The Go Programming Language Specification

The Go Programming Language Specification

Version of June 29, 2022

Introduction

This is the reference manual for the Go programming language. The pre-Go1.18 version, without generics, can be found here. For more information and other documents, see golang.org.

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 packages, whose properties allow efficient management of dependencies.

The syntax is compact and simple to parse, allowing for easy analysis by automatic tools such as integrated development environments.

Notation

The syntax is specified using a variant of Extended Backus-Naur Form (EBNF):

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 "}" .

Productions are expressions constructed from terms and the following operators, in increasing precedence:

|   alternation
()  grouping
[]  option (0 or 1 times)
{}  repetition (0 to n times)

""``

a … bab……...

Source code representation

Source code is Unicode text encoded in UTF-8. 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 character to refer to a Unicode code point in the source text.

Each code point is distinct; for instance, uppercase and lowercase letters are different characters.

Implementation restriction: For compatibility with other tools, a compiler may disallow the NUL character (U+0000) in the source text.

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.

Characters

The following terms are used to denote specific Unicode character categories:

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" */ .

In The Unicode Standard 8.0, 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.

Letters and digits

_

letter        = unicode_letter | "_" .
decimal_digit = "0" … "9" .
binary_digit  = "0" | "1" .
octal_digit   = "0" … "7" .
hex_digit     = "0" … "9" | "A" … "F" | "a" … "f" .

Lexical elements

Comments

Comments serve as program documentation. There are two forms:

///**/

A comment cannot start inside a rune or string literal, or inside a comment. A general comment containing no newlines acts like a space. Any other comment acts like a newline.

Tokens

Tokens form the vocabulary of the Go language. There are four classes: identifiers, keywords, operators and punctuation, and literals. White space, 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 semicolon. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.

Semicolons

";"

To reflect idiomatic use, code examples in this document elide semicolons using these rules.

Identifiers

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.

identifier = letter { letter | unicode_digit } .

a
_x9
ThisVariableIsExported
αβ

Some identifiers are predeclared.

Keywords

The following keywords are reserved and may not be used as identifiers.

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

Operators and punctuation

The following character sequences represent operators (including assignment operators) and punctuation:

+    &     +=    &=     &&    ==    !=    (    )
-    |     -=    |=     ||    <     <=    [    ]
*    ^     *=    ^=     <-    >     >=    {    }
/    <<    /=    <<=    ++    =     :=    ,    ;
%    >>    %=    >>=    --    !     ...   .    :
     &^          &^=          ~

Integer literals

0b0B00o0O0x0X0afAF

_

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 } .

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

Floating-point literals

A floating-point literal is a decimal or hexadecimal representation of a floating-point constant.

eE

0x0XpP

_

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 .

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

Imaginary literals

i

imaginary_lit = (decimal_digits | int_lit | float_lit) "i" .

0

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

Rune literals

'x''\n'

'a'a0x61'ä'0xc30xa4a0xe4

\x\u\U\

\u\U0x10FFFF

After a backslash, certain single-character escapes represent special values:

\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)

An unrecognized character following a backslash in a rune literal is illegal.

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" | `\` | "'" | `"` ) .

'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

String literals

A string literal represents a string constant obtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals.

`foo`

"bar"\'\"\\x\377\xFF0xFFÿ\u00FF\U000000FF\xc3\xbf0xc30xbf

string_lit             = raw_string_lit | interpreted_string_lit .
raw_string_lit         = "`" { unicode_char | newline } "`" .
interpreted_string_lit = `"` { unicode_value | byte_value } `"` .

`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

These examples all represent the same string:

"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

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.

Constants

There are boolean constants, rune constants, integer constants, floating-point constants, complex constants, and string constants. Rune, integer, floating-point, and complex constants are collectively called numeric constants.

unsafe.Sizeofcaplenrealimagcomplextruefalse

In general, complex constants are a form of constant expression and are discussed in that section.

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.

truefalseiota

A constant may be given a type explicitly by a constant declaration or conversion, or implicitly when used in a variable declaration or an assignment statement or as an operand in an expression. It is an error if the constant value cannot be represented 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.

i := 0boolruneintfloat64complex128string

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:

• Represent integer constants with at least 256 bits.
• 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.
• Give an error if unable to represent an integer constant precisely.
• Give an error if unable to represent a floating-point or complex constant due to overflow.
• Round to the nearest representable constant if unable to represent a floating-point or complex constant due to limits on precision.

These requirements apply both to literal constants and to the result of evaluating constant expressions.

Variables

A variable is a storage location for holding a value. The set of permissible values is determined by the variable's .

new

Structured variables of array, slice, and struct types have elements and fields that may be addressed individually. Each such element acts like a variable.

newnil

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

A variable's value is retrieved by referring to the variable in an expression; it is the most recent value assigned to the variable. If a variable has not yet been assigned a value, its value is the zero value for its type.

Types

A type determines a set of values together with operations and methods specific to those values. A type may be denoted by a type name, if it has one, which must be followed by type arguments if the type is generic. A type may also be specified using a type literal, which composes a type from existing types.

Type      = TypeName [ TypeArgs ] | TypeLit | "(" Type ")" .
TypeName  = identifier | QualifiedIdent .
TypeArgs  = "[" TypeList [ "," ] "]" .
TypeList  = Type { "," Type } .
TypeLit   = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
            SliceType | MapType | ChannelType .

The language predeclares certain type names. Others are introduced with type declarations or type parameter lists. Composite types—array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals.

Predeclared types, defined types, and type parameters are called named types. An alias denotes a named type if the type given in the alias declaration is a named type.

Boolean types

truefalsebool

Numeric types

An integer, floating-point, or complex type represents the set of integer, floating-point, or complex values, respectively. They are collectively called numeric types. The predeclared architecture-independent numeric types are:

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

The value of an n-bit integer is n bits wide and represented using two's complement arithmetic.

There is also a set of predeclared integer types with implementation-specific sizes:

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

byteuint8runeint32int32int

String types

string

slenlen(s)-1s[i]i&s[i]

Array types

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.

ArrayType   = "[" ArrayLength "]" ElementType .
ArrayLength = Expression .
ElementType = Type .

intalenlen(a)-1

[32]byte
[2*N] struct { x, y int32 }
[1000]*float64
[3][5]int
[2][2][2]float64  // same as [2]([2]([2]float64))

Slice types

nil

SliceType = "[" "]" ElementType .

slenlen(s)-1

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.

acap(a)

Tmakemake

make([]T, length, capacity)

produces the same slice as allocating an array and slicing it, so these two expressions are equivalent:

make([]int, 50, 100)
new([100]int)[0:50]

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.

Struct types

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-blank field names must be unique.

StructType    = "struct" "{" { FieldDecl ";" } "}" .
FieldDecl     = (IdentifierList Type | EmbeddedField) [ Tag ] .
EmbeddedField = [ "*" ] TypeName [ TypeArgs ] .
Tag           = string_lit .

