by a bitwise AND operation: </p> <pre> x x / 4 x % 4 x >> 2 x & 3 11 2 3 2 3 -11 -2 -3 -3 1 </pre> <p> The shift operators shift the left operand by the shift count specified by the right operand, which must be non-negative. If the shift count is negative at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. The shift operators implement arithmetic shifts if the left operand is a signed integer and logical shifts if it is an unsigned integer. There is no upper limit on the shift count. Shifts behave as if the left operand is shifted <code>n</code> times by 1 for a shift count of <code>n</code>. As a result, <code>x << 1</code> is the same as <code>x2</code> and <code>x >> 1</code> is the same as <code>x/2</code> but truncated towards negative infinity. </p> <p> For integer operands, the unary operators <code>+</code>, <code>-</code>, and <code>^</code> are defined as follows: </p> <pre class="grammar"> +x is 0 + x -x negation is 0 - x ^x bitwise complement is m ^ x with m = "all bits set to 1" for unsigned x and m = -1 for signed x </pre> <h4 id="Integer_overflow">Integer overflow</h4> <p> For <a href="#Numeric_types">unsigned integer</a> values, the operations <code>+</code>, <code>-</code>, <code></code>, and <code><<</code> are computed modulo 2<sup><i>n</i></sup>, where <i>n</i> is the bit width of the unsigned integer's type. Loosely speaking, these unsigned integer operations discard high bits upon overflow, and programs may rely on "wrap around". </p> <p> For signed integers, the operations <code>+</code>, <code>-</code>, <code></code>, <code>/</code>, and <code><<</code> may legally overflow and the resulting value exists and is deterministically defined by the signed integer representation, the operation, and its operands. Overflow does not cause a <a href="#Run_time_panics">run-time panic</a>. A compiler may not optimize code under the assumption that overflow does not occur. For instance, it may not assume that <code>x < x + 1</code> is always true. </p> <h4 id="Floating_point_operators">Floating-point operators</h4> <p> For floating-point and complex numbers, <code>+x</code> is the same as <code>x</code>, while <code>-x</code> is the negation of <code>x</code>. The result of a floating-point or complex division by zero is not specified beyond the IEEE-754 standard; whether a <a href="#Run_time_panics">run-time panic</a> occurs is implementation-specific. </p> <p> An implementation may combine multiple floating-point operations into a single fused operation, possibly across statements, and produce a result that differs from the value obtained by executing and rounding the instructions individually. An explicit <a href="#Numeric_types">floating-point type</a> <a href="#Conversions">conversion</a> rounds to the precision of the target type, preventing fusion that would discard that rounding. </p> <p> For instance, some architectures provide a "fused multiply and add" (FMA) instruction that computes <code>xy + z</code> without rounding the intermediate result <code>xy</code>. These examples show when a Go implementation can use that instruction: </p> <pre> // FMA allowed for computing r, because xy is not explicitly rounded: r = xy + z r = z; r += xy t = xy; r = t + z p = xy; r = p + z r = xy + float64(z) // FMA disallowed for computing r, because it would omit rounding of xy: r = float64(xy) + z r = z; r += float64(xy) t = float64(xy); r = t + z </pre> <h4 id="String_concatenation">String concatenation</h4> <p> Strings can be concatenated using the <code>+</code> operator or the <code>+=</code> assignment operator: </p> <pre> s := "hi" + string(c) s += " and good bye" </pre> <p> String addition creates a new string by concatenating the operands. </p> <h3 id="Comparison_operators">Comparison operators</h3> <p> Comparison operators compare two operands and yield an untyped boolean value. </p> <pre class="grammar"> == equal != not equal < less <= less or equal > greater >= greater or equal </pre> <p> In any comparison, the first operand must be <a href="#Assignability">assignable</a> to the type of the second operand, or vice versa. </p> <p> The equality operators <code>==</code> and <code>!=</code> apply to operands of <i>comparable</i> types. The ordering operators <code><</code>, <code><=</code>, <code>></code>, and <code>>=</code> apply to operands of <i>ordered</i> types. These terms and the result of the comparisons are defined as follows: </p> <ul> <li> Boolean types are comparable. Two boolean values are equal if they are either both <code>true</code> or both <code>false</code>. </li> <li> Integer types are comparable and ordered. Two integer values are compared in the usual way. </li> <li> Floating-point types are comparable and ordered. Two floating-point values are compared as defined by the IEEE-754 standard. </li> <li> Complex types are comparable. Two complex values <code>u</code> and <code>v</code> are equal if both <code>real(u) == real(v)</code> and <code>imag(u) == imag(v)</code>. </li> <li> String types are comparable and ordered. Two string values are compared lexically byte-wise. </li> <li> Pointer types are comparable. Two pointer values are equal if they point to the same variable or if both have value <code>nil</code>. Pointers to distinct <a href="#Size_and_alignment_guarantees">zero-size</a> variables may or may not be equal. </li> <li> Channel types are comparable. Two channel values are equal if they were created by the same call to <a href="#Making_slices_maps_and_channels"><code>make</code></a> or if both have value <code>nil</code>. </li> <li> Interface types that are not type parameters are comparable. Two interface values are equal if they have <a href="#Type_identity">identical</a> dynamic types and equal dynamic values or if both have value <code>nil</code>. </li> <li> A value <code>x</code> of non-interface type <code>X</code> and a value <code>t</code> of interface type <code>T</code> can be compared if type <code>X</code> is comparable and <code>X</code> <a href="#Implementing_an_interface">implements</a> <code>T</code>. They are equal if <code>t</code>'s dynamic type is identical to <code>X</code> and <code>t</code>'s dynamic value is equal to <code>x</code>. </li> <li> Struct types are comparable if all their field types are comparable. Two struct values are equal if their corresponding non-<a href="#Blank_identifier">blank</a> field values are equal. The fields are compared in source order, and comparison stops as soon as two field values differ (or all fields have been compared). </li> <li> Array types are comparable if their array element types are comparable. Two array values are equal if their corresponding element values are equal. The elements are compared in ascending index order, and comparison stops as soon as two element values differ (or all elements have been compared). </li> <li> Type parameters are comparable if they are strictly comparable (see below). </li> </ul> <p> A comparison of two interface values with identical dynamic types causes a <a href="#Run_time_panics">run-time panic</a> if that type is not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields. </p> <p> Slice, map, and function types are not comparable. However, as a special case, a slice, map, or function value may be compared to the predeclared identifier <code>nil</code>. Comparison of pointer, channel, and interface values to <code>nil</code> is also allowed and follows from the general rules above. </p> <pre> const c = 3 < 4 // c is the untyped boolean constant true type MyBool bool var x, y int var ( // The result of a comparison is an untyped boolean. // The usual assignment rules apply. b3 = x == y // b3 has type bool b4 bool = x == y // b4 has type bool b5 MyBool = x == y // b5 has type MyBool ) </pre> <p> A type is <i>strictly comparable</i> if it is comparable and not an interface type nor composed of interface types. Specifically: </p> <ul> <li> Boolean, numeric, string, pointer, and channel types are strictly comparable. </li> <li> Struct types are strictly comparable if all their field types are strictly comparable. </li> <li> Array types are strictly comparable if their array element types are strictly comparable. </li> <li> Type parameters are strictly comparable if all types in their type set are strictly comparable. </li> </ul> <h3 id="Logical_operators">Logical operators</h3> <p> Logical operators apply to <a href="#Boolean_types">boolean</a> values and yield a result of the same type as the operands. The left operand is evaluated, and then the right if the condition requires it. </p> <pre class="grammar"> && conditional AND p && q is "if p then q else false" \|\| conditional OR p \|\| q is "if p then true else q" ! NOT !p is "not p" </pre> <h3 id="Address_operators">Address operators</h3> <p> For an operand <code>x</code> of type <code>T</code>, the address operation <code>&x</code> generates a pointer of type <code>T</code> to <code>x</code>. The operand must be <i>addressable</i>, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, <code>x</code> may also be a (possibly parenthesized) <a href="#Composite_literals">composite literal</a>. If the evaluation of <code>x</code> would cause a <a href="#Run_time_panics">run-time panic</a>, then the evaluation of <code>&x</code> does too. </p> <p> For an operand <code>x</code> of pointer type <code>T</code>, the pointer indirection <code>x</code> denotes the <a href="#Variables">variable</a> of type <code>T</code> pointed to by <code>x</code>. If <code>x</code> is <code>nil</code>, an attempt to evaluate <code>x</code> will cause a <a href="#Run_time_panics">run-time panic</a>. </p> <pre> &x &a[f(2)] &Point{2, 3} p pf(x) var x int = nil x // causes a run-time panic &x // causes a run-time panic </pre> <h3 id="Receive_operator">Receive operator</h3> <p> For an operand <code>ch</code> whose <a href="#Core_types">core type</a> is a <a href="#Channel_types">channel</a>, the value of the receive operation <code><-ch</code> is the value received from the channel <code>ch</code>. The channel direction must permit receive operations, and the type of the receive operation is the element type of the channel. The expression blocks until a value is available. Receiving from a <code>nil</code> channel blocks forever. A receive operation on a <a href="#Close">closed</a> channel can always proceed immediately, yielding the element type's <a href="#The_zero_value">zero value</a> after any previously sent values have been received. </p> <pre> v1 := <-ch v2 = <-ch f(<-ch) <-strobe // wait until clock pulse and discard received value </pre> <p> A receive expression used in an <a href="#Assignment_statements">assignment statement</a> or initialization of the special form </p> <pre> x, ok = <-ch x, ok := <-ch var x, ok = <-ch var x, ok T = <-ch </pre> <p> yields an additional untyped boolean result reporting whether the communication succeeded. The value of <code>ok</code> is <code>true</code> if the value received was delivered by a successful send operation to the channel, or <code>false</code> if it is a zero value generated because the channel is closed and empty. </p> <h3 id="Conversions">Conversions</h3> <p> A conversion changes the <a href="#Types">type</a> of an expression to the type specified by the conversion. A conversion may appear literally in the source, or it may be <i>implied</i> by the context in which an expression appears. </p> <p> An <i>explicit</i> conversion is an expression of the form <code>T(x)</code> where <code>T</code> is a type and <code>x</code> is an expression that can be converted to type <code>T</code>. </p> <pre class="ebnf"> Conversion = Type "(" Expression [ "," ] ")" . </pre> <p> If the type starts with the operator <code></code> or <code><-</code>, or if the type starts with the keyword <code>func</code> and has no result list, it must be parenthesized when necessary to avoid ambiguity: </p> <pre> Point(p) // same as (Point(p)) (Point)(p) // p is converted to Point <-chan int(c) // same as <-(chan int(c)) (<-chan int)(c) // c is converted to <-chan int func()(x) // function signature func() x (func())(x) // x is converted to func() (func() int)(x) // x is converted to func() int func() int(x) // x is converted to func() int (unambiguous) </pre> <p> A <a href="#Constants">constant</a> value <code>x</code> can be converted to type <code>T</code> if <code>x</code> is <a href="#Representability">representable</a> by a value of <code>T</code>. As a special case, an integer constant <code>x</code> can be explicitly converted to a <a href="#String_types">string type</a> using the <a href="#Conversions_to_and_from_a_string_type">same rule</a> as for non-constant <code>x</code>. </p> <p> Converting a constant to a type that is not a <a href="#Type_parameter_declarations">type parameter</a> yields a typed constant. </p> <pre> uint(iota) // iota value of type uint float32(2.718281828) // 2.718281828 of type float32 complex128(1) // 1.0 + 0.0i of type complex128 float32(0.49999999) // 0.5 of type float32 float64(-1e-1000) // 0.0 of type float64 string('x') // "x" of type string string(0x266c) // "♬" of type string myString("foo" + "bar") // "foobar" of type myString string([]byte{'a'}) // not a constant: []byte{'a'} is not a constant (int)(nil) // not a constant: nil is not a constant, int is not a boolean, numeric, or string type int(1.2) // illegal: 1.2 cannot be represented as an int string(65.0) // illegal: 65.0 is not an integer constant </pre> <p> Converting a constant to a type parameter yields a <i>non-constant</i> value of that type, with the value represented as a value of the type argument that the type parameter is <a href="#Instantiations">instantiated</a> with. For example, given the function: </p> <pre> func f[P ~float32\|~float64]() { … P(1.1) … } </pre> <p> the conversion <code>P(1.1)</code> results in a non-constant value of type <code>P</code> and the value <code>1.1</code> is represented as a <code>float32</code> or a <code>float64</code> depending on the type argument for <code>f</code>. Accordingly, if <code>f</code> is instantiated with a <code>float32</code> type, the numeric value of the expression <code>P(1.1) + 1.2</code> will be computed with the same precision as the corresponding non-constant <code>float32</code> addition. </p> <p> A non-constant value <code>x</code> can be converted to type <code>T</code> in any of these cases: </p> <ul> <li> <code>x</code> is <a href="#Assignability">assignable</a> to <code>T</code>. </li> <li> ignoring struct tags (see below), <code>x</code>'s type and <code>T</code> are not <a href="#Type_parameter_declarations">type parameters</a> but have <a href="#Type_identity">identical</a> <a href="#Underlying_types">underlying types</a>. </li> <li> ignoring struct tags (see below), <code>x</code>'s type and <code>T</code> are pointer types that are not <a href="#Types">named types</a>, and their pointer base types are not type parameters but have identical underlying types. </li> <li> <code>x</code>'s type and <code>T</code> are both integer or floating point types. </li> <li> <code>x</code>'s type and <code>T</code> are both complex types. </li> <li> <code>x</code> is an integer or a slice of bytes or runes and <code>T</code> is a string type. </li> <li> <code>x</code> is a string and <code>T</code> is a slice of bytes or runes. </li> <li> <code>x</code> is a slice, <code>T</code> is an array [<a href="#Go_1.20">Go 1.20</a>] or a pointer to an array [<a href="#Go_1.17">Go 1.17</a>], and the slice and array types have <a href="#Type_identity">identical</a> element types. </li> </ul> <p> Additionally, if <code>T</code> or <code>x</code>'s type <code>V</code> are type parameters, <code>x</code> can also be converted to type <code>T</code> if one of the following conditions applies: </p> <ul> <li> Both <code>V</code> and <code>T</code> are type parameters and a value of each type in <code>V</code>'s type set can be converted to each type in <code>T</code>'s type