The Go Programming Language Specification - The Go Programming Language

Version of Aug 2, 2023

Introduction

This is the reference manual for the Go programming language. The pre-Go1.18 version, without generics, can be foundhere. 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 avariantof 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)

Lowercase production names are used to identify lexical (terminal) tokens. Non-terminals are in CamelCase. Lexical tokens are enclosed in double quotes "" or back quotes ``.

The form a … b represents the set of characters froma through b as alternatives. The horizontal ellipsis … is also used elsewhere in the spec to informally denote various enumerations or code snippets that are not further specified. The character …(as opposed to the three characters ...) is not a token of the Go language.

Source code representation

Source code is Unicode text encoded inUTF-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

The underscore character _ (U+005F) is considered a lowercase letter.

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 serve as program documentation. There are two forms:

Line comments start with the character sequence //and stop at the end of the line.
General comments start with the character sequence /*and stop with the first subsequent character sequence */.

A comment cannot start inside a rune orstring 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

The formal syntax uses semicolons ";" as terminators in a number of productions. Go programs may omit most of these semicolons using the following two rules:

When the input is broken into tokens, a semicolon is automatically inserted into the token stream immediately after a line's final token if that token is
- anidentifier
- aninteger,floating-point,imaginary,rune, orstring literal
- one of the keywords break,continue,fallthrough, orreturn
- one of the operators and punctuation ++,--,),], or}
To allow complex statements to occupy a single line, a semicolon may be omitted before a closing ")" or "}".

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

An integer literal is a sequence of digits representing aninteger constant. An optional prefix sets a non-decimal base: 0b or 0Bfor binary, 0, 0o, or 0O for octal, and 0x or 0X for hexadecimal. A single 0 is considered a decimal zero. In hexadecimal literals, letters a through fand A through F represent values 10 through 15.

For readability, an underscore character _ may appear after a base prefix or between successive digits; such underscores do not change the literal's value.

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 afloating-point constant.

A decimal floating-point literal consists of an integer part (decimal digits), a decimal point, a fractional part (decimal digits), and an exponent part (e or E followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; one of the decimal point or the exponent part may be elided. An exponent value exp scales the mantissa (integer and fractional part) by 10exp.

A hexadecimal floating-point literal consists of a 0x or 0Xprefix, an integer part (hexadecimal digits), a radix point, a fractional part (hexadecimal digits), and an exponent part (p or P followed by an optional sign and decimal digits). One of the integer part or the fractional part may be elided; the radix point may be elided as well, but the exponent part is required. (This syntax matches the one given in IEEE 754-2008 §5.12.3.) An exponent value exp scales the mantissa (integer and fractional part) by 2exp.

For readability, an underscore character _ may appear after a base prefix or between successive digits; such underscores do not change the literal value.

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 .

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

An imaginary literal represents the imaginary part of acomplex constant. It consists of an integer orfloating-point literal followed by the lowercase letter i. The value of an imaginary literal is the value of the respective integer or floating-point literal multiplied by the imaginary unit i.

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

For backward compatibility, an imaginary literal's integer part consisting entirely of decimal digits (and possibly underscores) is considered a decimal integer, even if it starts with a leading 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

A rune literal represents a rune constant, an integer value identifying a Unicode code point. A rune literal is expressed as one or more characters enclosed in single quotes, as in 'x' or '\n'. Within the quotes, any character may appear except newline and unescaped single quote. A single quoted character represents the Unicode value of the character itself, while multi-character sequences beginning with a backslash encode values in various formats.

The simplest form represents the single character within the quotes; since Go source text is Unicode characters encoded in UTF-8, multiple UTF-8-encoded bytes may represent a single integer value. For instance, the literal 'a' holds a single byte representing a literal a, Unicode U+0061, value 0x61, while'ä' holds two bytes (0xc3 0xa4) representing a literal a-dieresis, U+00E4, value 0xe4.

Several backslash escapes allow arbitrary values to be encoded as ASCII text. There are four ways to represent the integer value as a numeric constant: \x followed by exactly two hexadecimal digits; \u followed by exactly four hexadecimal digits;\U followed by exactly eight hexadecimal digits, and a plain backslash \ followed by exactly three octal digits. In each case the value of the literal is the value represented by the digits in the corresponding base.