// An empty struct.
struct {}

// A struct with 6 fields.
struct {
	x, y int
	u float32
	_ float32  // padding
	A *[]int
	F func()
}

T*TT

// 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
}

The following declaration is illegal because field names must be unique in a struct type:

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
}

fxx.ff

Promoted fields act like ordinary fields of a struct except that they cannot be used as field names in composite literals of the struct.

ST

STS*ST*S*TS*TS*ST*T

A field declaration may be followed by an optional string literal tag, 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 reflection interface and take part in type identity for structs but are otherwise ignored.

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"`
}

Pointer types

nil

PointerType = "*" BaseType .
BaseType    = Type .

*Point
*[4]int

Function types

nil

FunctionType   = "func" Signature .
Signature      = Parameters [ Result ] .
Result         = Parameters | Type .
Parameters     = "(" [ ParameterList [ "," ] ] ")" .
ParameterList  = ParameterDecl { "," ParameterDecl } .
ParameterDecl  = [ IdentifierList ] [ "..." ] Type .

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-blank names in the signature must be unique. 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.

...

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)

Interface types

nil

InterfaceType  = "interface" "{" { InterfaceElem ";" } "}" .
InterfaceElem  = MethodElem | TypeElem .
MethodElem     = MethodName Signature .
MethodName     = identifier .
TypeElem       = TypeTerm { "|" TypeTerm } .
TypeTerm       = Type | UnderlyingType .
UnderlyingType = "~" Type .

An interface type is specified by a list of interface elements. An interface element is either a method or a type element, where a type element is a union of one or more type terms. A type term is either a single type or a single underlying type.

Basic interfaces

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 method set consists exactly of the methods specified by the interface. Interfaces whose type sets can be defined entirely by a list of methods are called basic interfaces.

// A simple File interface.
interface {
	Read([]byte) (int, error)
	Write([]byte) (int, error)
	Close() error
}

The name of each explicitly specified method must be unique and not blank.

interface {
	String() string
	String() string  // illegal: String not unique
	_(x int)         // illegal: method must have non-blank name
}

S1S2

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

TS1S2FileS1S2S1S2

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 empty interface which stands for the set of all (non-interface) types:

interface{}

any

Locker

type Locker interface {
	Lock()
	Unlock()
}

S1S2

func (p T) Lock() { … }
func (p T) Unlock() { … }

LockerFile

Embedded interfaces

TEETTTTTTE

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
}

When embedding interfaces, methods with the same names must have identical signatures.

type ReadCloser interface {
	Reader   // includes methods of Reader in ReadCloser's method set
	Close()  // illegal: signatures of Reader.Close and Close are different
}

General interfaces

T~TTt1|t2|…|tn

~TTt1|t2|…|tn

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.

By construction, an interface's type set never contains an interface type.

// 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
}

~TTT

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
}

Union elements denote unions of type sets:

// 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
}

TT~TP

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
}

comparablecomparable

Interfaces that are not basic 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.

var x Float                     // illegal: Float is not a basic interface

var x interface{} = Float(nil)  // illegal

type Floatish struct {
	f Float                 // illegal
}

TT

// illegal: Bad cannot embed itself
type Bad interface {
	Bad
}

// illegal: Bad1 cannot embed itself using Bad2
type Bad1 interface {
	Bad2
}
type Bad2 interface {
	Bad1
}

// illegal: Bad3 cannot embed a union containing Bad3
type Bad3 interface {
	~int | ~string | Bad3
}

Implementing an interface

TI

TITTI

TT

Map types

nil

MapType     = "map" "[" KeyType "]" ElementType .
KeyType     = Type .

==!=

map[string]int
map[*T]struct{ x, y float64 }
map[string]interface{}

mlendelete

make

make(map[string]int)
make(map[string]int, 100)

nilnil

Channel types

nil

ChannelType = ( "chan" | "chan" "<-" | "<-" "chan" ) ElementType .

<-

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

<-chan

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)

make

make(chan int, 100)

nil

close

caplen

Properties of types and values

Underlying types

TTTTT

type (
	A1 = string
	A2 = A1
)

type (
	B1 string
	B2 B1
	B3 []B1
	B4 B3
)

func f[P any](x P) { … }

stringA1A2B1B2string[]B1B3B4[]B1Pinterface{}

Core types

TT

T

No other interfaces have a core type.

The core type of an interface is, depending on the condition that is satisfied, either:

Uchan ETchan<- E<-chan E

By definition, a core type is never a defined type, type parameter, or interface type.

Examples of interfaces with core types:

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

Examples of interfaces without core types:

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

appendcopy[]bytestringTTbytestring

bytestring

interface{ int }                          // int (same as ordinary core type)
interface{ []byte | string }              // bytestring
interface{ ~[]byte | myString }           // bytestring

bytestring

Type identity

Two types are either identical or different.

A named type is always different from any other type. Otherwise, two types are identical if their underlying type literals are structurally equivalent; that is, they have the same literal structure and corresponding components have identical types. In detail:

• Two array types are identical if they have identical element types and the same array length.
• Two slice types are identical if they have identical element types.
• 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. Non-exported field names from different packages are always different.
• Two pointer types are identical if they have identical base types.
• 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.
• Two interface types are identical if they define the same type set.
• Two map types are identical if they have identical key and element types.
• Two channel types are identical if they have identical element types and the same direction.
• Two instantiated types are identical if their defined types and all type arguments are identical.

Given the declarations

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]
)

these types are identical:

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

B0B1func(int, float64) *B0func(x int, y float64) *[]stringB0[]stringP1P2D0[int, string]struct{ x int; y string }

Assignability

xVTxT

xVTxT

xnilTxTVTxTVTVT

Representability

xTT

TxTxT

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

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

Method sets

The method set of a type determines the methods that can be called on an operand of that type. Every type has a (possibly empty) method set associated with it:

Further rules apply to structs (and pointer to structs) containing embedded fields, as described in the section on struct types. Any other type has an empty method set.

In a method set, each method must have a unique non-blank method name.

Blocks

A block is a possibly empty sequence of declarations and statements within matching brace brackets.

Block = "{" StatementList "}" .
StatementList = { Statement ";" } .

In addition to explicit blocks in the source code, there are implicit blocks:

Blocks nest and influence scoping.

Declarations and scope

A declaration binds a non-blank identifier to a constant, type, type parameter, variable, function, label, or package. 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.

initinit

Declaration   = ConstDecl | TypeDecl | VarDecl .
TopLevelDecl  = Declaration | FunctionDecl | MethodDecl .

The scope of a declared identifier is the extent of source text in which the identifier denotes the specified constant, type, variable, function, label, or package.

Go is lexically scoped using blocks:

1. The scope of a predeclared identifier is the universe block.
2. 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.
3. The scope of the package name of an imported package is the file block of the file containing the import declaration.
4. The scope of an identifier denoting a method receiver, function parameter, or result variable is the function body.
5. 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.
6. 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.
7. 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.
8. 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.

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.