set. </li> <li> Only <code>V</code> is a type parameter and a value of each type in <code>V</code>'s type set can be converted to <code>T</code>. </li> <li> Only <code>T</code> is a type parameter and <code>x</code> can be converted to each type in <code>T</code>'s type set. </li> </ul> <p> <a href="#Struct_types">Struct tags</a> are ignored when comparing struct types for identity for the purpose of conversion: </p> <pre> type Person struct { Name string Address struct { Street string City string } } var data struct { Name string `json:"name"` Address struct { Street string `json:"street"` City string `json:"city"` } `json:"address"` } var person = (Person)(data) // ignoring tags, the underlying types are identical </pre> <p> Specific rules apply to (non-constant) conversions between numeric types or to and from a string type. These conversions may change the representation of <code>x</code> and incur a run-time cost. All other conversions only change the type but not the representation of <code>x</code>. </p> <p> There is no linguistic mechanism to convert between pointers and integers. The package <a href="#Package_unsafe"><code>unsafe</code></a> implements this functionality under restricted circumstances. </p> <h4>Conversions between numeric types</h4> <p> For the conversion of non-constant numeric values, the following rules apply: </p> <ol> <li> When converting between <a href="#Numeric_types">integer types</a>, if the value is a signed integer, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type's size. For example, if <code>v := uint16(0x10F0)</code>, then <code>uint32(int8(v)) == 0xFFFFFFF0</code>. The conversion always yields a valid value; there is no indication of overflow. </li> <li> When converting a <a href="#Numeric_types">floating-point number</a> to an integer, the fraction is discarded (truncation towards zero). </li> <li> When converting an integer or floating-point number to a floating-point type, or a <a href="#Numeric_types">complex number</a> to another complex type, the result value is rounded to the precision specified by the destination type. For instance, the value of a variable <code>x</code> of type <code>float32</code> may be stored using additional precision beyond that of an IEEE-754 32-bit number, but float32(x) represents the result of rounding <code>x</code>'s value to 32-bit precision. Similarly, <code>x + 0.1</code> may use more than 32 bits of precision, but <code>float32(x + 0.1)</code> does not. </li> </ol> <p> In all non-constant conversions involving floating-point or complex values, if the result type cannot represent the value the conversion succeeds but the result value is implementation-dependent. </p> <h4 id="Conversions_to_and_from_a_string_type">Conversions to and from a string type</h4> <ol> <li> Converting a slice of bytes to a string type yields a string whose successive bytes are the elements of the slice. <pre> string([]byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø" string([]byte{}) // "" string([]byte(nil)) // "" type bytes []byte string(bytes{'h', 'e', 'l', 'l', '\xc3', '\xb8'}) // "hellø" type myByte byte string([]myByte{'w', 'o', 'r', 'l', 'd', '!'}) // "world!" myString([]myByte{'\xf0', '\x9f', '\x8c', '\x8d'}) // "🌍" </pre> </li> <li> Converting a slice of runes to a string type yields a string that is the concatenation of the individual rune values converted to strings. <pre> string([]rune{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔" string([]rune{}) // "" string([]rune(nil)) // "" type runes []rune string(runes{0x767d, 0x9d6c, 0x7fd4}) // "\u767d\u9d6c\u7fd4" == "白鵬翔" type myRune rune string([]myRune{0x266b, 0x266c}) // "\u266b\u266c" == "♫♬" myString([]myRune{0x1f30e}) // "\U0001f30e" == "🌎" </pre> </li> <li> Converting a value of a string type to a slice of bytes type yields a non-nil slice whose successive elements are the bytes of the string. <pre> []byte("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'} []byte("") // []byte{} bytes("hellø") // []byte{'h', 'e', 'l', 'l', '\xc3', '\xb8'} []myByte("world!") // []myByte{'w', 'o', 'r', 'l', 'd', '!'} []myByte(myString("🌏")) // []myByte{'\xf0', '\x9f', '\x8c', '\x8f'} </pre> </li> <li> Converting a value of a string type to a slice of runes type yields a slice containing the individual Unicode code points of the string. <pre> []rune(myString("白鵬翔")) // []rune{0x767d, 0x9d6c, 0x7fd4} []rune("") // []rune{} runes("白鵬翔") // []rune{0x767d, 0x9d6c, 0x7fd4} []myRune("♫♬") // []myRune{0x266b, 0x266c} []myRune(myString("🌐")) // []myRune{0x1f310} </pre> </li> <li> Finally, for historical reasons, an integer value may be converted to a string type. This form of conversion yields a string containing the (possibly multi-byte) UTF-8 representation of the Unicode code point with the given integer value. Values outside the range of valid Unicode code points are converted to <code>"\uFFFD"</code>. <pre> string('a') // "a" string(65) // "A" string('\xf8') // "\u00f8" == "ø" == "\xc3\xb8" string(-1) // "\ufffd" == "\xef\xbf\xbd" type myString string myString('\u65e5') // "\u65e5" == "日" == "\xe6\x97\xa5" </pre> Note: This form of conversion may eventually be removed from the language. The <a href="/pkg/cmd/vet"><code>go vet</code></a> tool flags certain integer-to-string conversions as potential errors. Library functions such as <a href="/pkg/unicode/utf8#AppendRune"><code>utf8.AppendRune</code></a> or <a href="/pkg/unicode/utf8#EncodeRune"><code>utf8.EncodeRune</code></a> should be used instead. </li> </ol> <h4 id="Conversions_from_slice_to_array_or_array_pointer">Conversions from slice to array or array pointer</h4> <p> Converting a slice to an array yields an array containing the elements of the underlying array of the slice. Similarly, converting a slice to an array pointer yields a pointer to the underlying array of the slice. In both cases, if the <a href="#Length_and_capacity">length</a> of the slice is less than the length of the array, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <pre> s := make([]byte, 2, 4) a0 := [0]byte(s) a1 := [1]byte(s[1:]) // a1[0] == s[1] a2 := [2]byte(s) // a2[0] == s[0] a4 := [4]byte(s) // panics: len([4]byte) > len(s) s0 := ([0]byte)(s) // s0 != nil s1 := ([1]byte)(s[1:]) // &s1[0] == &s[1] s2 := ([2]byte)(s) // &s2[0] == &s[0] s4 := ([4]byte)(s) // panics: len([4]byte) > len(s) var t []string t0 := [0]string(t) // ok for nil slice t t1 := ([0]string)(t) // t1 == nil t2 := ([1]string)(t) // panics: len([1]string) > len(t) u := make([]byte, 0) u0 := ([0]byte)(u) // u0 != nil </pre> <h3 id="Constant_expressions">Constant expressions</h3> <p> Constant expressions may contain only <a href="#Constants">constant</a> operands and are evaluated at compile time. </p> <p> Untyped boolean, numeric, and string constants may be used as operands wherever it is legal to use an operand of boolean, numeric, or string type, respectively. </p> <p> A constant <a href="#Comparison_operators">comparison</a> always yields an untyped boolean constant. If the left operand of a constant <a href="#Operators">shift expression</a> is an untyped constant, the result is an integer constant; otherwise it is a constant of the same type as the left operand, which must be of <a href="#Numeric_types">integer type</a>. </p> <p> Any other operation on untyped constants results in an untyped constant of the same kind; that is, a boolean, integer, floating-point, complex, or string constant. If the untyped operands of a binary operation (other than a shift) are of different kinds, the result is of the operand's kind that appears later in this list: integer, rune, floating-point, complex. For example, an untyped integer constant divided by an untyped complex constant yields an untyped complex constant. </p> <pre> const a = 2 + 3.0 // a == 5.0 (untyped floating-point constant) const b = 15 / 4 // b == 3 (untyped integer constant) const c = 15 / 4.0 // c == 3.75 (untyped floating-point constant) const Θ float64 = 3/2 // Θ == 1.0 (type float64, 3/2 is integer division) const Π float64 = 3/2. // Π == 1.5 (type float64, 3/2. is float division) const d = 1 << 3.0 // d == 8 (untyped integer constant) const e = 1.0 << 3 // e == 8 (untyped integer constant) const f = int32(1) << 33 // illegal (constant 8589934592 overflows int32) const g = float64(2) >> 1 // illegal (float64(2) is a typed floating-point constant) const h = "foo" > "bar" // h == true (untyped boolean constant) const j = true // j == true (untyped boolean constant) const k = 'w' + 1 // k == 'x' (untyped rune constant) const l = "hi" // l == "hi" (untyped string constant) const m = string(k) // m == "x" (type string) const Σ = 1 - 0.707i // (untyped complex constant) const Δ = Σ + 2.0e-4 // (untyped complex constant) const Φ = iota1i - 1/1i // (untyped complex constant) </pre> <p> Applying the built-in function <code>complex</code> to untyped integer, rune, or floating-point constants yields an untyped complex constant. </p> <pre> const ic = complex(0, c) // ic == 3.75i (untyped complex constant) const iΘ = complex(0, Θ) // iΘ == 1i (type complex128) </pre> <p> Constant expressions are always evaluated exactly; intermediate values and the constants themselves may require precision significantly larger than supported by any predeclared type in the language. The following are legal declarations: </p> <pre> const Huge = 1 << 100 // Huge == 1267650600228229401496703205376 (untyped integer constant) const Four int8 = Huge >> 98 // Four == 4 (type int8) </pre> <p> The divisor of a constant division or remainder operation must not be zero: </p> <pre> 3.14 / 0.0 // illegal: division by zero </pre> <p> The values of <i>typed</i> constants must always be accurately <a href="#Representability">representable</a> by values of the constant type. The following constant expressions are illegal: </p> <pre> uint(-1) // -1 cannot be represented as a uint int(3.14) // 3.14 cannot be represented as an int int64(Huge) // 1267650600228229401496703205376 cannot be represented as an int64 Four * 300 // operand 300 cannot be represented as an int8 (type of Four) Four * 100 // product 400 cannot be represented as an int8 (type of Four) </pre> <p> The mask used by the unary bitwise complement operator <code>^</code> matches the rule for non-constants: the mask is all 1s for unsigned constants and -1 for signed and untyped constants. </p> <pre> ^1 // untyped integer constant, equal to -2 uint8(^1) // illegal: same as uint8(-2), -2 cannot be represented as a uint8 ^uint8(1) // typed uint8 constant, same as 0xFF ^ uint8(1) = uint8(0xFE) int8(^1) // same as int8(-2) ^int8(1) // same as -1 ^ int8(1) = -2 </pre> <p> Implementation restriction: A compiler may use rounding while computing untyped floating-point or complex constant expressions; see the implementation restriction in the section on <a href="#Constants">constants</a>. This rounding may cause a floating-point constant expression to be invalid in an integer context, even if it would be integral when calculated using infinite precision, and vice versa. </p> <h3 id="Order_of_evaluation">Order of evaluation</h3> <p> At package level, <a href="#Package_initialization">initialization dependencies</a> determine the evaluation order of individual initialization expressions in <a href="#Variable_declarations">variable declarations</a>. Otherwise, when evaluating the <a href="#Operands">operands</a> of an expression, assignment, or <a href="#Return_statements">return statement</a>, all function calls, method calls, <a href="#Receive operator">receive operations</a>, and <a href="#Logical_operators">binary logical operations</a> are evaluated in lexical left-to-right order. </p> <p> For example, in the (function-local) assignment </p> <pre> y[f()], ok = g(z \|\| h(), i()+x[j()], <-c), k() </pre> <p> the function calls and communication happen in the order <code>f()</code>, <code>h()</code> (if <code>z</code> evaluates to false), <code>i()</code>, <code>j()</code>, <code><-c</code>, <code>g()</code>, and <code>k()</code>. However, the order of those events compared to the evaluation and indexing of <code>x</code> and the evaluation of <code>y</code> and <code>z</code> is not specified, except as required lexically. For instance, <code>g</code> cannot be called before its arguments are evaluated. </p> <pre> a := 1 f := func() int { a++; return a } x := []int{a, f()} // x may be [1, 2] or [2, 2]: evaluation order between a and f() is not specified m := map[int]int{a: 1, a: 2} // m may be {2: 1} or {2: 2}: evaluation order between the two map assignments is not specified n := map[int]int{a: f()} // n may be {2: 3} or {3: 3}: evaluation order between the key and the value is not specified </pre> <p> At package level, initialization dependencies override the left-to-right rule for individual initialization expressions, but not for operands within each expression: </p> <pre> var a, b, c = f() + v(), g(), sqr(u()) + v() func f() int { return c } func g() int { return a } func sqr(x int) int { return xx } // functions u and v are independent of all other variables and functions </pre> <p> The function calls happen in the order <code>u()</code>, <code>sqr()</code>, <code>v()</code>, <code>f()</code>, <code>v()</code>, and <code>g()</code>. </p> <p> Floating-point operations within a single expression are evaluated according to the associativity of the operators. Explicit parentheses affect the evaluation by overriding the default associativity. In the expression <code>x + (y + z)</code> the addition <code>y + z</code> is performed before adding <code>x</code>. </p> <h2 id="Statements">Statements</h2> <p> Statements control execution. </p> <pre class="ebnf"> Statement = Declaration \| LabeledStmt \| SimpleStmt \| GoStmt \| ReturnStmt \| BreakStmt \| ContinueStmt \| GotoStmt \| FallthroughStmt \| Block \| IfStmt \| SwitchStmt \| SelectStmt \| ForStmt \| DeferStmt . SimpleStmt = EmptyStmt \| ExpressionStmt \| SendStmt \| IncDecStmt \| Assignment \| ShortVarDecl . </pre> <h3 id="Terminating_statements">Terminating statements</h3> <p> A <i>terminating statement</i> interrupts the regular flow of control in a <a href="#Blocks">block</a>. The following statements are terminating: </p> <ol> <li> A <a href="#Return_statements">"return"</a> or <a href="#Goto_statements">"goto"</a> statement. <!-- ul below only for regular layout --> <ul> </ul> </li> <li> A call to the built-in function <a href="#Handling_panics"><code>panic</code></a>. <!-- ul below only for regular layout --> <ul> </ul> </li> <li> A <a href="#Blocks">block</a> in which the statement list ends in a terminating statement. <!-- ul below only for regular layout --> <ul> </ul> </li> <li> An <a href="#If_statements">"if" statement</a> in which: <ul> <li>the "else" branch is present, and</li> <li>both branches are terminating statements.</li> </ul> </li> <li> A <a href="#For_statements">"for" statement</a> in which: <ul> <li>there are no "break" statements referring to the "for" statement, and</li> <li>the loop condition is absent, and</li> <li>the "for" statement does not use a range clause.</li> </ul> </li> <li> A <a href="#Switch_statements">"switch" statement</a> in which: <ul> <li>there are no "break" statements referring to the "switch" statement,</li> <li>there is a default case, and</li> <li>the statement lists in each case, including the default, end in a terminating statement, or a possibly labeled <a href="#Fallthrough_statements">"fallthrough" statement</a>.</li> </ul> </li> <li> A <a href="#Select_statements">"select" statement</a> in which: <ul> <li>there are no "break" statements referring to the "select" statement, and</li> <li>the statement lists in each case, including the default if present, end in a terminating statement.