Although these representations all result in an integer, they have different valid ranges. Octal escapes must represent a value between 0 and 255 inclusive. Hexadecimal escapes satisfy this condition by construction. The escapes \u and \Urepresent Unicode code points so within them some values are illegal, in particular those above 0x10FFFF and surrogate halves.

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 constantobtained from concatenating a sequence of characters. There are two forms: raw string literals and interpreted string literals.

Raw string literals are character sequences between back quotes, as in`foo`. Within the quotes, any character may appear except back quote. The value of a raw string literal is the string composed of the uninterpreted (implicitly UTF-8-encoded) characters between the quotes; in particular, backslashes have no special meaning and the string may contain newlines. Carriage return characters ('\r') inside raw string literals are discarded from the raw string value.

Interpreted string literals are character sequences between double quotes, as in "bar". Within the quotes, any character may appear except newline and unescaped double quote. The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in rune literals (except that \' is illegal and\" is legal), with the same restrictions. The three-digit octal (\nnn) and two-digit hexadecimal (\xnn) escapes represent individual_bytes_ of the resulting string; all other escapes represent the (possibly multi-byte) UTF-8 encoding of individual characters. Thus inside a string literal \377 and \xFF represent a single byte of value 0xFF=255, while ÿ,\u00FF, \U000000FF and \xc3\xbf represent the two bytes 0xc3 0xbf of the UTF-8 encoding of character U+00FF.

string_lit = raw_string_lit | interpreted_string_lit . raw_string_lit = "" { [unicode_char](#unicode%5Fchar) | [newline](#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.

A constant value is represented by arune,integer,floating-point,imaginary, orstring literal, an identifier denoting a constant, a constant expression, a conversion with a result that is a constant, or the result value of some built-in functions such asmin or max applied to constant arguments,unsafe.Sizeof applied to certain values,cap or len applied tosome expressions,real and imag applied to a complex constant and complex applied to numeric constants. The boolean truth values are represented by the predeclared constantstrue and false. The predeclared identifieriota denotes an integer constant.

In general, complex constants are a form ofconstant expressionand 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.

Constants may be typed or untyped. Literal constants, true, false, iota, and certain constant expressionscontaining only untyped constant operands are untyped.

A constant may be given a type explicitly by a constant declarationor conversion, or implicitly when used in avariable declaration or anassignment 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.

An untyped constant has a default type which is the type to which the constant is implicitly converted in contexts where a typed value is required, for instance, in a short variable declarationsuch as i := 0 where there is no explicit type. The default type of an untyped constant is bool, rune,int, float64, complex128, or stringrespectively, depending on whether it is a boolean, rune, integer, floating-point, complex, or string constant.

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 type.

A variable declarationor, for function parameters and results, the signature of a function declarationor function literal reserves storage for a named variable. Calling the built-in function newor taking the address of a composite literalallocates storage for a variable at run time. Such an anonymous variable is referred to via a (possibly implicit)pointer indirection.

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

The static type (or just type) of a variable is the type given in its declaration, the type provided in thenew call or composite literal, or the type of an element of a structured variable. Variables of interface type also have a distinct dynamic type, which is the (non-interface) type of the value assigned to the variable at run time (unless the value is the predeclared identifier nil, which has no type). The dynamic type may vary during execution but values stored in interface variables are always assignableto the static type of the variable.

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 anexpression; it is the most recent valueassigned to the variable. If a variable has not yet been assigned a value, its value is thezero 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 declarationsor 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

A boolean type represents the set of Boolean truth values denoted by the predeclared constants trueand false. The predeclared boolean type is bool; it is a defined type.

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 usingtwo'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

To avoid portability issues all numeric types are defined types and thus distinct exceptbyte, which is an alias for uint8, andrune, which is an alias for int32. Explicit conversions are required when different numeric types are mixed in an expression or assignment. For instance, int32 and intare not the same type even though they may have the same size on a particular architecture.

String types

A string type represents the set of string values. A string value is a (possibly empty) sequence of bytes. The number of bytes is called the length of the string and is never negative. Strings are immutable: once created, it is impossible to change the contents of a string. The predeclared string type is string; it is a defined type.

The length of a string s can be discovered using the built-in function len. The length is a compile-time constant if the string is a constant. A string's bytes can be accessed by integer indices0 through len(s)-1. It is illegal to take the address of such an element; ifs[i] is the i'th byte of a string, &s[i] is invalid.