The package clause is not a declaration; the package name does not appear in any scope. Its purpose is to identify the files belonging to the same package and to specify the default package name for import declarations.

Label scopes

Labels are declared by labeled statements and are used in the "break", "continue", and "goto" 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.

Blank identifier

_

Predeclared identifiers

The following identifiers are implicitly declared in the universe block:

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 close complex copy delete imag len
	make new panic print println real recover

Exported identifiers

An identifier may be exported to permit access to it from another package. An identifier is exported if both:

All other identifiers are not exported.

Uniqueness of identifiers

Given a set of identifiers, an identifier is called unique if it is different from every other in the set. Two identifiers are different if they are spelled differently, or if they appear in different packages and are not exported. Otherwise, they are the same.

Constant declarations

A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of constant expressions. The number of identifiers must be equal to the number of expressions, and the nth identifier on the left is bound to the value of the nth expression on the right.

ConstDecl      = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) .
ConstSpec      = IdentifierList [ [ Type ] "=" ExpressionList ] .

IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .

If the type is present, all constants take the type specified, and the expressions must be assignable 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 constants, 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.

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

constiota

const (
	Sunday = iota
	Monday
	Tuesday
	Wednesday
	Thursday
	Friday
	Partyday
	numberOfDays  // this constant is not exported
)

Iota

iota

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

iota

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)
)

This last example exploits the implicit repetition of the last non-empty expression list.

Type declarations

A type declaration binds an identifier, the type name, to a type. Type declarations come in two forms: alias declarations and type definitions.

TypeDecl = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec = AliasDecl | TypeDef .

Alias declarations

An alias declaration binds an identifier to the given type.

AliasDecl = identifier "=" Type .

Within the scope of the identifier, it serves as an alias for the type.

type (
	nodeList = []*Node  // nodeList and []*Node are identical types
	Polar    = polar    // Polar and polar denote identical types
)

Type definitions

A type definition creates a new, distinct type with the same underlying type and operations as the given type and binds an identifier, the type name, to it.

TypeDef = identifier [ TypeParameters ] Type .

The new type is called a defined type. It is different from any other type, including the type it is created from.

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)
}

A defined type may have methods associated with it. It does not inherit any methods bound to the given type, but the method set of an interface type or of elements of a composite type remains unchanged:

// 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

Type definitions may be used to define different boolean, numeric, or string types and associate methods with them:

type TimeZone int

const (
	EST TimeZone = -(5 + iota)
	CST
	MST
	PST
)

func (tz TimeZone) String() string {
	return fmt.Sprintf("GMT%+dh", tz)
}

If the type definition specifies type parameters, the type name denotes a generic type. Generic types must be instantiated when they are used.

type List[T any] struct {
	next  *List[T]
	value T
}

In a type definition the given type cannot be a type parameter.

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
}

A generic type may also have methods associated with it. In this case, the method receivers must declare the same number of type parameters as present in the generic type definition.

// The method Len returns the number of elements in the linked list l.
func (l *List[T]) Len() int  { … }

Type parameter declarations

A type parameter list declares the type parameters of a generic function or type declaration. The type parameter list looks like an ordinary function parameter list except that the type parameter names must all be present and the list is enclosed in square brackets rather than parentheses.

TypeParameters  = "[" TypeParamList [ "," ] "]" .
TypeParamList   = TypeParamDecl { "," TypeParamDecl } .
TypeParamDecl   = IdentifierList TypeConstraint .

All non-blank names in the list must be unique. Each name declares a type parameter, which is a new and different named type that acts as a place holder for an (as of yet) unknown type in the declaration. The type parameter is replaced with a type argument upon instantiation of the generic function or type.

[P any]
[S interface{ ~[]byte|string }]
[S ~[]E, E any]
[P Constraint[int]]
[_ any]

Just as each ordinary function parameter has a parameter type, each type parameter has a corresponding (meta-)type which is called its type constraint.

PCP C

type T[P *C] …
type T[P (C)] …
type T[P *C|Q] …
…

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 interface or use a trailing comma:

type T[P interface{*C}] …
type T[P *C,] …

Type parameters may also be declared by the receiver specification of a method declaration associated with a generic type.

Type constraints

A type constraint is an interface that defines the set of permissible type arguments for the respective type parameter and controls the operations supported by values of that type parameter.

interface{E}Einterface{ … }

[T []P]                      // = [T interface{[]P}]
[T ~int]                     // = [T interface{~int}]
[T int|string]               // = [T interface{int|string}]
type Constraint ~int         // illegal: ~int is not inside a type parameter list

comparableTcomparable

TT==!=TTcomparable

comparable

int                          // implements 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
interface{ comparable }      // type parameter only: implements comparable
interface{ ~int | ~[]byte }  // type parameter only: does not implement comparable (not all types in the type set are comparable)

comparablecomparable

Variable declarations

A variable declaration creates one or more variables, binds corresponding identifiers to them, and gives each a type and an initial value.

VarDecl     = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarSpec     = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .

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"

If a list of expressions is given, the variables are initialized with the expressions following the rules for assignment statements. Otherwise, each variable is initialized to its zero value.

boolnil

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

Implementation restriction: A compiler may make it illegal to declare a variable inside a function body if the variable is never used.

Short variable declarations

A short variable declaration uses the syntax:

ShortVarDecl = IdentifierList ":=" ExpressionList .

It is shorthand for a regular variable declaration with initializer expressions but no types:

"var" IdentifierList "=" ExpressionList .

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

:=

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 :=

Short variable declarations may appear only inside functions. In some contexts such as the initializers for "if", "for", or "switch" statements, they can be used to declare local temporary variables.

Function declarations

A function declaration binds an identifier, the function name, to a function.

FunctionDecl = "func" FunctionName [ TypeParameters ] Signature [ FunctionBody ] .
FunctionName = identifier .
FunctionBody = Block .

If the function's signature declares result parameters, the function body's statement list must end in a terminating statement.

func IndexRune(s string, r rune) int {
	for i, c := range s {
		if c == r {
			return i
		}
	}
	// invalid: missing return statement
}

If the function declaration specifies type parameters, the function name denotes a generic function. A generic function must be instantiated before it can be called or used as a value.

func min[T ~int|~float64](x, y T) T {
	if x < y {
		return x
	}
	return y
}

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.

func flushICache(begin, end uintptr)  // implemented externally

Method declarations

A method is a function with a receiver. A method declaration binds an identifier, the method name, to a method, and associates the method with the receiver's base type.

MethodDecl = "func" Receiver MethodName Signature [ FunctionBody ] .
Receiver   = Parameters .

TT[P1, P2, …]TT*T

A non-blank receiver identifier must be unique 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.

For a base type, the non-blank names of methods bound to it must be unique. If the base type is a struct type, the non-blank method and field names must be distinct.

Point

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
}

LengthScale*PointPoint

If the receiver base type is a generic type, 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 instantiation 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.

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

Expressions

An expression specifies the computation of a value by applying operators and functions to operands.