</li> </ul> </li> <li> A <a href="#Labeled_statements">labeled statement</a> labeling a terminating statement. </li> </ol> <p> All other statements are not terminating. </p> <p> A <a href="#Blocks">statement list</a> ends in a terminating statement if the list is not empty and its final non-empty statement is terminating. </p> <h3 id="Empty_statements">Empty statements</h3> <p> The empty statement does nothing. </p> <pre class="ebnf"> EmptyStmt = . </pre> <h3 id="Labeled_statements">Labeled statements</h3> <p> A labeled statement may be the target of a <code>goto</code>, <code>break</code> or <code>continue</code> statement. </p> <pre class="ebnf"> LabeledStmt = Label ":" Statement . Label = identifier . </pre> <pre> Error: log.Panic("error encountered") </pre> <h3 id="Expression_statements">Expression statements</h3> <p> With the exception of specific built-in functions, function and method <a href="#Calls">calls</a> and <a href="#Receive_operator">receive operations</a> can appear in statement context. Such statements may be parenthesized. </p> <pre class="ebnf"> ExpressionStmt = Expression . </pre> <p> The following built-in functions are not permitted in statement context: </p> <pre> append cap complex imag len make new real unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice unsafe.SliceData unsafe.String unsafe.StringData </pre> <pre> h(x+y) f.Close() <-ch (<-ch) len("foo") // illegal if len is the built-in function </pre> <h3 id="Send_statements">Send statements</h3> <p> A send statement sends a value on a channel. The channel expression's <a href="#Core_types">core type</a> must be a <a href="#Channel_types">channel</a>, the channel direction must permit send operations, and the type of the value to be sent must be <a href="#Assignability">assignable</a> to the channel's element type. </p> <pre class="ebnf"> SendStmt = Channel "<-" Expression . Channel = Expression . </pre> <p> Both the channel and the value expression are evaluated before communication begins. Communication blocks until the send can proceed. A send on an unbuffered channel can proceed if a receiver is ready. A send on a buffered channel can proceed if there is room in the buffer. A send on a closed channel proceeds by causing a <a href="#Run_time_panics">run-time panic</a>. A send on a <code>nil</code> channel blocks forever. </p> <pre> ch <- 3 // send value 3 to channel ch </pre> <h3 id="IncDec_statements">IncDec statements</h3> <p> The "++" and "--" statements increment or decrement their operands by the untyped <a href="#Constants">constant</a> <code>1</code>. As with an assignment, the operand must be <a href="#Address_operators">addressable</a> or a map index expression. </p> <pre class="ebnf"> IncDecStmt = Expression ( "++" \| "--" ) . </pre> <p> The following <a href="#Assignment_statements">assignment statements</a> are semantically equivalent: </p> <pre class="grammar"> IncDec statement Assignment x++ x += 1 x-- x -= 1 </pre> <h3 id="Assignment_statements">Assignment statements</h3> <p> An <i>assignment</i> replaces the current value stored in a <a href="#Variables">variable</a> with a new value specified by an <a href="#Expressions">expression</a>. An assignment statement may assign a single value to a single variable, or multiple values to a matching number of variables. </p> <pre class="ebnf"> Assignment = ExpressionList assign_op ExpressionList . assign_op = [ add_op \| mul_op ] "=" . </pre> <p> Each left-hand side operand must be <a href="#Address_operators">addressable</a>, a map index expression, or (for <code>=</code> assignments only) the <a href="#Blank_identifier">blank identifier</a>. Operands may be parenthesized. </p> <pre> x = 1 p = f() a[i] = 23 (k) = <-ch // same as: k = <-ch </pre> <p> An <i>assignment operation</i> <code>x</code> <i>op</i><code>=</code> <code>y</code> where <i>op</i> is a binary <a href="#Arithmetic_operators">arithmetic operator</a> is equivalent to <code>x</code> <code>=</code> <code>x</code> <i>op</i> <code>(y)</code> but evaluates <code>x</code> only once. The <i>op</i><code>=</code> construct is a single token. In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression, and the left-hand expression must not be the blank identifier. </p> <pre> a[i] <<= 2 i &^= 1<<n </pre> <p> A tuple assignment assigns the individual elements of a multi-valued operation to a list of variables. There are two forms. In the first, the right hand operand is a single multi-valued expression such as a function call, a <a href="#Channel_types">channel</a> or <a href="#Map_types">map</a> operation, or a <a href="#Type_assertions">type assertion</a>. The number of operands on the left hand side must match the number of values. For instance, if <code>f</code> is a function returning two values, </p> <pre> x, y = f() </pre> <p> assigns the first value to <code>x</code> and the second to <code>y</code>. In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the <i>n</i>th expression on the right is assigned to the <i>n</i>th operand on the left: </p> <pre> one, two, three = '一', '二', '三' </pre> <p> The <a href="#Blank_identifier">blank identifier</a> provides a way to ignore right-hand side values in an assignment: </p> <pre> _ = x // evaluate x but ignore it x, _ = f() // evaluate f() but ignore second result value </pre> <p> The assignment proceeds in two phases. First, the operands of <a href="#Index_expressions">index expressions</a> and <a href="#Address_operators">pointer indirections</a> (including implicit pointer indirections in <a href="#Selectors">selectors</a>) on the left and the expressions on the right are all <a href="#Order_of_evaluation">evaluated in the usual order</a>. Second, the assignments are carried out in left-to-right order. </p> <pre> a, b = b, a // exchange a and b x := []int{1, 2, 3} i := 0 i, x[i] = 1, 2 // set i = 1, x[0] = 2 i = 0 x[i], i = 2, 1 // set x[0] = 2, i = 1 x[0], x[0] = 1, 2 // set x[0] = 1, then x[0] = 2 (so x[0] == 2 at end) x[1], x[3] = 4, 5 // set x[1] = 4, then panic setting x[3] = 5. type Point struct { x, y int } var p Point x[2], p.x = 6, 7 // set x[2] = 6, then panic setting p.x = 7 i = 2 x = []int{3, 5, 7} for i, x[i] = range x { // set i, x[2] = 0, x[0] break } // after this loop, i == 0 and x is []int{3, 5, 3} </pre> <p> In assignments, each value must be <a href="#Assignability">assignable</a> to the type of the operand to which it is assigned, with the following special cases: </p> <ol> <li> Any typed value may be assigned to the blank identifier. </li> <li> If an untyped constant is assigned to a variable of interface type or the blank identifier, the constant is first implicitly <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>. </li> <li> If an untyped boolean value is assigned to a variable of interface type or the blank identifier, it is first implicitly converted to type <code>bool</code>. </li> </ol> <h3 id="If_statements">If statements</h3> <p> "If" statements specify the conditional execution of two branches according to the value of a boolean expression. If the expression evaluates to true, the "if" branch is executed, otherwise, if present, the "else" branch is executed. </p> <pre class="ebnf"> IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt \| Block ) ] . </pre> <pre> if x > max { x = max } </pre> <p> The expression may be preceded by a simple statement, which executes before the expression is evaluated. </p> <pre> if x := f(); x < y { return x } else if x > z { return z } else { return y } </pre> <h3 id="Switch_statements">Switch statements</h3> <p> "Switch" statements provide multi-way execution. An expression or type is compared to the "cases" inside the "switch" to determine which branch to execute. </p> <pre class="ebnf"> SwitchStmt = ExprSwitchStmt \| TypeSwitchStmt . </pre> <p> There are two forms: expression switches and type switches. In an expression switch, the cases contain expressions that are compared against the value of the switch expression. In a type switch, the cases contain types that are compared against the type of a specially annotated switch expression. The switch expression is evaluated exactly once in a switch statement. </p> <h4 id="Expression_switches">Expression switches</h4> <p> In an expression switch, the switch expression is evaluated and the case expressions, which need not be constants, are evaluated left-to-right and top-to-bottom; the first one that equals the switch expression triggers execution of the statements of the associated case; the other cases are skipped. If no case matches and there is a "default" case, its statements are executed. There can be at most one default case and it may appear anywhere in the "switch" statement. A missing switch expression is equivalent to the boolean value <code>true</code>. </p> <pre class="ebnf"> ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" . ExprCaseClause = ExprSwitchCase ":" StatementList . ExprSwitchCase = "case" ExpressionList \| "default" . </pre> <p> If the switch expression evaluates to an untyped constant, it is first implicitly <a href="#Conversions">converted</a> to its <a href="#Constants">default type</a>. The predeclared untyped value <code>nil</code> cannot be used as a switch expression. The switch expression type must be <a href="#Comparison_operators">comparable</a>. </p> <p> If a case expression is untyped, it is first implicitly <a href="#Conversions">converted</a> to the type of the switch expression. For each (possibly converted) case expression <code>x</code> and the value <code>t</code> of the switch expression, <code>x == t</code> must be a valid <a href="#Comparison_operators">comparison</a>. </p> <p> In other words, the switch expression is treated as if it were used to declare and initialize a temporary variable <code>t</code> without explicit type; it is that value of <code>t</code> against which each case expression <code>x</code> is tested for equality. </p> <p> In a case or default clause, the last non-empty statement may be a (possibly <a href="#Labeled_statements">labeled</a>) <a href="#Fallthrough_statements">"fallthrough" statement</a> to indicate that control should flow from the end of this clause to the first statement of the next clause. Otherwise control flows to the end of the "switch" statement. A "fallthrough" statement may appear as the last statement of all but the last clause of an expression switch. </p> <p> The switch expression may be preceded by a simple statement, which executes before the expression is evaluated. </p> <pre> switch tag { default: s3() case 0, 1, 2, 3: s1() case 4, 5, 6, 7: s2() } switch x := f(); { // missing switch expression means "true" case x < 0: return -x default: return x } switch { case x < y: f1() case x < z: f2() case x == 4: f3() } </pre> <p> Implementation restriction: A compiler may disallow multiple case expressions evaluating to the same constant. For instance, the current compilers disallow duplicate integer, floating point, or string constants in case expressions. </p> <h4 id="Type_switches">Type switches</h4> <p> A type switch compares types rather than values. It is otherwise similar to an expression switch. It is marked by a special switch expression that has the form of a <a href="#Type_assertions">type assertion</a> using the keyword <code>type</code> rather than an actual type: </p> <pre> switch x.(type) { // cases } </pre> <p> Cases then match actual types <code>T</code> against the dynamic type of the expression <code>x</code>. As with type assertions, <code>x</code> must be of <a href="#Interface_types">interface type</a>, but not a <a href="#Type_parameter_declarations">type parameter</a>, and each non-interface type <code>T</code> listed in a case must implement the type of <code>x</code>. The types listed in the cases of a type switch must all be <a href="#Type_identity">different</a>. </p> <pre class="ebnf"> TypeSwitchStmt = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" . TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" . TypeCaseClause = TypeSwitchCase ":" StatementList . TypeSwitchCase = "case" TypeList \| "default" . </pre> <p> The TypeSwitchGuard may include a <a href="#Short_variable_declarations">short variable declaration</a>. When that form is used, the variable is declared at the end of the TypeSwitchCase in the <a href="#Blocks">implicit block</a> of each clause. In clauses with a case listing exactly one type, the variable has that type; otherwise, the variable has the type of the expression in the TypeSwitchGuard. </p> <p> Instead of a type, a case may use the predeclared identifier <a href="#Predeclared_identifiers"><code>nil</code></a>; that case is selected when the expression in the TypeSwitchGuard is a <code>nil</code> interface value. There may be at most one <code>nil</code> case. </p> <p> Given an expression <code>x</code> of type <code>interface{}</code>, the following type switch: </p> <pre> switch i := x.(type) { case nil: printString("x is nil") // type of i is type of x (interface{}) case int: printInt(i) // type of i is int case float64: printFloat64(i) // type of i is float64 case func(int) float64: printFunction(i) // type of i is func(int) float64 case bool, string: printString("type is bool or string") // type of i is type of x (interface{}) default: printString("don't know the type") // type of i is type of x (interface{}) } </pre> <p> could be rewritten: </p> <pre> v := x // x is evaluated exactly once if v == nil { i := v // type of i is type of x (interface{}) printString("x is nil") } else if i, isInt := v.(int); isInt { printInt(i) // type of i is int } else if i, isFloat64 := v.(float64); isFloat64 { printFloat64(i) // type of i is float64 } else if i, isFunc := v.(func(int) float64); isFunc { printFunction(i) // type of i is func(int) float64 } else { _, isBool := v.(bool) _, isString := v.(string) if isBool \|\| isString { i := v // type of i is type of x (interface{}) printString("type is bool or string") } else { i := v // type of i is type of x (interface{}) printString("don't know the type") } } </pre> <p> A <a href="#Type_parameter_declarations">type parameter</a> or a <a href="#Type_declarations">generic type</a> may be used as a type in a case. If upon <a href="#Instantiations">instantiation</a> that type turns out to duplicate another entry in the switch, the first matching case is chosen. </p> <pre> func f[P any](x any) int { switch x.