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 .

The length is part of the array's type; it must evaluate to a non-negative constant representable by a value of type int. The length of array a can be discovered using the built-in function len. The elements can be addressed by integer indices0 through len(a)-1. Array types are always one-dimensional but may be composed to form multi-dimensional types.

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

An array type T may not have an element of type T, or of a type containing T as a component, directly or indirectly, if those containing types are only array or struct types.

// invalid array types type ( T1 [10]T1 // element type of T1 is T1 T2 [10]struct{ f T2 } // T2 contains T2 as component of a struct T3 [10]T4 // T3 contains T3 as component of a struct in T4 T4 struct{ f T3 } // T4 contains T4 as component of array T3 in a struct )

// valid array types type ( T5 [10]*T5 // T5 contains T5 as component of a pointer T6 [10]func() T6 // T6 contains T6 as component of a function type T7 [10]struct{ f []T7 } // T7 contains T7 as component of a slice in a struct )

Slice types

A slice is a descriptor for a contiguous segment of an underlying array and provides access to a numbered sequence of elements from that array. A slice type denotes the set of all slices of arrays of its element type. The number of elements is called the length of the slice and is never negative. The value of an uninitialized slice is nil.

SliceType = "[" "]" ElementType .

The length of a slice s can be discovered by the built-in functionlen; unlike with arrays it may change during execution. The elements can be addressed by integer indices0 through len(s)-1. The slice index of a given element may be less than the index of the same element in the underlying array.

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.

The array underlying a slice may extend past the end of the slice. The capacity is a measure of that extent: it is the sum of the length of the slice and the length of the array beyond the slice; a slice of length up to that capacity can be created byslicing a new one from the original slice. The capacity of a slice a can be discovered using the built-in function cap(a).

A new, initialized slice value for a given element type T may be made using the built-in functionmake, which takes a slice type and parameters specifying the length and optionally the capacity. A slice created with make always allocates a new, hidden array to which the returned slice value refers. That is, executing

make([]T, length, capacity)

produces the same slice as allocating an array and slicingit, 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() }

A field declared with a type but no explicit field name is called an embedded field. An embedded field must be specified as a type name T or as a pointer to a non-interface type name *T, and T itself may not be a pointer type. The unqualified type name acts as the field name.

// 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 }

A field or method f of an embedded field in a struct x is called promoted ifx.f is a legal selector that denotes that field or method f.

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

Given a struct type S and a named type T, promoted methods are included in the method set of the struct as follows:

If S contains an embedded field T, the method sets of S and *S both include promoted methods with receiverT. The method set of *S also includes promoted methods with receiver *T.
If S contains an embedded field *T, the method sets of S and *S both include promoted methods with receiver T or*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 interfaceand 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" }

A struct type T may not contain a field of type T, or of a type containing T as a component, directly or indirectly, if those containing types are only array or struct types.

// invalid struct types type ( T1 struct{ T1 } // T1 contains a field of T1 T2 struct{ f [10]T2 } // T2 contains T2 as component of an array T3 struct{ T4 } // T3 contains T3 as component of an array in struct T4 T4 struct{ f [10]T3 } // T4 contains T4 as component of struct T3 in an array )

// valid struct types type ( T5 struct{ f *T5 } // T5 contains T5 as component of a pointer T6 struct{ f func() T6 } // T6 contains T6 as component of a function type T7 struct{ f [10][]T7 } // T7 contains T7 as component of a slice in an array )

Pointer types

A pointer type denotes the set of all pointers to variables of a given type, called the base type of the pointer. The value of an uninitialized pointer is nil.

PointerType = "*" BaseType . BaseType = Type .

*Point *[4]int

Function types

A function type denotes the set of all functions with the same parameter and result types. The value of an uninitialized variable of function type is 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.

The final incoming parameter in a function signature may have a type prefixed with .... A function with such a parameter is called variadic and may be invoked with zero or more arguments for that parameter.

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

An interface type defines a type set. A variable of interface type can store a value of any type that is in the type set of the interface. Such a type is said toimplement the interface. The value of an uninitialized variable of interface type is 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 uniqueand not blank.

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

More than one type may implement an interface. For instance, if two types S1 and S2have the method set

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

(where T stands for either S1 or S2) then the File interface is implemented by both S1 andS2, regardless of what other methodsS1 and S2 may have or share.