Operands

Operands denote the elementary values in an expression. An operand may be a literal, a (possibly qualified) non-blank identifier denoting a constant, variable, or function, or a parenthesized expression.

Operand     = Literal | OperandName [ TypeArgs ] | "(" Expression ")" .
Literal     = BasicLit | CompositeLit | FunctionLit .
BasicLit    = int_lit | float_lit | imaginary_lit | rune_lit | string_lit .
OperandName = identifier | QualifiedIdent .

An operand name denoting a generic function may be followed by a list of type arguments; the resulting operand is an instantiated function.

The blank identifier may appear as an operand only on the left-hand side of an assignment statement.

Implementation restriction: A compiler need not report an error if an operand's type is a type parameter with an empty type set. Functions with such type parameters cannot be instantiated; any attempt will lead to an error at the instantiation site.

Qualified identifiers

A qualified identifier is an identifier qualified with a package name prefix. Both the package name and the identifier must not be blank.

QualifiedIdent = PackageName "." identifier .

A qualified identifier accesses an identifier in a different package, which must be imported. The identifier must be exported and declared in the package block of that package.

math.Sin // denotes the Sin function in package math

Composite literals

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.

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 .

TT

For struct literals the following rules apply:

• A key must be a field name declared in the struct type.
• 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.
• If any element has a key, every element must have a key.
• 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.
• A literal may omit the element list; such a literal evaluates to the zero value for its type.
• It is an error to specify an element for a non-exported field of a struct belonging to a different package.

Given the declarations

type Point3D struct { x, y, z float64 }
type Line struct { p, q Point3D }

one may write

origin := Point3D{}                            // zero value for Point3D
line := Line{origin, Point3D{y: -4, z: 12.3}}  // zero value for line.q.x

For array and slice literals the following rules apply:

int

Taking the address of a composite literal generates a pointer to a unique variable initialized with the literal's value.

var pointer *Point3D = &Point3D{y: 1000}

Note that the zero value 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 new.

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

...

buffer := [10]string{}             // len(buffer) == 10
intSet := [6]int{1, 2, 3, 5}       // len(intSet) == 6
days := [...]string{"Sat", "Sun"}  // len(days) == 2

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

[]T{x1, x2, … xn}

and is shorthand for a slice operation applied to an array:

tmp := [n]T{x1, x2, … xn}
tmp[0 : n]

TT&T*T

[...]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{})}

A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword 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.

if x == (T{a,b,c}[i]) { … }
if (x == T{a,b,c}[i]) { … }

Examples of valid array, slice, and map literals:

// 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,
}

Function literals

A function literal represents an anonymous function. Function literals cannot declare type parameters.

FunctionLit = "func" Signature FunctionBody .

func(a, b int, z float64) bool { return a*b < int(z) }

A function literal can be assigned to a variable or invoked directly.

f := func(x, y int) int { return x + y }
func(ch chan int) { ch <- ACK }(replyChan)

Function literals are closures: 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.

Primary expressions

Primary expressions are the operands for unary and binary expressions.

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 ] ) [ "..." ] [ "," ] ] ")" .

x
2
(s + ".txt")
f(3.1415, true)
Point{1, 2}
m["foo"]
s[i : j + 1]
obj.color
f.p[i].x()

Selectors

x

x.f

fx*xffx

ffTfTfTfTfATfA

The following rules apply to selectors:

For example, given the declarations:

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

one may write:

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

but the following is invalid:

q.M0()       // (*q).M0 is valid but not a field selector

Method expressions

MTT.MM

MethodExpr    = ReceiverType "." MethodName .
ReceiverType  = Type .

TMvTMp*T

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

The expression

T.Mv

Mv

func(tv T, a int) int

That function may be called normally with an explicit receiver, so these five invocations are equivalent:

t.Mv(7)
T.Mv(t, 7)
(T).Mv(t, 7)
f1 := T.Mv; f1(t, 7)
f2 := (T).Mv; f2(t, 7)

Similarly, the expression

(*T).Mp

Mp

func(tp *T, f float32) float32

For a method with a value receiver, one can derive a function with an explicit pointer receiver, so

(*T).Mv

Mv

func(tv *T, a int) int

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.

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.

f := T.Mvff(t, 7)t.f(7)

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.

Method values

xTMTx.Mx.Mx.Mx

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

T

TMvTMp*T

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

The expression

t.Mv

yields a function value of type

func(int) int

These two invocations are equivalent:

t.Mv(7)
f := t.Mv; f(7)

Similarly, the expression

pt.Mp

yields a function value of type

func(float32) float32

pt.Mv(*pt).Mv

t.Mp(&t).Mp

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

Although the examples above use non-interface types, it is also legal to create a method value from a value of interface type.

var i interface { M(int) } = myVal
f := i.M; f(7)  // like i.M(7)

Index expressions

A primary expression of the form

a[x]

axx

a

xintintx0 <= x < len(a)

aA

xa[x]xa[x]A

a

a[x](*a)[x]

aS

xa[x]xa[x]S

axa[x]xa[x]bytea[x]

aM

xMxa[x]xa[x]Mnila[x]M

a[x]PPbytePa[x]xxPa[x]a[x]P

a[x]

amap[K]V

v, ok = a[x]
v, ok := a[x]
var v, ok = a[x]

oktruexfalse

nil

Slice expressions

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.

Simple slice expressions

The primary expression

a[low : high]

abytestringlowhighahighlowa

a := [5]int{1, 2, 3, 4, 5}
s := a[1:4]

s[]int

s[0] == 2
s[1] == 3
s[2] == 4

lowhigh

a[2:]  // same as a[2 : len(a)]
a[:3]  // same as a[0 : 3]
a[:]   // same as a[0 : len(a)]

aa[low : high](*a)[low : high]

0lowhighlen(a)cap(a)intlow <= high

string

nilnil

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

Full slice expressions

The primary expression

a[low : high : max]

a[low : high]max - lowaa

a := [5]int{1, 2, 3, 4, 5}
t := a[1:3:5]

t[]int

t[0] == 2
t[1] == 3

aa[low : high : max](*a)[low : high : max]

0 <= low <= high <= max <= cap(a)int

Type assertions

xT

x.(T)

xnilxTx.(T)

Tx.(T)xTTxxTTx.(T)xT

xTxx.(T)T

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
	…
}

A type assertion used in an assignment statement or initialization of the special form

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

oktruefalsevT

Calls

fF

f(a1, a2, … an)

fa1, a2, … anFF

math.Atan2(x, y)  // function call
var pt *Point
pt.Scale(3.5)     // method call with receiver pt

f

In a function call, the function value and arguments are evaluated in the usual order. 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.

nil

gff(g(parameters_of_g))fgffggf...g

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")
}

x.m()xmmx&xmx.m()(&x).m()

var p Point
p.Scale(3.5)

There is no distinct method type and there are no method literals.