(type) { case P: return 0 case string: return 1 case []P: return 2 case []byte: return 3 default: return 4 } } var v1 = f[string]("foo") // v1 == 0 var v2 = f[byte]([]byte{}) // v2 == 2 </pre> <p> The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated. </p> <p> The "fallthrough" statement is not permitted in a type switch. </p> <h3 id="For_statements">For statements</h3> <p> A "for" statement specifies repeated execution of a block. There are three forms: The iteration may be controlled by a single condition, a "for" clause, or a "range" clause. </p> <pre class="ebnf"> ForStmt = "for" [ Condition \| ForClause \| RangeClause ] Block . Condition = Expression . </pre> <h4 id="For_condition">For statements with single condition</h4> <p> In its simplest form, a "for" statement specifies the repeated execution of a block as long as a boolean condition evaluates to true. The condition is evaluated before each iteration. If the condition is absent, it is equivalent to the boolean value <code>true</code>. </p> <pre> for a < b { a = 2 } </pre> <h4 id="For_clause">For statements with <code>for</code> clause</h4> <p> A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an <i>init</i> and a <i>post</i> statement, such as an assignment, an increment or decrement statement. The init statement may be a <a href="#Short_variable_declarations">short variable declaration</a>, but the post statement must not. </p> <pre class="ebnf"> ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] . InitStmt = SimpleStmt . PostStmt = SimpleStmt . </pre> <pre> for i := 0; i < 10; i++ { f(i) } </pre> <p> If non-empty, the init statement is executed once before evaluating the condition for the first iteration; the post statement is executed after each execution of the block (and only if the block was executed). Any element of the ForClause may be empty but the <a href="#Semicolons">semicolons</a> are required unless there is only a condition. If the condition is absent, it is equivalent to the boolean value <code>true</code>. </p> <pre> for cond { S() } is the same as for ; cond ; { S() } for { S() } is the same as for true { S() } </pre> <p> Each iteration has its own separate declared variable (or variables) [<a href="#Go_1.22">Go 1.22</a>]. The variable used by the first iteration is declared by the init statement. The variable used by each subsequent iteration is declared implicitly before executing the post statement and initialized to the value of the previous iteration's variable at that moment. </p> <pre> var prints []func() for i := 0; i < 5; i++ { prints = append(prints, func() { println(i) }) i++ } for _, p := range prints { p() } </pre> <p> prints </p> <pre> 1 3 5 </pre> <p> Prior to [<a href="#Go_1.22">Go 1.22</a>], iterations share one set of variables instead of having their own separate variables. In that case, the example above prints </p> <pre> 6 6 6 </pre> <h4 id="For_range">For statements with <code>range</code> clause</h4> <p> A "for" statement with a "range" clause iterates through all entries of an array, slice, string or map, values received on a channel, or integer values from zero to an upper limit [<a href="#Go_1.22">Go 1.22</a>]. For each entry it assigns <i>iteration values</i> to corresponding <i>iteration variables</i> if present and then executes the block. </p> <pre class="ebnf"> RangeClause = [ ExpressionList "=" \| IdentifierList ":=" ] "range" Expression . </pre> <p> The expression on the right in the "range" clause is called the <i>range expression</i>, its <a href="#Core_types">core type</a> must be an array, pointer to an array, slice, string, map, channel permitting <a href="#Receive_operator">receive operations</a>, or an integer. As with an assignment, if present the operands on the left must be <a href="#Address_operators">addressable</a> or map index expressions; they denote the iteration variables. If the range expression is a channel or integer, at most one iteration variable is permitted, otherwise there may be up to two. If the last iteration variable is the <a href="#Blank_identifier">blank identifier</a>, the range clause is equivalent to the same clause without that identifier. </p> <p> The range expression <code>x</code> is evaluated once before beginning the loop, with one exception: if at most one iteration variable is present and <code>len(x)</code> is <a href="#Length_and_capacity">constant</a>, the range expression is not evaluated. </p> <p> Function calls on the left are evaluated once per iteration. For each iteration, iteration values are produced as follows if the respective iteration variables are present: </p> <pre class="grammar"> Range expression 1st value 2nd value array or slice a [n]E, [n]E, or []E index i int a[i] E string s string type index i int see below rune map m map[K]V key k K m[k] V channel c chan E, <-chan E element e E integer n integer type value i see below </pre> <ol> <li> For an array, pointer to array, or slice value <code>a</code>, the index iteration values are produced in increasing order, starting at element index 0. If at most one iteration variable is present, the range loop produces iteration values from 0 up to <code>len(a)-1</code> and does not index into the array or slice itself. For a <code>nil</code> slice, the number of iterations is 0. </li> <li> For a string value, the "range" clause iterates over the Unicode code points in the string starting at byte index 0. On successive iterations, the index value will be the index of the first byte of successive UTF-8-encoded code points in the string, and the second value, of type <code>rune</code>, will be the value of the corresponding code point. If the iteration encounters an invalid UTF-8 sequence, the second value will be <code>0xFFFD</code>, the Unicode replacement character, and the next iteration will advance a single byte in the string. </li> <li> The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If a map entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced. If a map entry is created during iteration, that entry may be produced during the iteration or may be skipped. The choice may vary for each entry created and from one iteration to the next. If the map is <code>nil</code>, the number of iterations is 0. </li> <li> For channels, the iteration values produced are the successive values sent on the channel until the channel is <a href="#Close">closed</a>. If the channel is <code>nil</code>, the range expression blocks forever. </li> <li> For an integer value <code>n</code>, the iteration values 0 through <code>n-1</code> are produced in increasing order. If <code>n</code> &lt= 0, the loop does not run any iterations. </li> </ol> <p> The iteration variables may be declared by the "range" clause using a form of <a href="#Short_variable_declarations">short variable declaration</a> (<code>:=</code>). In this case their <a href="#Declarations_and_scope">scope</a> is the block of the "for" statement and each iteration has its own new variables [<a href="#Go_1.22">Go 1.22</a>] (see also <a href="#For_clause">"for" statements with a ForClause</a>). If the range expression is a (possibly untyped) integer expression <code>n</code>, the variable has the same type as if it was <a href="#Variable_declarations">declared</a> with initialization expression <code>n</code>. Otherwise, the variables have the types of their respective iteration values. </p> <p> If the iteration variables are not explicitly declared by the "range" clause, they must be preexisting. In this case, the iteration values are assigned to the respective variables as in an <a href="#Assignment_statements">assignment statement</a>. If the range expression is a (possibly untyped) integer expression <code>n</code>, <code>n</code> too must be <a href="#Assignability">assignable</a> to the iteration variable; if there is no iteration variable, <code>n</code> must be assignable to <code>int</code>. </p> <pre> var testdata struct { a [7]int } for i, _ := range testdata.a { // testdata.a is never evaluated; len(testdata.a) is constant // i ranges from 0 to 6 f(i) } var a [10]string for i, s := range a { // type of i is int // type of s is string // s == a[i] g(i, s) } var key string var val interface{} // element type of m is assignable to val m := map[string]int{"mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6} for key, val = range m { h(key, val) } // key == last map key encountered in iteration // val == map[key] var ch chan Work = producer() for w := range ch { doWork(w) } // empty a channel for range ch {} // call f(0), f(1), ... f(9) for i := range 10 { // type of i is int (default type for untyped constant 10) f(i) } // invalid: 256 cannot be assigned to uint8 var u uint8 for u = range 256 { } </pre> <h3 id="Go_statements">Go statements</h3> <p> A "go" statement starts the execution of a function call as an independent concurrent thread of control, or <i>goroutine</i>, within the same address space. </p> <pre class="ebnf"> GoStmt = "go" Expression . </pre> <p> The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for <a href="#Expression_statements">expression statements</a>. </p> <p> The function value and parameters are <a href="#Calls">evaluated as usual</a> in the calling goroutine, but unlike with a regular call, program execution does not wait for the invoked function to complete. Instead, the function begins executing independently in a new goroutine. When the function terminates, its goroutine also terminates. If the function has any return values, they are discarded when the function completes. </p> <pre> go Server() go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c) </pre> <h3 id="Select_statements">Select statements</h3> <p> A "select" statement chooses which of a set of possible <a href="#Send_statements">send</a> or <a href="#Receive_operator">receive</a> operations will proceed. It looks similar to a <a href="#Switch_statements">"switch"</a> statement but with the cases all referring to communication operations. </p> <pre class="ebnf"> SelectStmt = "select" "{" { CommClause } "}" . CommClause = CommCase ":" StatementList . CommCase = "case" ( SendStmt \| RecvStmt ) \| "default" . RecvStmt = [ ExpressionList "=" \| IdentifierList ":=" ] RecvExpr . RecvExpr = Expression . </pre> <p> A case with a RecvStmt may assign the result of a RecvExpr to one or two variables, which may be declared using a <a href="#Short_variable_declarations">short variable declaration</a>. The RecvExpr must be a (possibly parenthesized) receive operation. There can be at most one default case and it may appear anywhere in the list of cases. </p> <p> Execution of a "select" statement proceeds in several steps: </p> <ol> <li> For all the cases in the statement, the channel operands of receive operations and the channel and right-hand-side expressions of send statements are evaluated exactly once, in source order, upon entering the "select" statement. The result is a set of channels to receive from or send to, and the corresponding values to send. Any side effects in that evaluation will occur irrespective of which (if any) communication operation is selected to proceed. Expressions on the left-hand side of a RecvStmt with a short variable declaration or assignment are not yet evaluated. </li> <li> If one or more of the communications can proceed, a single one that can proceed is chosen via a uniform pseudo-random selection. Otherwise, if there is a default case, that case is chosen. If there is no default case, the "select" statement blocks until at least one of the communications can proceed. </li> <li> Unless the selected case is the default case, the respective communication operation is executed. </li> <li> If the selected case is a RecvStmt with a short variable declaration or an assignment, the left-hand side expressions are evaluated and the received value (or values) are assigned. </li> <li> The statement list of the selected case is executed. </li> </ol> <p> Since communication on <code>nil</code> channels can never proceed, a select with only <code>nil</code> channels and no default case blocks forever. </p> <pre> var a []int var c, c1, c2, c3, c4 chan int var i1, i2 int select { case i1 = <-c1: print("received ", i1, " from c1\n") case c2 <- i2: print("sent ", i2, " to c2\n") case i3, ok := (<-c3): // same as: i3, ok := <-c3 if ok { print("received ", i3, " from c3\n") } else { print("c3 is closed\n") } case a[f()] = <-c4: // same as: // case t := <-c4 // a[f()] = t default: print("no communication\n") } for { // send random sequence of bits to c select { case c <- 0: // note: no statement, no fallthrough, no folding of cases case c <- 1: } } select {} // block forever </pre> <h3 id="Return_statements">Return statements</h3> <p> A "return" statement in a function <code>F</code> terminates the execution of <code>F</code>, and optionally provides one or more result values. Any functions <a href="#Defer_statements">deferred</a> by <code>F</code> are executed before <code>F</code> returns to its caller. </p> <pre class="ebnf"> ReturnStmt = "return" [ ExpressionList ] . </pre> <p> In a function without a result type, a "return" statement must not specify any result values. </p> <pre> func noResult() { return } </pre> <p> There are three ways to return values from a function with a result type: </p> <ol> <li>The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and <a href="#Assignability">assignable</a> to the corresponding element of the function's result type. <pre> func simpleF() int { return 2 } func complexF1() (re float64, im float64) { return -7.0, -4.0 } </pre> </li> <li>The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply. <pre> func complexF2() (re float64, im float64) { return complexF1() } </pre> </li> <li>The expression list may be empty if the function's result type specifies names for its <a href="#Function_types">result parameters</a>. The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables. <pre> func complexF3() (re float64, im float64) { re = 7.0 im = 4.0 return } func (devnull) Write(p []byte) (n int, _ error) { n = len(p) return } </pre> </li> </ol> <p> Regardless of how they are declared, all the result values are initialized to the <a href="#The_zero_value">zero values</a> for their type upon entry to the function. A "return" statement that specifies results sets the result parameters before any deferred functions are executed. </p> <p> Implementation restriction: A compiler may disallow an empty expression list in a "return" statement if a different entity (constant, type, or variable) with the same name as a result parameter is in <a href="#Declarations_and_scope">scope</a> at the place of the return. </p> <pre> func f(n int) (res int, err error) { if _, err := f(n-1); err != nil { return // invalid return statement: err is shadowed } return } </pre> <h3 id="Break_statements">Break statements</h3> <p> A "break" statement terminates execution of the innermost <a href="#For_statements">"for"</a>, <a href="#Switch_statements">"switch"</a>, or <a href="#Select_statements">"select"</a> statement within the same function. </p> <pre class="ebnf"> BreakStmt = "break" [ Label ] . </pre> <p> If there is a label, it must be that of an enclosing "for", "switch", or "select" statement, and that is the one whose execution terminates. </p> <pre> OuterLoop: for i = 0; i < n; i++ { for j = 0; j < m; j++ { switch a[i][j] { case nil: state = Error break OuterLoop case item: state = Found break OuterLoop } } } </pre> <h3 id="Continue_statements">Continue statements</h3> <p> A "continue" statement begins the next iteration of the innermost enclosing <a href="#For_statements">"for" loop</a> by advancing control to the end of the loop block. The "for" loop must be within the same function. </p> <pre class="ebnf"> ContinueStmt = "continue" [ Label ] . </pre> <p> If there is a label, it must be that of an enclosing "for" statement, and that is the one whose execution advances. </p> <pre> RowLoop: for y, row := range rows { for x, data := range row { if data == endOfRow { continue RowLoop } row[x] = data + bias(x, y) } } </pre> <h3 id="Goto_statements">Goto statements</h3> <p> A "goto" statement transfers control to the statement with the corresponding label within the same function. </p> <pre class="ebnf"> GotoStmt = "goto" Label . </pre> <pre> goto Error </pre> <p> Executing the "goto" statement must not cause any variables to come into <a href="#Declarations_and_scope">scope</a> that were not already in scope at the point of the goto. For instance, this example: </p> <pre> goto L // BAD v := 3 L: </pre> <p> is erroneous because the jump to label <code>L</code> skips the creation of <code>v</code>. </p> <p> A "goto" statement outside a <a href="#Blocks">block</a> cannot jump to a label inside that block. For instance, this example: </p> <pre> if n%2 == 1 { goto L1 } for n > 0 { f() n-- L1: f() n-- } </pre> <p> is erroneous because the label <code>L1</code> is inside the "for" statement's block but the <code>goto</code> is not. </p> <h3 id="Fallthrough_statements">Fallthrough statements</h3> <p> A "fallthrough" statement transfers control to the first statement of the next case clause in an <a href="#Expression_switches">expression "switch" statement</a>. It may be used only as the final non-empty statement in such a clause. </p> <pre class="ebnf"> FallthroughStmt = "fallthrough" . </pre> <h3 id="Defer_statements">Defer statements</h3> <p> A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a <a href="#Return_statements">return statement</a>, reached the end of its <a href="#Function_declarations">function body</a>, or because the corresponding goroutine is <a href="#Handling_panics">panicking</a>. </p> <pre class="ebnf"> DeferStmt = "defer" Expression . </pre> <p> The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for <a href="#Expression_statements">expression statements</a>. </p> <p> Each time a "defer" statement executes, the function value and parameters to the call are <a href="#Calls">evaluated as usual</a> and saved anew but the actual function is not invoked. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. That is, if the surrounding function returns through an explicit <a href="#Return_statements">return statement</a>, deferred functions are executed <i>after</i> any result parameters are set by that return statement but <i>before</i> the function returns to its caller. If a deferred function value evaluates to <code>nil</code>, execution <a href="#Handling_panics">panics</a> when the function is invoked, not when the "defer" statement is executed. </p> <p> For instance, if the deferred function is a <a href="#Function_literals">function literal</a> and the surrounding function has <a href="#Function_types">named result parameters</a> that are in scope within the literal, the deferred function may access and modify the result parameters before they are returned. If the deferred function has any return values, they are discarded when the function completes. (See also the section on <a href="#Handling_panics">handling panics</a>.) </p> <pre> lock(l) defer unlock(l) // unlocking happens before surrounding function returns // prints 3 2 1 0 before surrounding function returns for i := 0; i <= 3; i++ { defer fmt.Print(i) } // f returns 42 func f() (result int) { defer func() { // result is accessed after it was set to 6 by the return statement result = 7 }() return 6 } </pre> <h2 id="Built-in_functions">Built-in functions</h2> <p> Built-in functions are <a href="#Predeclared_identifiers">predeclared</a>. They are called like any other function but some of them accept a type instead of an expression as the first argument. </p> <p> The built-in functions do not have standard Go types, so they can only appear in <a href="#Calls">call expressions</a>; they cannot be used as function values. </p> <h3 id="Appending_and_copying_slices">Appending to and copying slices</h3> <p> The built-in functions <code>append</code> and <code>copy</code> assist in common slice operations. For both functions, the result is independent of whether the memory referenced by the arguments overlaps. </p> <p> The <a href="#Function_types">variadic</a> function <code>append</code> appends zero or more values <code>x</code> to a slice <code>s</code> and returns the resulting slice of the same type as <code>s</code>. The <a href="#Core_types">core type</a> of <code>s</code> must be a slice of type <code>[]E</code>. The values <code>x</code> are passed to a parameter of type <code>...E</code> and the respective <a href="#Passing_arguments_to_..._parameters">parameter passing rules</a> apply. As a special case, if the core type of <code>s</code> is <code>[]byte</code>, <code>append</code> also accepts a second argument with core type <a href="#Core_types"><code>bytestring</code></a> followed by <code>...</code>. This form appends the bytes of the byte slice or string. </p> <pre class="grammar"> append(s S, x ...E) S // core type of S is []E </pre> <p> If the capacity of <code>s</code> is not large enough to fit the additional values, <code>append</code> <a href="#Allocation">allocates</a> a new, sufficiently large underlying array that fits both the existing slice elements and the additional values. Otherwise, <code>append</code> re-uses the underlying array. </p> <pre> s0 := []int{0, 0} s1 := append(s0, 2) // append a single element s1 is []int{0, 0, 2} s2 := append(s1, 3, 5, 7) // append multiple elements s2 is []int{0, 0, 2, 3, 5, 7} s3 := append(s2, s0...) // append a slice s3 is []int{0, 0, 2, 3, 5, 7, 0, 0} s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 is []int{3, 5, 7, 2, 3, 5, 7, 0, 0} var t []interface{} t = append(t, 42, 3.1415, "foo") // t is []interface{}{42, 3.1415, "foo"} var b []byte b = append(b, "bar"...) // append string contents b is []byte{'b', 'a', 'r' } </pre> <p> The function <code>copy</code> copies slice elements from a source <code>src</code> to a destination <code>dst</code> and returns the number of elements copied. The <a href="#Core_types">core types</a> of both arguments must be slices with <a href="#Type_identity">identical</a> element type. The number of elements copied is the minimum of <code>len(src)</code> and <code>len(dst)</code>. As a special case, if the destination's core type is <code>[]byte</code>, <code>copy</code> also accepts a source argument with core type </a> <a href="#Core_types"><code>bytestring</code></a>. This form copies the bytes from the byte slice or string into the byte slice. </p> <pre class="grammar"> copy(dst, src []T) int copy(dst []byte, src string) int </pre> <p> Examples: </p> <pre> var a = [...]int{0, 1, 2, 3, 4, 5, 6, 7} var s = make([]int, 6) var b = make([]byte, 5) n1 := copy(s, a[0:]) // n1 == 6, s is []int{0, 1, 2, 3, 4, 5} n2 := copy(s, s[2:]) // n2 == 4, s is []int{2, 3, 4, 5, 4, 5} n3 := copy(b, "Hello, World!") // n3 == 5, b is []byte("Hello") </pre> <h3 id="Clear">Clear</h3> <p> The built-in function <code>clear</code> takes an argument of <a href="#Map_types">map</a>, <a href="#Slice_types">slice</a>, or <a href="#Type_parameter_declarations">type parameter</a> type, and deletes or zeroes out all elements [<a href="#Go_1.21">Go 1.21</a>]. </p> <pre class="grammar"> Call Argument type Result clear(m) map[K]T deletes all entries, resulting in an empty map (len(m) == 0) clear(s) []T sets all elements up to the length of <code>s</code> to the zero value of T clear(t) type parameter see below </pre> <p> If the type of the argument to <code>clear</code> is a <a href="#Type_parameter_declarations">type parameter</a>, all types in its type set must be maps or slices, and <code>clear</code> performs the operation corresponding to the actual type argument. </p> <p> If the map or slice is <code>nil</code>, <code>clear</code> is a no-op. </p> <h3 id="Close">Close</h3> <p> For an argument <code>ch</code> with a <a href="#Core_types">core type</a> that is a <a href="#Channel_types">channel</a>, the built-in function <code>close</code> records that no more values will be sent on the channel. It is an error if <code>ch</code> is a receive-only channel. Sending to or closing a closed channel causes a <a href="#Run_time_panics">run-time panic</a>. Closing the nil channel also causes a <a href="#Run_time_panics">run-time panic</a>. After calling <code>close</code>, and after any previously sent values have been received, receive operations will return the zero value for the channel's type without blocking. The multi-valued <a href="#Receive_operator">receive operation</a> returns a received value along with an indication of whether the channel is closed. </p> <h3 id="Complex_numbers">Manipulating complex numbers</h3> <p> Three functions assemble and disassemble complex numbers. The built-in function <code>complex</code> constructs a complex value from a floating-point real and imaginary part, while <code>real</code> and <code>imag</code> extract the real and imaginary parts of a complex value. </p> <pre class="grammar"> complex(realPart, imaginaryPart floatT) complexT real(complexT) floatT imag(complexT) floatT </pre> <p> The type of the arguments and return value correspond. For <code>complex</code>, the two arguments must be of the same <a href="#Numeric_types">floating-point type</a> and the return type is the <a href="#Numeric_types">complex type</a> with the corresponding floating-point constituents: <code>complex64</code> for <code>float32</code> arguments, and <code>complex128</code> for <code>float64</code> arguments. If one of the arguments evaluates to an untyped constant, it is first implicitly <a href="#Conversions">converted</a> to the type of the other argument. If both arguments evaluate to untyped constants, they must be non-complex numbers or their imaginary parts must be zero, and the return value of the function is an untyped complex constant. </p> <p> For <code>real</code> and <code>imag</code>, the argument must be of complex type, and the return type is the corresponding floating-point type: <code>float32</code> for a <code>complex64</code> argument, and <code>float64</code> for a <code>complex128</code> argument. If the argument evaluates to an untyped constant, it must be a number, and the return value of the function is an untyped floating-point constant. </p> <p> The <code>real</code> and <code>imag</code> functions together form the inverse of <code>complex</code>, so for a value <code>z</code> of a complex type <code>Z</code>, <code>z == Z(complex(real(z), imag(z)))</code>. </p> <p> If the operands of these functions are all constants, the return value is a constant. </p> <pre> var a = complex(2, -2) // complex128 const b = complex(1.0, -1.4) // untyped complex constant 1 - 1.4i x := float32(math.Cos(math.Pi/2)) // float32 var c64 = complex(5, -x) // complex64 var s int = complex(1, 0) // untyped complex constant 1 + 0i can be converted to int _ = complex(1, 2<<s) // illegal: 2 assumes floating-point type, cannot shift var rl = real(c64) // float32 var im = imag(a) // float64 const c = imag(b) // untyped constant -1.4 _ = imag(3 << s) // illegal: 3 assumes complex type, cannot shift </pre> <p> Arguments of type parameter type are not permitted. </p> <h3 id="Deletion_of_map_elements">Deletion of map elements</h3> <p> The built-in function <code>delete</code> removes the element with key <code>k</code> from a <a href="#Map_types">map</a> <code>m</code>. The value <code>k</code> must be <a href="#Assignability">assignable</a> to the key type of <code>m</code>. </p> <pre class="grammar"> delete(m, k) // remove element m[k] from map m </pre> <p> If the type of <code>m</code> is a <a href="#Type_parameter_declarations">type parameter</a>, all types in that type set must be maps, and they must all have identical key types. </p> <p> If the map <code>m</code> is <code>nil</code> or the element <code>m[k]</code> does not exist, <code>delete</code> is a no-op. </p> <h3 id="Length_and_capacity">Length and capacity</h3> <p> The built-in functions <code>len</code> and <code>cap</code> take arguments of various types and return a result of type <code>int</code>. The implementation guarantees that the result always fits into an <code>int</code>. </p> <pre class="grammar"> Call Argument type Result len(s) string type string length in bytes [n]T, [n]T array length (== n) []T slice length map[K]T map length (number of defined keys) chan T number of elements queued in channel buffer type parameter see below cap(s) [n]T, [n]T array length (== n) []T slice capacity chan T channel buffer capacity type parameter see below </pre> <p> If the argument type is a <a href="#Type_parameter_declarations">type parameter</a> <code>P</code>, the call <code>len(e)</code> (or <code>cap(e)</code> respectively) must be valid for each type in <code>P</code>'s type set. The result is the length (or capacity, respectively) of the argument whose type corresponds to the type argument with which <code>P</code> was <a href="#Instantiations">instantiated</a>. </p> <p> The capacity of a slice is the number of elements for which there is space allocated in the underlying array. At any time the following relationship holds: </p> <pre> 0 <= len(s) <= cap(s) </pre> <p> The length of a <code>nil</code> slice, map or channel is 0. The capacity of a <code>nil</code> slice or channel is 0. </p> <p> The expression <code>len(s)</code> is <a href="#Constants">constant</a> if <code>s</code> is a string constant. The expressions <code>len(s)</code> and <code>cap(s)</code> are constants if the type of <code>s</code> is an array or pointer to an array and the expression <code>s</code> does not contain <a href="#Receive_operator">channel receives</a> or (non-constant) <a href="#Calls">function calls</a>; in this case <code>s</code> is not evaluated. Otherwise, invocations of <code>len</code> and <code>cap</code> are not constant and <code>s</code> is evaluated. </p> <pre> const ( c1 = imag(2i) // imag(2i) = 2.0 is a constant c2 = len([10]float64{2}) // [10]float64{2} contains no function calls c3 = len([10]float64{c1}) // [10]float64{c1} contains no function calls c4 = len([10]float64{imag(2i)}) // imag(2i) is a constant and no function call is issued c5 = len([10]float64{imag(z)}) // invalid: imag(z) is a (non-constant) function call ) var z complex128 </pre> <h3 id="Making_slices_maps_and_channels">Making slices, maps and channels</h3> <p> The built-in function <code>make</code> takes a type <code>T</code>, optionally followed by a type-specific list of expressions. The <a href="#Core_types">core type</a> of <code>T</code> must be a slice, map or channel. It returns a value of type <code>T</code> (not <code>T</code>). The memory is initialized as described in the section on <a href="#The_zero_value">initial values</a>. </p> <pre class="grammar"> Call Core type Result make(T, n) slice slice of type T with length n and capacity n make(T, n, m) slice slice of type T with length n and capacity m make(T) map map of type T make(T, n) map map of type T with initial space for approximately n elements make(T) channel unbuffered channel of type T make(T, n) channel buffered channel of type T, buffer size n </pre> <p> Each of the size arguments <code>n</code> and <code>m</code> must be of <a href="#Numeric_types">integer type</a>, have a <a href="#Interface_types">type set</a> containing only integer types, or be an untyped <a href="#Constants">constant</a>. A constant size argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>. If both <code>n</code> and <code>m</code> are provided and are constant, then <code>n</code> must be no larger than <code>m</code>. For slices and channels, if <code>n</code> is negative or larger than <code>m</code> at run time, a <a href="#Run_time_panics">run-time panic</a> occurs. </p> <pre> s := make([]int, 10, 100) // slice with len(s) == 10, cap(s) == 100 s := make([]int, 1e3) // slice with len(s) == cap(s) == 1000 s := make([]int, 1<<63) // illegal: len(s) is not representable by a value of type int s := make([]int, 10, 0) // illegal: len(s) > cap(s) c := make(chan int, 10) // channel with a buffer size of 10 m := make(map[string]int, 100) // map with initial space for approximately 100 elements </pre> <p> Calling <code>make</code> with a map type and size hint <code>n</code> will create a map with initial space to hold <code>n</code> map elements. The precise behavior is implementation-dependent. </p> <h3 id="Min_and_max">Min and max</h3> <p> The built-in functions <code>min</code> and <code>max</code> compute the smallest—or largest, respectively—value of a fixed number of arguments of <a href="#Comparison_operators">ordered types</a>. There must be at least one argument [<a href="#Go_1.21">Go 1.21</a>]. </p> <p> The same type rules as for <a href="#Operators">operators</a> apply: for <a href="#Comparison_operators">ordered</a> arguments <code>x</code> and <code>y</code>, <code>min(x, y)</code> is valid if <code>x + y</code> is valid, and the type of <code>min(x, y)</code> is the type of <code>x + y</code> (and similarly for <code>max</code>). If all arguments are constant, the result is constant. </p> <pre> var x, y int m := min(x) // m == x m := min(x, y) // m is the smaller of x and y m := max(x, y, 10) // m is the larger of x and y but at least 10 c := max(1, 2.0, 10) // c == 10.0 (floating-point kind) f := max(0, float32(x)) // type of f is float32 var s []string _ = min(s...) // invalid: slice arguments are not permitted t := max("", "foo", "bar") // t == "foo" (string kind) </pre> <p> For numeric arguments, assuming all NaNs are equal, <code>min</code> and <code>max</code> are commutative and associative: </p> <pre> min(x, y) == min(y, x) min(x, y, z) == min(min(x, y), z) == min(x, min(y, z)) </pre> <p> For floating-point arguments negative zero, NaN, and infinity the following rules apply: </p> <pre> x y min(x, y) max(x, y) -0.0 0.0 -0.0 0.0 // negative zero is smaller than (non-negative) zero -Inf y -Inf y // negative infinity is smaller than any other number +Inf y y +Inf // positive infinity is larger than any other number NaN y NaN NaN // if any argument is a NaN, the result is a NaN </pre> <p> For string arguments the result for <code>min</code> is the first argument with the smallest (or for <code>max</code>, largest) value, compared lexically byte-wise: </p> <pre> min(x, y) == if x <= y then x else y min(x, y, z) == min(min(x, y), z) </pre> <h3 id="Allocation">Allocation</h3> <p> The built-in function <code>new</code> takes a type <code>T</code>, allocates storage for a <a href="#Variables">variable</a> of that type at run time, and returns a value of type <code>T</code> <a href="#Pointer_types">pointing</a> to it. The variable is initialized as described in the section on <a href="#The_zero_value">initial values</a>. </p> <pre class="grammar"> new(T) </pre> <p> For instance </p> <pre> type S struct { a int; b float64 } new(S) </pre> <p> allocates storage for a variable of type <code>S</code>, initializes it (<code>a=0</code>, <code>b=0.0</code>), and returns a value of type <code>S</code> containing the address of the location. </p> <h3 id="Handling_panics">Handling panics</h3> <p> Two built-in functions, <code>panic</code> and <code>recover</code>, assist in reporting and handling <a href="#Run_time_panics">run-time panics</a> and program-defined error conditions. </p> <pre class="grammar"> func panic(interface{}) func recover() interface{} </pre> <p> While executing a function <code>F</code>, an explicit call to <code>panic</code> or a <a href="#Run_time_panics">run-time panic</a> terminates the execution of <code>F</code>. Any functions <a href="#Defer_statements">deferred</a> by <code>F</code> are then executed as usual. Next, any deferred functions run by <code>F</code>'s caller are run, and so on up to any deferred by the top-level function in the executing goroutine. At that point, the program is terminated and the error condition is reported, including the value of the argument to <code>panic</code>. This termination sequence is called <i>panicking</i>. </p> <pre> panic(42) panic("unreachable") panic(Error("cannot parse")) </pre> <p> The <code>recover</code> function allows a program to manage behavior of a panicking goroutine. Suppose a function <code>G</code> defers a function <code>D</code> that calls <code>recover</code> and a panic occurs in a function on the same goroutine in which <code>G</code> is executing. When the running of deferred functions reaches <code>D</code>, the return value of <code>D</code>'s call to <code>recover</code> will be the value passed to the call of <code>panic</code>. If <code>D</code> returns normally, without starting a new <code>panic</code>, the panicking sequence stops. In that case, the state of functions called between <code>G</code> and the call to <code>panic</code> is discarded, and normal execution resumes. Any functions deferred by <code>G</code> before <code>D</code> are then run and <code>G</code>'s execution terminates by returning to its caller. </p> <p> The return value of <code>recover</code> is <code>nil</code> when the goroutine is not panicking or <code>recover</code> was not called directly by a deferred function. Conversely, if a goroutine is panicking and <code>recover</code> was called directly by a deferred function, the return value of <code>recover</code> is guaranteed not to be <code>nil</code>. To ensure this, calling <code>panic</code> with a <code>nil</code> interface value (or an untyped <code>nil</code>) causes a <a href="#Run_time_panics">run-time panic</a>. </p> <p> The <code>protect</code> function in the example below invokes the function argument <code>g</code> and protects callers from run-time panics raised by <code>g</code>. </p> <pre> func protect(g func()) { defer func() { log.Println("done") // Println executes normally even if there is a panic if x := recover(); x != nil { log.Printf("run time panic: %v", x) } }() log.Println("start") g() } </pre> <h3 id="Bootstrapping">Bootstrapping</h3> <p> Current implementations provide several built-in functions useful during bootstrapping. These functions are documented for completeness but are not guaranteed to stay in the language. They do not return a result. </p> <pre class="grammar"> Function Behavior print prints all arguments; formatting of arguments is implementation-specific println like print but prints spaces between arguments and a newline at the end </pre> <p> Implementation restriction: <code>print</code> and <code>println</code> need not accept arbitrary argument types, but printing of boolean, numeric, and string <a href="#Types">types</a> must be supported. </p> <h2 id="Packages">Packages</h2> <p> Go programs are constructed by linking together <i>packages</i>. A package in turn is constructed from one or more source files that together declare constants, types, variables and functions belonging to the package and which are accessible in all files of the same package. Those elements may be <a href="#Exported_identifiers">exported</a> and used in another package. </p> <h3 id="Source_file_organization">Source file organization</h3> <p> Each source file consists of a package clause defining the package to which it belongs, followed by a possibly empty set of import declarations that declare packages whose contents it wishes to use, followed by a possibly empty set of declarations of functions, types, variables, and constants. </p> <pre class="ebnf"> SourceFile = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } . </pre> <h3 id="Package_clause">Package clause</h3> <p> A package clause begins each source file and defines the package to which the file belongs. </p> <pre class="ebnf"> PackageClause = "package" PackageName . PackageName = identifier . </pre> <p> The PackageName must not be the <a href="#Blank_identifier">blank identifier</a>. </p> <pre> package math </pre> <p> A set of files sharing the same PackageName form the implementation of a package. An implementation may require that all source files for a package inhabit the same directory. </p> <h3 id="Import_declarations">Import declarations</h3> <p> An import declaration states that the source file containing the declaration depends on functionality of the <i>imported</i> package (<a href="#Program_initialization_and_execution">§Program initialization and execution</a>) and enables access to <a href="#Exported_identifiers">exported</a> identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported. </p> <pre class="ebnf"> ImportDecl = "import" ( ImportSpec \| "(" { ImportSpec ";" } ")" ) . ImportSpec = [ "." \| PackageName ] ImportPath . ImportPath = string_lit . </pre> <p> The PackageName is used in <a href="#Qualified_identifiers">qualified identifiers</a> to access exported identifiers of the package within the importing source file. It is declared in the <a href="#Blocks">file block</a>. If the PackageName is omitted, it defaults to the identifier specified in the <a href="#Package_clause">package clause</a> of the imported package. If an explicit period (<code>.</code>) appears instead of a name, all the package's exported identifiers declared in that package's <a href="#Blocks">package block</a> will be declared in the importing source file's file block and must be accessed without a qualifier. </p> <p> The interpretation of the ImportPath is implementation-dependent but it is typically a substring of the full file name of the compiled package and may be relative to a repository of installed packages. </p> <p> Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging to <a href="https://www.unicode.org/versions/Unicode6.3.0/">Unicode's</a> L, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters <code>!"#$%&'(),:;<=>?[\]^`{\|}</code> and the Unicode replacement character U+FFFD. </p> <p> Consider a compiled a package containing the package clause <code>package math</code>, which exports function <code>Sin</code>, and installed the compiled package in the file identified by <code>"lib/math"</code>. This table illustrates how <code>Sin</code> is accessed in files that import the package after the various types of import declaration. </p> <pre class="grammar"> Import declaration Local name of Sin import "lib/math" math.Sin import m "lib/math" m.Sin import . "lib/math" Sin </pre> <p> An import declaration declares a dependency relation between the importing and imported package. It is illegal for a package to import itself, directly or indirectly, or to directly import a package without referring to any of its exported identifiers. To import a package solely for its side-effects (initialization), use the <a href="#Blank_identifier">blank</a> identifier as explicit package name: </p> <pre> import _ "lib/math" </pre> <h3 id="An_example_package">An example package</h3> <p> Here is a complete Go package that implements a concurrent prime sieve. </p> <pre> package main import "fmt" // Send the sequence 2, 3, 4, … to channel 'ch'. func generate(ch chan<- int) { for i := 2; ; i++ { ch <- i // Send 'i' to channel 'ch'. } } // Copy the values from channel 'src' to channel 'dst', // removing those divisible by 'prime'. func filter(src <-chan int, dst chan<- int, prime int) { for i := range src { // Loop over values received from 'src'. if i%prime != 0 { dst <- i // Send 'i' to channel 'dst'. } } } // The prime sieve: Daisy-chain filter processes together. func sieve() { ch := make(chan int) // Create a new channel. go generate(ch) // Start generate() as a subprocess. for { prime := <-ch fmt.Print(prime, "\n") ch1 := make(chan int) go filter(ch, ch1, prime) ch = ch1 } } func main() { sieve() } </pre> <h2 id="Program_initialization_and_execution">Program initialization and execution</h2> <h3 id="The_zero_value">The zero value</h3> <p> When storage is allocated for a <a href="#Variables">variable</a>, either through a declaration or a call of <code>new</code>, or when a new value is created, either through a composite literal or a call of <code>make</code>, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the <i>zero value</i> for its type: <code>false</code> for booleans, <code>0</code> for numeric types, <code>""</code> for strings, and <code>nil</code> for pointers, functions, interfaces, slices, channels, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified. </p> <p> These two simple declarations are equivalent: </p> <pre> var i int var i int = 0 </pre> <p> After </p> <pre> type T struct { i int; f float64; next T } t := new(T) </pre> <p> the following holds: </p> <pre> t.i == 0 t.f == 0.0 t.next == nil </pre> <p> The same would also be true after </p> <pre> var t T </pre> <h3 id="Package_initialization">Package initialization</h3> <p> Within a package, package-level variable initialization proceeds stepwise, with each step selecting the variable earliest in <i>declaration order</i> which has no dependencies on uninitialized variables. </p> <p> More precisely, a package-level variable is considered <i>ready for initialization</i> if it is not yet initialized and either has no <a href="#Variable_declarations">initialization expression</a> or its initialization expression has no <i>dependencies</i> on uninitialized variables. Initialization proceeds by repeatedly initializing the next package-level variable that is earliest in declaration order and ready for initialization, until there are no variables ready for initialization. </p> <p> If any variables are still uninitialized when this process ends, those variables are part of one or more initialization cycles, and the program is not valid. </p> <p> Multiple variables on the left-hand side of a variable declaration initialized by single (multi-valued) expression on the right-hand side are initialized together: If any of the variables on the left-hand side is initialized, all those variables are initialized in the same step. </p> <pre> var x = a var a, b = f() // a and b are initialized together, before x is initialized </pre> <p> For the purpose of package initialization, <a href="#Blank_identifier">blank</a> variables are treated like any other variables in declarations. </p> <p> The declaration order of variables declared in multiple files is determined by the order in which the files are presented to the compiler: Variables declared in the first file are declared before any of the variables declared in the second file, and so on. To ensure reproducible initialization behavior, build systems are encouraged to present multiple files belonging to the same package in lexical file name order to a compiler. </p> <p> Dependency analysis does not rely on the actual values of the variables, only on lexical <i>references</i> to them in the source, analyzed transitively. For instance, if a variable <code>x</code>'s initialization expression refers to a function whose body refers to variable <code>y</code> then <code>x</code> depends on <code>y</code>. Specifically: </p> <ul> <li> A reference to a variable or function is an identifier denoting that variable or function. </li> <li> A reference to a method <code>m</code> is a <a href="#Method_values">method value</a> or <a href="#Method_expressions">method expression</a> of the form <code>t.m</code>, where the (static) type of <code>t</code> is not an interface type, and the method <code>m</code> is in the <a href="#Method_sets">method set</a> of <code>t</code>. It is immaterial whether the resulting function value <code>t.m</code> is invoked. </li> <li> A variable, function, or method <code>x</code> depends on a variable <code>y</code> if <code>x</code>'s initialization expression or body (for functions and methods) contains a reference to <code>y</code> or to a function or method that depends on <code>y</code>. </li> </ul> <p> For example, given the declarations </p> <pre> var ( a = c + b // == 9 b = f() // == 4 c = f() // == 5 d = 3 // == 5 after initialization has finished ) func f() int { d++ return d } </pre> <p> the initialization order is <code>d</code>, <code>b</code>, <code>c</code>, <code>a</code>. Note that the order of subexpressions in initialization expressions is irrelevant: <code>a = c + b</code> and <code>a = b + c</code> result in the same initialization order in this example. </p> <p> Dependency analysis is performed per package; only references referring to variables, functions, and (non-interface) methods declared in the current package are considered. If other, hidden, data dependencies exists between variables, the initialization order between those variables is unspecified. </p> <p> For instance, given the declarations </p> <pre> var x = I(T{}).ab() // x has an undetected, hidden dependency on a and b var _ = sideEffect() // unrelated to x, a, or b var a = b var b = 42 type I interface { ab() []int } type T struct{} func (T) ab() []int { return []int{a, b} } </pre> <p> the variable <code>a</code> will be initialized after <code>b</code> but whether <code>x</code> is initialized before <code>b</code>, between <code>b</code> and <code>a</code>, or after <code>a</code>, and thus also the moment at which <code>sideEffect()</code> is called (before or after <code>x</code> is initialized) is not specified. </p> <p> Variables may also be initialized using functions named <code>init</code> declared in the package block, with no arguments and no result parameters. </p> <pre> func init() { … } </pre> <p> Multiple such functions may be defined per package, even within a single source file. In the package block, the <code>init</code> identifier can be used only to declare <code>init</code> functions, yet the identifier itself is not <a href="#Declarations_and_scope">declared</a>. Thus <code>init</code> functions cannot be referred to from anywhere in a program. </p> <p> The entire package is initialized by assigning initial values to all its package-level variables followed by calling all <code>init</code> functions in the order they appear in the source, possibly in multiple files, as presented to the compiler. </p> <h3 id="Program_initialization">Program initialization</h3> <p> The packages of a complete program are initialized stepwise, one package at a time. If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package, the imported package will be initialized only once. The importing of packages, by construction, guarantees that there can be no cyclic initialization dependencies. More precisely: </p> <p> Given the list of all packages, sorted by import path, in each step the first uninitialized package in the list for which all imported packages (if any) are already initialized is <a href="#Package_initialization">initialized</a>. This step is repeated until all packages are initialized. </p> <p> Package initialization—variable initialization and the invocation of <code>init</code> functions—happens in a single goroutine, sequentially, one package at a time. An <code>init</code> function may launch other goroutines, which can run concurrently with the initialization code. However, initialization always sequences the <code>init</code> functions: it will not invoke the next one until the previous one has returned. </p> <h3 id="Program_execution">Program execution</h3> <p> A complete program is created by linking a single, unimported package called the <i>main package</i> with all the packages it imports, transitively. The main package must have package name <code>main</code> and declare a function <code>main</code> that takes no arguments and returns no value. </p> <pre> func main() { … } </pre> <p> Program execution begins by <a href="#Program_initialization">initializing the program</a> and then invoking the function <code>main</code> in package <code>main</code>. When that function invocation returns, the program exits. It does not wait for other (non-<code>main</code>) goroutines to complete. </p> <h2 id="Errors">Errors</h2> <p> The predeclared type <code>error</code> is defined as </p> <pre> type error interface { Error() string } </pre> <p> It is the conventional interface for representing an error condition, with the nil value representing no error. For instance, a function to read data from a file might be defined: </p> <pre> func Read(f File, b []byte) (n int, err error) </pre> <h2 id="Run_time_panics">Run-time panics</h2> <p> Execution errors such as attempting to index an array out of bounds trigger a <i>run-time panic</i> equivalent to a call of the built-in function <a href="#Handling_panics"><code>panic</code></a> with a value of the implementation-defined interface type <code>runtime.Error</code>. That type satisfies the predeclared interface type <a href="#Errors"><code>error</code></a>. The exact error values that represent distinct run-time error conditions are unspecified. </p> <pre> package runtime type Error interface { error // and perhaps other methods } </pre> <h2 id="System_considerations">System considerations</h2> <h3 id="Package_unsafe">Package <code>unsafe</code></h3> <p> The built-in package <code>unsafe</code>, known to the compiler and accessible through the <a href="#Import_declarations">import path</a> <code>"unsafe"</code>, provides facilities for low-level programming including operations that violate the type system. A package using <code>unsafe</code> must be vetted manually for type safety and may not be portable. The package provides the following interface: </p> <pre class="grammar"> package unsafe type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type type Pointer ArbitraryType func Alignof(variable ArbitraryType) uintptr func Offsetof(selector ArbitraryType) uintptr func Sizeof(variable ArbitraryType) uintptr type IntegerType int // shorthand for an integer type; it is not a real type func Add(ptr Pointer, len IntegerType) Pointer func Slice(ptr ArbitraryType, len IntegerType) []ArbitraryType func SliceData(slice []ArbitraryType) ArbitraryType func String(ptr byte, len IntegerType) string func StringData(str string) byte </pre> <!-- These conversions also apply to type parameters with suitable core types. Determine if we can simply use core type instead of underlying type here, of if the general conversion rules take care of this. --> <p> A <code>Pointer</code> is a <a href="#Pointer_types">pointer type</a> but a <code>Pointer</code> value may not be <a href="#Address_operators">dereferenced</a>. Any pointer or value of <a href="#Core_types">core type</a> <code>uintptr</code> can be <a href="#Conversions">converted</a> to a type of core type <code>Pointer</code> and vice versa. The effect of converting between <code>Pointer</code> and <code>uintptr</code> is implementation-defined. </p> <pre> var f float64 bits = (uint64)(unsafe.Pointer(&f)) type ptr unsafe.Pointer bits = (uint64)(ptr(&f)) func f[P ~B, B any](p P) uintptr { return uintptr(unsafe.Pointer(p)) } var p ptr = nil </pre> <p> The functions <code>Alignof</code> and <code>Sizeof</code> take an expression <code>x</code> of any type and return the alignment or size, respectively, of a hypothetical variable <code>v</code> as if <code>v</code> was declared via <code>var v = x</code>. </p> <p> The function <code>Offsetof</code> takes a (possibly parenthesized) <a href="#Selectors">selector</a> <code>s.f</code>, denoting a field <code>f</code> of the struct denoted by <code>s</code> or <code>s</code>, and returns the field offset in bytes relative to the struct's address. If <code>f</code> is an <a href="#Struct_types">embedded field</a>, it must be reachable without pointer indirections through fields of the struct. For a struct <code>s</code> with field <code>f</code>: </p> <pre> uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f)) </pre> <p> Computer architectures may require memory addresses to be <i>aligned</i>; that is, for addresses of a variable to be a multiple of a factor, the variable's type's <i>alignment</i>. The function <code>Alignof</code> takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable <code>x</code>: </p> <pre> uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0 </pre> <p> A (variable of) type <code>T</code> has <i>variable size</i> if <code>T</code> is a <a href="#Type_parameter_declarations">type parameter</a>, or if it is an array or struct type containing elements or fields of variable size. Otherwise the size is <i>constant</i>. Calls to <code>Alignof</code>, <code>Offsetof</code>, and <code>Sizeof</code> are compile-time <a href="#Constant_expressions">constant expressions</a> of type <code>uintptr</code> if their arguments (or the struct <code>s</code> in the selector expression <code>s.f</code> for <code>Offsetof</code>) are types of constant size. </p> <p> The function <code>Add</code> adds <code>len</code> to <code>ptr</code> and returns the updated pointer <code>unsafe.Pointer(uintptr(ptr) + uintptr(len))</code> [<a href="#Go_1.17">Go 1.17</a>]. The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>. A constant <code>len</code> argument must be <a href="#Representability">representable</a> by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>. The rules for <a href="/pkg/unsafe#Pointer">valid uses</a> of <code>Pointer</code> still apply. </p> <p> The function <code>Slice</code> returns a slice whose underlying array starts at <code>ptr</code> and whose length and capacity are <code>len</code>. <code>Slice(ptr, len)</code> is equivalent to </p> <pre> ([len]ArbitraryType)(unsafe.Pointer(ptr))[:] </pre> <p> except that, as a special case, if <code>ptr</code> is <code>nil</code> and <code>len</code> is zero, <code>Slice</code> returns <code>nil</code> [<a href="#Go_1.17">Go 1.17</a>]. </p> <p> The <code>len</code> argument must be of <a href="#Numeric_types">integer type</a> or an untyped <a href="#Constants">constant</a>. A constant <code>len</code> argument must be non-negative and <a href="#Representability">representable</a> by a value of type <code>int</code>; if it is an untyped constant it is given type <code>int</code>. At run time, if <code>len</code> is negative, or if <code>ptr</code> is <code>nil</code> and <code>len</code> is not zero, a <a href="#Run_time_panics">run-time panic</a> occurs [<a href="#Go_1.17">Go 1.17</a>]. </p> <p> The function <code>SliceData</code> returns a pointer to the underlying array of the <code>slice</code> argument. If the slice's capacity <code>cap(slice)</code> is not zero, that pointer is <code>&slice[:1][0]</code>. If <code>slice</code> is <code>nil</code>, the result is <code>nil</code>. Otherwise it is a non-<code>nil</code> pointer to an unspecified memory address [<a href="#Go_1.20">Go 1.20</a>]. </p> <p> The function <code>String</code> returns a <code>string</code> value whose underlying bytes start at <code>ptr</code> and whose length is <code>len</code>. The same requirements apply to the <code>ptr</code> and <code>len</code> argument as in the function <code>Slice</code>. If <code>len</code> is zero, the result is the empty string <code>""</code>. Since Go strings are immutable, the bytes passed to <code>String</code> must not be modified afterwards. [<a href="#Go_1.20">Go 1.20</a>] </p> <p> The function <code>StringData</code> returns a pointer to the underlying bytes of the <code>str</code> argument. For an empty string the return value is unspecified, and may be <code>nil</code>. Since Go strings are immutable, the bytes returned by <code>StringData</code> must not be modified [<a href="#Go_1.20">Go 1.20</a>]. </p> <h3 id="Size_and_alignment_guarantees">Size and alignment guarantees</h3> <p> For the <a href="#Numeric_types">numeric types</a>, the following sizes are guaranteed: </p> <pre class="grammar"> type size in bytes byte, uint8, int8 1 uint16, int16 2 uint32, int32, float32 4 uint64, int64, float64, complex64 8 complex128 16 </pre> <p> The following minimal alignment properties are guaranteed: </p> <ol> <li>For a variable <code>x</code> of any type: <code>unsafe.Alignof(x)</code> is at least 1. </li> <li>For a variable <code>x</code> of struct type: <code>unsafe.Alignof(x)</code> is the largest of all the values <code>unsafe.Alignof(x.f)</code> for each field <code>f</code> of <code>x</code>, but at least 1. </li> <li>For a variable <code>x</code> of array type: <code>unsafe.Alignof(x)</code> is the same as the alignment of a variable of the array's element type. </li> </ol> <p> A struct or array type has size zero if it contains no fields (or elements, respectively) that have a size greater than zero. Two distinct zero-size variables may have the same address in memory. </p> <h2 id="Appendix">Appendix</h2> <h3 id="Language_versions">Language versions</h3> <p> The <a href="/doc/go1compat">Go 1 compatibility guarantee</a> ensures that programs written to the Go 1 specification will continue to compile and run correctly, unchanged, over the lifetime of that specification. More generally, as adjustments are made and features added to the language, the compatibility guarantee ensures that a Go program that works with a specific Go language version will continue to work with any subsequent version. </p> <p> For instance, the ability to use the prefix <code>0b</code> for binary integer literals was introduced with Go 1.13, indicated by [<a href="#Go_1.13">Go 1.13</a>] in the section on <a href="#Integer_literals">integer literals</a>. Source code containing an integer literal such as <code>0b1011</code> will be rejected if the implied or required language version used by the compiler is older than Go 1.13. </p> <p> The following table describes the minimum language version required for features introduced after Go 1. </p> <h4 id="Go_1.9">Go 1.9</h4> <ul> <li> An <a href="#Alias_declarations">alias declaration</a> may be used to declare an alias name for a type. </li> </ul> <h4 id="Go_1.13">Go 1.13</h4> <ul> <li> <a href="#Integer_literals">Integer literals</a> may use the prefixes <code>0b</code>, <code>0B</code>, <code>0o</code>, and <code>0O</code> for binary, and octal literals, respectively. </li> <li> Hexadecimal <a href="#Floating-point_literals">floating-point literals</a> may be written using the prefixes <code>0x</code> and <code>0X</code>. </li> <li> The <a href="#Imaginary_literals">imaginary suffix</a> <code>i</code> may be used with any (binary, decimal, hexadecimal) integer or floating-point literal, not just decimal literals. </li> <li> The digits of any number literal may be <a href="#Integer_literals">separated</a> (grouped) using underscores <code>_</code>. </li> <li> The shift count in a <a href="#Operators">shift operation</a> may be a signed integer type. </li> </ul> <h4 id="Go_1.14">Go 1.14</h4> <ul> <li> Emdedding a method more than once through different <a href="#Embedded_interfaces">embedded interfaces</a> is not an error. </li> </ul> <h4 id="Go_1.17">Go 1.17</h4> <ul> <li> A slice may be <a href="#Conversions">converted</a> to an array pointer if the slice and array element types match, and the array is not longer than the slice. </li> <li> The built-in <a href="#Package_unsafe">package <code>unsafe</code></a> includes the new functions <code>Add</code> and <code>Slice</code>. </li> </ul> <h4 id="Go_1.18">Go 1.18</h4> <p> The 1.18 release adds polymorphic functions and types ("generics") to the language. Specifically: </p> <ul> <li> The set of <a href="#Operators_and_punctuation">operators and punctuation</a> includes the new token <code>~</code>. </li> <li> Function and type declarations may declare <a href="#Type_parameter_declarations">type parameters</a>. </li> <li> Interface types may <a href="#General_interfaces">embed arbitrary types</a> (not just type names of interfaces) as well as union and <code>~T</code> type elements. </li> <li> The set of <a href="#Predeclared_identifiers">predeclared</a> types includes the new types <code>any</code> and <code>comparable</code>. </li> </ul> <h4 id="Go_1.20">Go 1.20</h4> <ul> <li> A slice may be <a href="#Conversions">converted</a> to an array if the slice and array element types match and the array is not longer than the slice. </li> <li> The built-in <a href="#Package_unsafe">package <code>unsafe</code></a> includes the new functions <code>SliceData</code>, <code>String</code>, and <code>StringData</code>. </li> <li> <a href="#Comparison_operators">Comparable types</a> (such as ordinary interfaces) may satisfy <code>comparable</code> constraints, even if the type arguments are not strictly comparable. </li> </ul> <h4 id="Go_1.21">Go 1.21</h4> <ul> <li> The set of <a href="#Predeclared_identifiers">predeclared</a> functions includes the new functions <code>min</code>, <code>max</code>, and <code>clear</code>. </li> <li> <a href="#Type_inference">Type inference</a> uses the types of interface methods for inference. It also infers type arguments for generic functions assigned to variables or passed as arguments to other (possibly generic) functions. </li> </ul> <h4 id="Go_1.22">Go 1.22</h4> <ul> <li> In a <a href="#For_statements">"for" statement</a>, each iteration has its own set of iteration variables rather than sharing the same variables in each iteration. </li> <li> A "for" statement with <a href="#For_range">"range" clause</a> may iterate over integer values from zero to an upper limit. </li> </ul> <h3 id="Type_unification_rules">Type unification rules</h3> <p> The type unification rules describe if and how two types unify. The precise details are relevant for Go implementations, affect the specifics of error messages (such as whether a compiler reports a type inference or other error), and may explain why type inference fails in unusual code situations. But by and large these rules can be ignored when writing Go code: type inference is designed to mostly "work as expected", and the unification rules are fine-tuned accordingly. </p> <p> Type unification is controlled by a <i>matching mode</i>, which may be <i>exact</i> or <i>loose</i>. As unification recursively descends a composite type structure, the matching mode used for elements