...

fp...Tfp[]Tfppnil[]TTp

Given the function and calls

func Greeting(prefix string, who ...string)
Greeting("nobody")
Greeting("hello:", "Joe", "Anna", "Eileen")

Greetingwhonil[]string{"Joe", "Anna", "Eileen"}

[]T......T

s

s := []string{"James", "Jasmine"}
Greeting("goodbye:", s...)

Greetingwhos

Instantiations

A generic function or type is instantiated by substituting type arguments for the type parameters. Instantiation proceeds in two steps:

Instantiating a type results in a new non-generic named type; instantiating a function produces a new non-generic function.

type parameter list    type arguments    after substitution

[P any]                int               int implements any
[S ~[]E, E any]        []int, int        []int implements ~[]int, int implements any
[P io.Writer]          string            illegal: string doesn't implement io.Writer

For a generic function, type arguments may be provided explicitly, or they may be partially or completely inferred. A generic function that is not called requires a type argument list for instantiation; if the list is partial, all remaining type arguments must be inferrable. A generic function that is called may provide a (possibly partial) type argument list, or may omit it entirely if the omitted type arguments are inferrable from the ordinary (non-type) function arguments.

func min[T ~int|~float64](x, y T) T { … }

f := min                   // illegal: min must be instantiated with type arguments when used without being called
minInt := min[int]         // minInt has type func(x, y int) int
a := minInt(2, 3)          // a has value 2 of type int
b := min[float64](2.0, 3)  // b has value 2.0 of type float64
c := min(b, -1)            // c has value -1.0 of type float64

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".

func apply[S ~[]E, E any](s S, f(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

For a generic type, all type arguments must always be provided explicitly.

Type inference

Missing function type arguments may be inferred by a series of steps, described below. Each step attempts to use known information to infer additional type arguments. Type inference stops as soon as all type arguments are known. After type inference is complete, it is still necessary to substitute all type arguments for type parameters and verify that each type argument implements the relevant constraint; it is possible for an inferred type argument to fail to implement a constraint, in which case instantiation fails.

Type inference is based on

• a substitution map M initialized with the known type arguments, if any
• a (possibly empty) list of ordinary function arguments (in case of a function call only)

and then proceeds with the following steps:

1. apply function argument type inference to all typed ordinary function arguments
2. apply function argument type inference to all untyped ordinary function arguments using the default type for each of the untyped function arguments
3. apply constraint type inference

If there are no ordinary or untyped function arguments, the respective steps are skipped. Constraint type inference is skipped if the previous step didn't infer any new type arguments, but it is run at least once if there are missing type arguments.

The substitution map M is carried through all steps, and each step may add entries to M. The process stops as soon as M has a type argument for each type parameter or if an inference step fails. If an inference step fails, or if M is still missing type arguments after the last step, type inference fails.

Type unification

PAPA

For unification, two types that don't contain any type parameters from the current type parameter list are equivalent if they are identical, or if they are channel types that are identical ignoring channel direction, or if their underlying types are equivalent.

Unification works by comparing the structure of pairs of types: their structure disregarding type parameters must be identical, and types other than type parameters must be equivalent. A type parameter in one type may match any complete subtype in the other type; each successful match causes an entry to be added to the substitution map. If the structure differs, or types other than type parameters are not equivalent, unification fails.

T1T2[]map[int]bool

[]map[int]bool   // types are identical
T1               // adds T1 → []map[int]bool to substitution map
[]T1             // adds T1 → map[int]bool to substitution map
[]map[T1]T2      // adds T1 → int and T2 → bool to substitution map

[]map[int]bool

int              // int is not a slice
struct{}         // a struct is not a slice
[]struct{}       // a struct is not a map
[]map[T1]string  // map element types don't match

DLDL

type Vector []float64

[]E[]float64[]EEfloat64

Function argument type inference

TTT

For instance, given the generic function

func scale[Number ~int64|~float64|~complex128](v []Number, s Number) []Number

and the call

var vector []float64
scaledVector := scale(vector, 42)

Numbervectorvector[]float64[]Numberfloat64NumberNumberfloat6442

Inference happens in two separate phases; each phase operates on a specific list of (parameter, argument) pairs:

1. The list Lt contains all (parameter, argument) pairs where the parameter type uses type parameters and where the function argument is typed.
2. The list Lu contains all remaining pairs where the parameter type is a single type parameter. In this list, the respective function arguments are untyped.

Any other (parameter, argument) pair is ignored.

By construction, the arguments of the pairs in Lu are untyped constants (or the untyped boolean result of a comparison). And because default types of untyped values are always predeclared non-composite types, they can never match against a composite type, so it is sufficient to only consider parameter types that are single type parameters.

Each list is processed in a separate phase:

1. In the first phase, the parameter and argument types of each pair in Lt are unified. If unification succeeds for a pair, it may yield new entries that are added to the substitution map M. If unification fails, type inference fails.
2. The second phase considers the entries of list Lu. Type parameters for which the type argument has already been determined are ignored in this phase. For each remaining pair, the parameter type (which is a single type parameter) and the default type of the corresponding untyped argument is unified. If unification fails, type inference fails.

While unification is successful, processing of each list continues until all list elements are considered, even if all type arguments are inferred before the last list element has been processed.

Example:

func min[T ~int|~float64](x, y T) T

var x int
min(x, 2.0)    // T is int, inferred from typed argument x; 2.0 is assignable to int
min(1.0, 2.0)  // T is float64, inferred from default type for 1.0 and matches default type for 2.0
min(1.0, 2)    // illegal: default type float64 (for 1.0) doesn't match default type int (for 2)

min(1.0, 2)1.0Tfloat64

Constraint type inference

PCPCPC

ListElem

[List ~[]Elem, Elem any]

ElemListElem[]ElemListBytes

type Bytes []byte

Bytes[]byte[]ElemElembyte

NNN

Generally, constraint type inference proceeds in two phases: Starting with a given substitution map M

PAAQQBQBA

PAPA

For instance, given the type parameter list

[A any, B []C, C *A]

intAAint

BCB[]CC*A

[]C*A

AintB[]CC*A

Aint

AintB[]CC*int

C*int

AintB[]*intC*int

M

Operators

Operators combine operands into expressions.

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   = "+" | "-" | "!" | "^" | "*" | "&" | "<-" .

Comparisons are discussed elsewhere. For other binary operators, the operand types must be identical unless the operation involves shifts or untyped constants. For operations involving constants only, see the section on constant expressions.

Except for shift operations, if one operand is an untyped constant and the other operand is not, the constant is implicitly converted to the type of the other operand.

uint

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

Operator precedence

++--*p++(*p)++

&&||

Precedence    Operator
    5             *  /  %  <<  >>  &  &^
    4             +  -  |  ^
    3             ==  !=  <  <=  >  >=
    2             &&
    1             ||

x / y * z(x / y) * z

+x
23 + 3*x[i]
x <= f()
^a >> b
f() || g()
x == y+1 && <-chanInt > 0

Arithmetic operators

+-*/+

+    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

If the operand type is a type parameter, 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 instantiated with, and the operation is computed with the precision of that type argument. For example, given the function:

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
}

x * ys += x * yfloat32float64F

Integer operators

xyq = x / yr = x % y

x = q*y + r  and  |r| < |y|

x / y

 x     y     x / y     x % y
 5     3       1         2
-5     3      -1        -2
 5    -3      -1         2
-5    -3       1        -2

xxq = x / -1xr = 0

                         x, q
int8                     -128
int16                  -32768
int32             -2147483648
int64    -9223372036854775808

If the divisor is a constant, it must not be zero. If the divisor is zero at run time, a run-time panic 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:

 x     x / 4     x % 4     x >> 2     x & 3
 11      2         3         2          3
-11     -2        -3        -3          1

nnx << 1x*2x >> 1x/2

+-^

+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

Integer overflow

+-*<<

+-*/<

Floating-point operators

+xx-xx

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 conversion rounds to the precision of the target type, preventing fusion that would discard that rounding.

x*y + zx*y

// FMA allowed for computing r, because x*y is not explicitly rounded:
r  = x*y + z
r  = z;   r += x*y
t  = x*y; r = t + z
*p = x*y; r = *p + z
r  = x*y + float64(z)

// FMA disallowed for computing r, because it would omit rounding of x*y:
r  = float64(x*y) + z
r  = z; r += float64(x*y)
t  = float64(x*y); r = t + z

String concatenation

++=

s := "hi" + string(c)
s += " and good bye"

String addition creates a new string by concatenating the operands.

Comparison operators

Comparison operators compare two operands and yield an untyped boolean value.

==    equal
!=    not equal
<     less
<=    less or equal
>     greater
>=    greater or equal

In any comparison, the first operand must be assignable to the type of the second operand, or vice versa.

==!=<<=>>=

truefalseuvreal(u) == real(v)imag(u) == imag(v)nilmakenilnilxXtTXXTtXtx

A comparison of two interface values with identical dynamic types causes a run-time panic 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.

nilnil

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
)

Logical operators

Logical operators apply to boolean values and yield a result of the same type as the operands. The right operand is evaluated conditionally.

&&    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"

Address operators

xT&x*Txxx&x

x*T*xTxxnil*x

&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

Receive operator

ch<-chchnil

v1 := <-ch
v2 = <-ch
f(<-ch)
<-strobe  // wait until clock pulse and discard received value

A receive expression used in an assignment statement or initialization of the special form

x, ok = <-ch
x, ok := <-ch
var x, ok = <-ch
var x, ok T = <-ch

oktruefalse

Conversions

A conversion changes the type of an expression to the type specified by the conversion. A conversion may appear literally in the source, or it may be implied by the context in which an expression appears.

T(x)TxT

Conversion = Type "(" Expression [ "," ] ")" .

*<-func

*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)

xTxTxx

Converting a constant to a type that is not a type parameter yields a typed constant.

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

Converting a constant to a type parameter yields a non-constant value of that type, with the value represented as a value of the type argument that the type parameter is instantiated with. For example, given the function:

func f[P ~float32|~float64]() {
	… P(1.1) …
}

P(1.1)P1.1float32float64fffloat32P(1.1) + 1.2float32

xT

xTxTxTxTxTxTxTxT

TxVxT

VTVTVVTTxT

Struct tags are ignored when comparing struct types for identity for the purpose of conversion:

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

xx

unsafe

Conversions between numeric types

For the conversion of non-constant numeric values, the following rules apply:

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.

Conversions to and from a string type

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"

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'})   // "🌍"

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" == "🌎"

[]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'}

[]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}

Conversions from slice to array pointer

Converting a slice to an array pointer yields a pointer to the underlying array of the slice. If the length of the slice is less than the length of the array, a run-time panic occurs.

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

Constant expressions

Constant expressions may contain only constant operands and are evaluated at compile time.

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.

A constant comparison always yields an untyped boolean constant. If the left operand of a constant shift expression 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 integer type.

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.

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 = iota*1i - 1/1i   //            (untyped complex constant)

complex

const ic = complex(0, c)   // ic == 3.75i  (untyped complex constant)
const iΘ = complex(0, Θ)   // iΘ == 1i     (type complex128)

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:

const Huge = 1 << 100         // Huge == 1267650600228229401496703205376  (untyped integer constant)
const Four int8 = Huge >> 98  // Four == 4                                (type int8)

The divisor of a constant division or remainder operation must not be zero:

3.14 / 0.0   // illegal: division by zero

The values of typed constants must always be accurately representable by values of the constant type. The following constant expressions are illegal:

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)

^

^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

Implementation restriction: A compiler may use rounding while computing untyped floating-point or complex constant expressions; see the implementation restriction in the section on constants. 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.

Order of evaluation

At package level, initialization dependencies determine the evaluation order of individual initialization expressions in variable declarations. Otherwise, when evaluating the operands of an expression, assignment, or return statement, all function calls, method calls, and communication operations are evaluated in lexical left-to-right order.

For example, in the (function-local) assignment

y[f()], ok = g(h(), i()+x[j()], <-c), k()

f()h()i()j()<-cg()k()xy

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

At package level, initialization dependencies override the left-to-right rule for individual initialization expressions, but not for operands within each expression:

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 x*x }

// functions u and v are independent of all other variables and functions

u()sqr()v()f()v()g()

x + (y + z)y + zx

Statements

Statements control execution.

Statement =
	Declaration | LabeledStmt | SimpleStmt |
	GoStmt | ReturnStmt | BreakStmt | ContinueStmt | GotoStmt |
	FallthroughStmt | Block | IfStmt | SwitchStmt | SelectStmt | ForStmt |
	DeferStmt .

SimpleStmt = EmptyStmt | ExpressionStmt | SendStmt | IncDecStmt | Assignment | ShortVarDecl .

Terminating statements

A terminating statement interrupts the regular flow of control in a block. The following statements are terminating:

panic

All other statements are not terminating.

A statement list ends in a terminating statement if the list is not empty and its final non-empty statement is terminating.

Empty statements

The empty statement does nothing.

Labeled statements

gotobreakcontinue

LabeledStmt = Label ":" Statement .
Label       = identifier .

Error: log.Panic("error encountered")

Expression statements

With the exception of specific built-in functions, function and method calls and receive operations can appear in statement context. Such statements may be parenthesized.

ExpressionStmt = Expression .

The following built-in functions are not permitted in statement context:

append cap complex imag len make new real
unsafe.Add unsafe.Alignof unsafe.Offsetof unsafe.Sizeof unsafe.Slice

h(x+y)
f.Close()
<-ch
(<-ch)
len("foo")  // illegal if len is the built-in function

Send statements

A send statement sends a value on a channel. The channel expression's core type must be a channel, the channel direction must permit send operations, and the type of the value to be sent must be assignable to the channel's element type.

SendStmt = Channel "<-" Expression .
Channel  = Expression .

nil

ch <- 3  // send value 3 to channel ch

IncDec statements

1

IncDecStmt = Expression ( "++" | "--" ) .