of the type, the <i>element matching mode</i>, remains the same as the matching mode except when two types are unified for <a href="#Assignability">assignability</a> (<code>≡<sub>A</sub></code>): in this case, the matching mode is <i>loose</i> at the top level but then changes to <i>exact</i> for element types, reflecting the fact that types don't have to be identical to be assignable. </p> <p> Two types that are not bound type parameters unify exactly if any of following conditions is true: </p> <ul> <li> Both types are <a href="#Type_identity">identical</a>. </li> <li> Both types have identical structure and their element types unify exactly. </li> <li> Exactly one type is an <a href="#Type_inference">unbound</a> type parameter with a <a href="#Core_types">core type</a>, and that core type unifies with the other type per the unification rules for <code>≡<sub>A</sub></code> (loose unification at the top level and exact unification for element types). </li> </ul> <p> If both types are bound type parameters, they unify per the given matching modes if: </p> <ul> <li> Both type parameters are identical. </li> <li> At most one of the type parameters has a known type argument. In this case, the type parameters are <i>joined</i>: they both stand for the same type argument. If neither type parameter has a known type argument yet, a future type argument inferred for one the type parameters is simultaneously inferred for both of them. </li> <li> Both type parameters have a known type argument and the type arguments unify per the given matching modes. </li> </ul> <p> A single bound type parameter <code>P</code> and another type <code>T</code> unify per the given matching modes if: </p> <ul> <li> <code>P</code> doesn't have a known type argument. In this case, <code>T</code> is inferred as the type argument for <code>P</code>. </li> <li> <code>P</code> does have a known type argument <code>A</code>, <code>A</code> and <code>T</code> unify per the given matching modes, and one of the following conditions is true: <ul> <li> Both <code>A</code> and <code>T</code> are interface types: In this case, if both <code>A</code> and <code>T</code> are also <a href="#Type_definitions">defined</a> types, they must be <a href="#Type_identity">identical</a>. Otherwise, if neither of them is a defined type, they must have the same number of methods (unification of <code>A</code> and <code>T</code> already established that the methods match). </li> <li> Neither <code>A</code> nor <code>T</code> are interface types: In this case, if <code>T</code> is a defined type, <code>T</code> replaces <code>A</code> as the inferred type argument for <code>P</code>. </li> </ul> </li> </ul> <p> Finally, two types that are not bound type parameters unify loosely (and per the element matching mode) if: </p> <ul> <li> Both types unify exactly. </li> <li> One type is a <a href="#Type_definitions">defined type</a>, the other type is a type literal, but not an interface, and their underlying types unify per the element matching mode. </li> <li> Both types are interfaces (but not type parameters) with identical <a href="#Interface_types">type terms</a>, both or neither embed the predeclared type <a href="#Predeclared_identifiers">comparable</a>, corresponding method types unify exactly, and the method set of one of the interfaces is a subset of the method set of the other interface. </li> <li> Only one type is an interface (but not a type parameter), corresponding methods of the two types unify per the element matching mode, and the method set of the interface is a subset of the method set of the other type. </li> <li> Both types have the same structure and their element types unify per the element matching mode. </li> </ul>	279.32 KB	2024-02-02 18:09
📄 godebug.md --- title: "Go, Backwards Compatibility, and GODEBUG" layout: article --- <!-- This document is kept in the Go repo, not x/website, because it documents the full list of known GODEBUG settings, which are tied to a specific release. --> ## Introduction {#intro} Go's emphasis on backwards compatibility is one of its key strengths. There are, however, times when we cannot maintain complete compatibility. If code depends on buggy (including insecure) behavior, then fixing the bug will break that code. New features can also have similar impacts: enabling the HTTP/2 use by the HTTP client broke programs connecting to servers with buggy HTTP/2 implementations. These kinds of changes are unavoidable and [permitted by the Go 1 compatibility rules](/doc/go1compat). Even so, Go provides a mechanism called GODEBUG to reduce the impact such changes have on Go developers using newer toolchains to compile old code. A GODEBUG setting is a `key=value` pair that controls the execution of certain parts of a Go program. The environment variable `GODEBUG` can hold a comma-separated list of these settings. For example, if a Go program is running in an environment that contains GODEBUG=http2client=0,http2server=0 then that Go program will disable the use of HTTP/2 by default in both the HTTP client and the HTTP server. It is also possible to set the default `GODEBUG` for a given program (discussed below). When preparing any change that is permitted by Go 1 compatibility but may nonetheless break some existing programs, we first engineer the change to keep as many existing programs working as possible. For the remaining programs, we define a new GODEBUG setting that allows individual programs to opt back in to the old behavior. A GODEBUG setting may not be added if doing so is infeasible, but that should be extremely rare. GODEBUG settings added for compatibility will be maintained for a minimum of two years (four Go releases). Some, such as `http2client` and `http2server`, will be maintained much longer, even indefinitely. When possible, each GODEBUG setting has an associated [runtime/metrics](/pkg/runtime/metrics/) counter named `/godebug/non-default-behavior/<name>:events` that counts the number of times a particular program's behavior has changed based on a non-default value for that setting. For example, when `GODEBUG=http2client=0` is set, `/godebug/non-default-behavior/http2client:events` counts the number of HTTP transports that the program has configured without HTTP/2 support. ## Default GODEBUG Values {#default} When a GODEBUG setting is not listed in the environment variable, its value is derived from three sources: the defaults for the Go toolchain used to build the program, amended to match the Go version listed in `go.mod`, and then overridden by explicit `//go:debug` lines in the program. The [GODEBUG History](#history) gives the exact defaults for each Go toolchain version. For example, Go 1.21 introduces the `panicnil` setting, controlling whether `panic(nil)` is allowed; it defaults to `panicnil=0`, making `panic(nil)` a run-time error. Using `panicnil=1` restores the behavior of Go 1.20 and earlier. When compiling a work module or workspace that declares an older Go version, the Go toolchain amends its defaults to match that older Go version as closely as possible. For example, when a Go 1.21 toolchain compiles a program, if the work module's `go.mod` or the workspace's `go.work` says `go` `1.20`, then the program defaults to `panicnil=1`, matching Go 1.20 instead of Go 1.21. Because this method of setting GODEBUG defaults was introduced only in Go 1.21, programs listing versions of Go earlier than Go 1.20 are configured to match Go 1.20, not the older version. To override these defaults, a main package's source files can include one or more `//go:debug` directives at the top of the file (preceding the `package` statement). Continuing the `panicnil` example, if the module or workspace is updated to say `go` `1.21`, the program can opt back into the old `panic(nil)` behavior by including this directive: //go:debug panicnil=1 Starting in Go 1.21, the Go toolchain treats a `//go:debug` directive with an unrecognized GODEBUG setting as an invalid program. Programs with more than one `//go:debug` line for a given setting are also treated as invalid. (Older toolchains ignore `//go:debug` directives entirely.) The defaults that will be compiled into a main package are reported by the command: {{raw ` go list -f '{{.DefaultGODEBUG}}' my/main/package `}} Only differences from the base Go toolchain defaults are reported. When testing a package, `//go:debug` lines in the `*_test.go` files are treated as directives for the test's main package. In any other context, `//go:debug` lines are ignored by the toolchain; `go` `vet` reports such lines as misplaced. ## GODEBUG History {#history} This section documents the GODEBUG settings introduced and removed in each major Go release for compatibility reasons. Packages or programs may define additional settings for internal debugging purposes; for example, see the [runtime documentation](/pkg/runtime#hdr-Environment_Variables) and the [go command documentation](/cmd/go#hdr-Build_and_test_caching). ### Go 1.22 Go 1.22 adds a configurable limit to control the maximum acceptable RSA key size that can be used in TLS handshakes, controlled by the [`tlsmaxrsasize` setting](/pkg/crypto/tls#Conn.Handshake). The default is tlsmaxrsasize=8192, limiting RSA to 8192-bit keys. To avoid denial of service attacks, this setting and default was backported to Go 1.19.13, Go 1.20.8, and Go 1.21.1. Go 1.22 made it an error for a request or response read by a net/http client or server to have an empty Content-Length header. This behavior is controlled by the `httplaxcontentlength` setting. Go 1.22 changed the behavior of ServeMux to accept extended patterns and unescape both patterns and request paths by segment. This behavior can be controlled by the [`httpmuxgo121` setting](/pkg/net/http/#ServeMux). Go 1.22 added the [Alias type](/pkg/go/types#Alias) to [go/types](/pkg/go/types) for the explicit representation of [type aliases](/ref/spec#Type_declarations). Whether the type checker produces `Alias` types or not is controlled by the [`gotypesalias` setting](/pkg/go/types#Alias). For Go 1.22 it defaults to `gotypesalias=0`. For Go 1.23, `gotypealias=1` will become the default. This setting will be removed in a future release, Go 1.24 at the earliest. Go 1.22 changed the default minimum TLS version supported by both servers and clients to TLS 1.2. The default can be reverted to TLS 1.0 using the [`tls10server` setting](/pkg/crypto/tls/#Config). Go 1.22 changed the default TLS cipher suites used by clients and servers when not explicitly configured, removing the cipher suites which used RSA based key exchange. The default can be revert using the [`tlsrsakex` setting](/pkg/crypto/tls/#Config). Go 1.22 disabled [`ConnectionState.ExportKeyingMaterial`](/pkg/crypto/tls/#ConnectionState.ExportKeyingMaterial) when the connection supports neither TLS 1.3 nor Extended Master Secret (implemented in Go 1.21). It can be reenabled with the [`tlsunsafeekm` setting](/pkg/crypto/tls/#ConnectionState.ExportKeyingMaterial). Go 1.22 changed how the runtime interacts with transparent huge pages on Linux. In particular, a common default Linux kernel configuration can result in significant memory overheads, and Go 1.22 no longer works around this default. To work around this issue without adjusting kernel settings, transparent huge pages can be disabled for Go memory with the [`disablethp` setting](/pkg/runtime#hdr-Environment_Variable). This behavior was backported to Go 1.21.1, but the setting is only available starting with Go 1.21.6. This setting may be removed in a future release, and users impacted by this issue should adjust their Linux configuration according to the recommendations in the [GC guide](/doc/gc-guide#Linux_transparent_huge_pages), or switch to a Linux distribution that disables transparent huge pages altogether. Go 1.22 added contention on runtime-internal locks to the [`mutex` profile](/pkg/runtime/pprof#Profile). Contention on these locks is always reported at `runtime._LostContendedRuntimeLock`. Complete stack traces of runtime locks can be enabled with the [`runtimecontentionstacks` setting](/pkg/runtime#hdr-Environment_Variable). These stack traces have non-standard semantics, see setting documentation for details. Go 1.22 added a new [`crypto/x509.Certificate`](/pkg/crypto/x509/#Certificate) field, [`Policies`](/pkg/crypto/x509/#Certificate.Policies), which supports certificate policy OIDs with components larger than 31 bits. By default this field is only used during parsing, when it is populated with policy OIDs, but not used during marshaling. It can be used to marshal these larger OIDs, instead of the existing PolicyIdentifiers field, by using the [`x509usepolicies` setting.](/pkg/crypto/x509/#CreateCertificate). ### Go 1.21 Go 1.21 made it a run-time error to call `panic` with a nil interface value, controlled by the [`panicnil` setting](/pkg/builtin/#panic). Go 1.21 made it an error for html/template actions to appear inside of an ECMAScript 6 template literal, controlled by the [`jstmpllitinterp` setting](/pkg/html/template#hdr-Security_Model). This behavior was backported to Go 1.19.8+ and Go 1.20.3+. Go 1.21 introduced a limit on the maximum number of MIME headers and multipart forms, controlled by the [`multipartmaxheaders` and `multipartmaxparts` settings](/pkg/mime/multipart#hdr-Limits) respectively. This behavior was backported to Go 1.19.8+ and Go 1.20.3+. Go 1.21 adds the support of Multipath TCP but it is only used if the application explicitly asked for it. This behavior can be controlled by the [`multipathtcp` setting](/pkg/net#Dialer.SetMultipathTCP). There is no plan to remove any of these settings. ### Go 1.20 Go 1.20 introduced support for rejecting insecure paths in tar and zip archives, controlled by the [`tarinsecurepath` setting](/pkg/archive/tar/#Reader.Next) and the [`zipinsecurepath` setting](/pkg/archive/zip/#NewReader). These default to `tarinsecurepath=1` and `zipinsecurepath=1`, preserving the behavior of earlier versions of Go. A future version of Go may change the defaults to `tarinsecurepath=0` and `zipinsecurepath=0`. Go 1.20 introduced automatic seeding of the [`math/rand`](/pkg/math/rand) global random number generator, controlled by the [`randautoseed` setting](/pkg/math/rand/#Seed). Go 1.20 introduced the concept of fallback roots for use during certificate verification, controlled by the [`x509usefallbackroots` setting](/pkg/crypto/x509/#SetFallbackRoots). Go 1.20 removed the preinstalled `.a` files for the standard library from the Go distribution. Installations now build and cache the standard library like packages in other modules. The [`installgoroot` setting](/cmd/go#hdr-Compile_and_install_packages_and_dependencies) restores the installation and use of preinstalled `.a` files. There is no plan to remove any of these settings. ### Go 1.19 Go 1.19 made it an error for path lookups to resolve to binaries in the current directory, controlled by the [`execerrdot` setting](/pkg/os/exec#hdr-Executables_in_the_current_directory). There is no plan to remove this setting. ### Go 1.18 Go 1.18 removed support for SHA1 in most X.509 certificates, controlled by the [`x509sha1` setting](/pkg/crypto/x509#InsecureAlgorithmError). This setting will be removed in a future release, Go 1.22 at the earliest. ### Go 1.10 Go 1.10 changed how build caching worked and added test caching, along with the [`gocacheverify`, `gocachehash`, and `gocachetest` settings](/cmd/go/#hdr-Build_and_test_caching). There is no plan to remove these settings. ### Go 1.6 Go 1.6 introduced transparent support for HTTP/2, controlled by the [`http2client`, `http2server`, and `http2debug` settings](/pkg/net/http/#hdr-HTTP_2). There is no plan to remove these settings. ### Go 1.5 Go 1.5 introduced a pure Go DNS resolver, controlled by the [`netdns` setting](/pkg/net/#hdr-Name_Resolution). There is no plan to remove this setting.	11.94 KB	2024-02-02 18:09

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