The following assignment statements are semantically equivalent:

IncDec statement    Assignment
x++                 x += 1
x--                 x -= 1

Assignment statements

An assignment replaces the current value stored in a variable with a new value specified by an expression. An assignment statement may assign a single value to a single variable, or multiple values to a matching number of variables.

Assignment = ExpressionList assign_op ExpressionList .

assign_op = [ add_op | mul_op ] "=" .

=

x = 1
*p = f()
a[i] = 23
(k) = <-ch  // same as: k = <-ch

x=yx=x(y)x=

a[i] <<= 2
i &^= 1<<n

f

x, y = f()

xy

one, two, three = '一', '二', '三'

The blank identifier provides a way to ignore right-hand side values in an assignment:

_ = x       // evaluate x but ignore it
x, _ = f()  // evaluate f() but ignore second result value

The assignment proceeds in two phases. First, the operands of index expressions and pointer indirections (including implicit pointer indirections in selectors) on the left and the expressions on the right are all evaluated in the usual order. Second, the assignments are carried out in left-to-right order.

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}

In assignments, each value must be assignable to the type of the operand to which it is assigned, with the following special cases:

bool

If statements

"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.

IfStmt = "if" [ SimpleStmt ";" ] Expression Block [ "else" ( IfStmt | Block ) ] .

if x > max {
	x = max
}

The expression may be preceded by a simple statement, which executes before the expression is evaluated.

if x := f(); x < y {
	return x
} else if x > z {
	return z
} else {
	return y
}

Switch statements

"Switch" statements provide multi-way execution. An expression or type is compared to the "cases" inside the "switch" to determine which branch to execute.

SwitchStmt = ExprSwitchStmt | TypeSwitchStmt .

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.

Expression switches

true

ExprSwitchStmt = "switch" [ SimpleStmt ";" ] [ Expression ] "{" { ExprCaseClause } "}" .
ExprCaseClause = ExprSwitchCase ":" StatementList .
ExprSwitchCase = "case" ExpressionList | "default" .

nil

xtx == t

ttx

In a case or default clause, the last non-empty statement may be a (possibly labeled) "fallthrough" statement 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.

The switch expression may be preceded by a simple statement, which executes before the expression is evaluated.

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()
}

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.

Type switches

type

switch x.(type) {
// cases
}

TxxTx

TypeSwitchStmt  = "switch" [ SimpleStmt ";" ] TypeSwitchGuard "{" { TypeCaseClause } "}" .
TypeSwitchGuard = [ identifier ":=" ] PrimaryExpr "." "(" "type" ")" .
TypeCaseClause  = TypeSwitchCase ":" StatementList .
TypeSwitchCase  = "case" TypeList | "default" .

The TypeSwitchGuard may include a short variable declaration. When that form is used, the variable is declared at the end of the TypeSwitchCase in the implicit block 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.

nilnilnil

xinterface{}

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{})
}

could be rewritten:

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")
	}
}

A type parameter or a generic type may be used as a type in a case. If upon instantiation that type turns out to duplicate another entry in the switch, the first matching case is chosen.

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

The type switch guard may be preceded by a simple statement, which executes before the guard is evaluated.

The "fallthrough" statement is not permitted in a type switch.

For statements

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.

ForStmt = "for" [ Condition | ForClause | RangeClause ] Block .
Condition = Expression .

For statements with single condition

true

for a < b {
	a *= 2
}

for

A "for" statement with a ForClause is also controlled by its condition, but additionally it may specify an init and a post statement, such as an assignment, an increment or decrement statement. The init statement may be a short variable declaration, but the post statement must not. Variables declared by the init statement are re-used in each iteration.

ForClause = [ InitStmt ] ";" [ Condition ] ";" [ PostStmt ] .
InitStmt = SimpleStmt .
PostStmt = SimpleStmt .

for i := 0; i < 10; i++ {
	f(i)
}

true

for cond { S() }    is the same as    for ; cond ; { S() }
for      { S() }    is the same as    for true     { S() }

range

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 iteration values to corresponding iteration variables if present and then executes the block.

RangeClause = [ ExpressionList "=" | IdentifierList ":=" ] "range" Expression .

The expression on the right in the "range" clause is called the range expression, its core type must be an array, pointer to an array, slice, string, map, or channel permitting receive operations. As with an assignment, if present the operands on the left must be addressable 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 blank identifier, the range clause is equivalent to the same clause without that identifier.

xlen(x)

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:

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

alen(a)-1nilrune0xFFFDnilnil

The iteration values are assigned to the respective iteration variables as in an assignment statement.

:=

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 {}

Go statements

A "go" statement starts the execution of a function call as an independent concurrent thread of control, or goroutine, within the same address space.

GoStmt = "go" Expression .

The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for expression statements.

The function value and parameters are evaluated as usual 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.

go Server()
go func(ch chan<- bool) { for { sleep(10); ch <- true }} (c)

Select statements

A "select" statement chooses which of a set of possible send or receive operations will proceed. It looks similar to a "switch" statement but with the cases all referring to communication operations.

SelectStmt = "select" "{" { CommClause } "}" .
CommClause = CommCase ":" StatementList .
CommCase   = "case" ( SendStmt | RecvStmt ) | "default" .
RecvStmt   = [ ExpressionList "=" | IdentifierList ":=" ] RecvExpr .
RecvExpr   = Expression .

A case with a RecvStmt may assign the result of a RecvExpr to one or two variables, which may be declared using a short variable declaration. 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.

Execution of a "select" statement proceeds in several steps:

1. 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.
2. 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.
3. Unless the selected case is the default case, the respective communication operation is executed.
4. 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.
5. The statement list of the selected case is executed.

nilnil

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

Return statements

FFFF

ReturnStmt = "return" [ ExpressionList ] .

In a function without a result type, a "return" statement must not specify any result values.

func noResult() {
	return
}

There are three ways to return values from a function with a result type:

func simpleF() int {
	return 2
}

func complexF1() (re float64, im float64) {
	return -7.0, -4.0
}

func complexF2() (re float64, im float64) {
	return complexF1()
}

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
}

Regardless of how they are declared, all the result values are initialized to the zero values for their type upon entry to the function. A "return" statement that specifies results sets the result parameters before any deferred functions are executed.

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 scope at the place of the return.

func f(n int) (res int, err error) {
	if _, err := f(n-1); err != nil {
		return  // invalid return statement: err is shadowed
	}
	return
}

Break statements

A "break" statement terminates execution of the innermost "for", "switch", or "select" statement within the same function.

BreakStmt = "break" [ Label ] .

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.

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
			}
		}
	}

Continue statements

A "continue" statement begins the next iteration of the innermost enclosing "for" loop by advancing control to the end of the loop block. The "for" loop must be within the same function.

ContinueStmt = "continue" [ Label ] .

If there is a label, it must be that of an enclosing "for" statement, and that is the one whose execution advances.

RowLoop:
	for y, row := range rows {
		for x, data := range row {
			if data == endOfRow {
				continue RowLoop
			}
			row[x] = data + bias(x, y)
		}
	}

Goto statements

A "goto" statement transfers control to the statement with the corresponding label within the same function.

GotoStmt = "goto" Label .

goto Error

Executing the "goto" statement must not cause any variables to come into scope that were not already in scope at the point of the goto. For instance, this example:

	goto L  // BAD
	v := 3
L:

Lv

A "goto" statement outside a block cannot jump to a label inside that block. For instance, this example:

if n%2 == 1 {
	goto L1
}
for n > 0 {
	f()
	n--
L1:
	f()
	n--
}

L1goto

Fallthrough statements

A "fallthrough" statement transfers control to the first statement of the next case clause in an expression "switch" statement. It may be used only as the final non-empty statement in such a clause.

FallthroughStmt = "fallthrough" .

Defer statements

A "defer" statement invokes a function whose execution is deferred to the moment the surrounding function returns, either because the surrounding function executed a return statement, reached the end of its function body, or because the corresponding goroutine is panicking.

DeferStmt = "defer" Expression .

The expression must be a function or method call; it cannot be parenthesized. Calls of built-in functions are restricted as for expression statements.

nil

For instance, if the deferred function is a function literal and the surrounding function has named result parameters 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 handling panics.)

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
}

Built-in functions

Built-in functions are predeclared. They are called like any other function but some of them accept a type instead of an expression as the first argument.

The built-in functions do not have standard Go types, so they can only appear in call expressions; they cannot be used as function values.

Close

chclosechclose

Length and capacity

lencapintint

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

Plen(e)cap(e)PP

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:

0 <= len(s) <= cap(s)

nilnil

len(s)slen(s)cap(s)ssslencaps

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

Allocation

newT*T

new(T)

For instance

type S struct { a int; b float64 }
new(S)

Sa=0b=0.0*S

Making slices, maps and channels

makeTTT*T

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

nmintintnmnmnm

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

makenn

Appending to and copying slices

appendcopy

appendxsss[]Ex...Es[]byteappendbytestring...

append(s S, x ...E) S  // core type of S is []E

sappendappend

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' }

copysrcdstlen(src)len(dst)[]bytecopybytestring

copy(dst, src []T) int
copy(dst []byte, src string) int

Examples:

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")

Deletion of map elements

deletekmkm

delete(m, k)  // remove element m[k] from map m

m

mnilm[k]delete

Manipulating complex numbers

complexrealimag

complex(realPart, imaginaryPart floatT) complexT
real(complexT) floatT
imag(complexT) floatT

complexcomplex64float32complex128float64

realimagfloat32complex64float64complex128

realimagcomplexzZz == Z(complex(real(z), imag(z)))

If the operands of these functions are all constants, the return value is a constant.

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

Arguments of type parameter type are not permitted.

Handling panics

panicrecover

func panic(interface{})
func recover() interface{}

FpanicFFFpanic

panic(42)
panic("unreachable")
panic(Error("cannot parse"))

recoverGDrecoverGDDrecoverpanicDpanicGpanicGDG

recovernil

panicnilrecover

protectgg

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()
}

Bootstrapping

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.

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

printprintln

Packages

Go programs are constructed by linking together packages. 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 exported and used in another package.

Source file organization

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.

SourceFile       = PackageClause ";" { ImportDecl ";" } { TopLevelDecl ";" } .

Package clause

A package clause begins each source file and defines the package to which the file belongs.

PackageClause  = "package" PackageName .
PackageName    = identifier .

The PackageName must not be the blank identifier.

package math

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.

Import declarations

An import declaration states that the source file containing the declaration depends on functionality of the imported package (§Program initialization and execution) and enables access to exported 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.

ImportDecl       = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
ImportSpec       = [ "." | PackageName ] ImportPath .
ImportPath       = string_lit .

.

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.

!"#$%&'()*,:;<=>?[\]^`{|}

package mathSin"lib/math"Sin

Import declaration          Local name of Sin

import   "lib/math"         math.Sin
import m "lib/math"         m.Sin
import . "lib/math"         Sin

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 blank identifier as explicit package name:

import _ "lib/math"

An example package

Here is a complete Go package that implements a concurrent prime sieve.

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()
}

Program initialization and execution

The zero value

newmakefalse0""nil

These two simple declarations are equivalent:

var i int
var i int = 0

After

type T struct { i int; f float64; next *T }
t := new(T)

the following holds:

t.i == 0
t.f == 0.0
t.next == nil

The same would also be true after

var t T

Package initialization

Within a package, package-level variable initialization proceeds stepwise, with each step selecting the variable earliest in declaration order which has no dependencies on uninitialized variables.

More precisely, a package-level variable is considered ready for initialization if it is not yet initialized and either has no initialization expression or its initialization expression has no dependencies 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.

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.

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.

var x = a
var a, b = f() // a and b are initialized together, before x is initialized

For the purpose of package initialization, blank variables are treated like any other variables in declarations.

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.

xyxy

For example, given the declarations

var (
	a = c + b  // == 9
	b = f()    // == 4
	c = f()    // == 5
	d = 3      // == 5 after initialization has finished
)

func f() int {
	d++
	return d
}

dbcaa = c + ba = b + c

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.

For instance, given the declarations

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} }

abxbbaasideEffect()x

init

func init() { … }

initinitinit

init

initinitinit

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.

Program execution

mainmain

func main() { … }

mainmain

Errors

error

type error interface {
	Error() string
}

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:

func Read(f *File, b []byte) (n int, err error)

Run-time panics

panicruntime.Errorerror

package runtime

type Error interface {
	error
	// and perhaps other methods
}

System considerations

unsafe

unsafe"unsafe"unsafe

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

PointerPointeruintptrPointerPointeruintptr

var f float64
bits = *(*uint64)(unsafe.Pointer(&f))

type ptr unsafe.Pointer
bits = *(*uint64)(ptr(&f))

var p ptr = nil

AlignofSizeofxvvvar v = x

Offsetofs.ffs*sfsf

uintptr(unsafe.Pointer(&s)) + unsafe.Offsetof(s.f) == uintptr(unsafe.Pointer(&s.f))

Alignofx

uintptr(unsafe.Pointer(&x)) % unsafe.Alignof(x) == 0

TTAlignofOffsetofSizeofuintptrss.fOffsetof

Addlenptrunsafe.Pointer(uintptr(ptr) + uintptr(len))lenlenintintPointer

SliceptrlenSlice(ptr, len)

(*[len]ArbitraryType)(unsafe.Pointer(ptr))[:]

ptrnillenSlicenil

lenlenintintlenptrnillen

Size and alignment guarantees

For the numeric types, the following sizes are guaranteed:

type                                 size in bytes

byte, uint8, int8                     1
uint16, int16                         2
uint32, int32, float32                4
uint64, int64, float64, complex64     8
complex128                           16

The following minimal alignment properties are guaranteed:

xunsafe.Alignof(x)xunsafe.Alignof(x)unsafe.Alignof(x.f)fxxunsafe.Alignof(x)

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.