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{}

For convenience, the predeclared type any is an alias for the empty interface.

Similarly, consider this interface specification, which appears within a type declarationto define an interface called Locker:

type Locker interface { Lock() Unlock() }

If S1 and S2 also implement

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

they implement the Locker interface as well as the File interface.

Embedded interfaces

In a slightly more general form an interface T may use a (possibly qualified) interface type name E as an interface element. This is called_embedding_ interface E in T. The type set of T is the intersection of the type sets defined by T's explicitly declared methods and the type sets of T’s embedded interfaces. In other words, the type set of T is the set of all types that implement all the explicitly declared methods of T and also all the methods ofE.

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 thesame 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

In their most general form, an interface element may also be an arbitrary type termT, or a term of the form ~T specifying the underlying type T, or a union of terms t1|t2|…|tn. Together with method specifications, these elements enable the precise definition of an interface's type set as follows:

The type set of the empty interface is the set of all non-interface types.
The type set of a non-empty interface is the intersection of the type sets of its interface elements.
The type set of a method specification is the set of all non-interface types whose method sets include that method.
The type set of a non-interface type term is the set consisting of just that type.
The type set of a term of the form ~T is the set of all types whose underlying type is T.
The type set of a union of termst1|t2|…|tn is the union of the type sets of the terms.

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 }

In a term of the form ~T, the underlying type of Tmust be itself, and T cannot be an interface.

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 }

The type T in a term of the form T or ~T cannot be a type parameter, and the type sets of all non-interface terms must be pairwise disjoint (the pairwise intersection of the type sets must be empty). Given a type parameter P:

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 }

Implementation restriction: A union (with more than one term) cannot contain thepredeclared identifier comparableor interfaces that specify methods, or embed comparable or interfaces that specify methods.

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 }

An interface type T may not embed a type element that is, contains, or embeds T, directly or indirectly.

// illegal: Bad may not embed itself type Bad interface { Bad }

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

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

// illegal: Bad4 may not embed an array containing Bad4 as element type type Bad4 interface { [10]Bad4 }

Implementing an interface

A type T implements an interface I if

T is not an interface and is an element of the type set of I; or
T is an interface and the type set of T is a subset of the type set of I.

A value of type T implements an interface if Timplements the interface.

Map types

A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keys of another type, called the key type. The value of an uninitialized map is nil.

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

The comparison operators == and != must be fully defined for operands of the key type; thus the key type must not be a function, map, or slice. If the key type is an interface type, these comparison operators must be defined for the dynamic key values; failure will cause a run-time panic.

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

The number of map elements is called its length. For a map m, it can be discovered using the built-in function lenand may change during execution. Elements may be added during execution using assignments and retrieved withindex expressions; they may be removed with thedelete andclear built-in function.

A new, empty map value is made using the built-in function make, which takes the map type and an optional capacity hint as arguments:

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

The initial capacity does not bound its size: maps grow to accommodate the number of items stored in them, with the exception of nil maps. A nil map is equivalent to an empty map except that no elements may be added.

Channel types

A channel provides a mechanism forconcurrently executing functionsto communicate bysending andreceivingvalues of a specified element type. The value of an uninitialized channel is nil.

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

The optional <- operator specifies the channel direction,send or receive. If a direction is given, the channel is directional, otherwise it is bidirectional. A channel may be constrained only to send or only to receive byassignment or explicit conversion.

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

The <- operator associates with the leftmost chanpossible:

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)

A new, initialized channel value can be made using the built-in functionmake, which takes the channel type and an optional capacity as arguments:

make(chan int, 100)

The capacity, in number of elements, sets the size of the buffer in the channel. If the capacity is zero or absent, the channel is unbuffered and communication succeeds only when both a sender and receiver are ready. Otherwise, the channel is buffered and communication succeeds without blocking if the buffer is not full (sends) or not empty (receives). A nil channel is never ready for communication.

A channel may be closed with the built-in functionclose. The multi-valued assignment form of thereceive operatorreports whether a received value was sent before the channel was closed.

A single channel may be used insend statements,receive operations, and calls to the built-in functionscap andlenby any number of goroutines without further synchronization. Channels act as first-in-first-out queues. For example, if one goroutine sends values on a channel and a second goroutine receives them, the values are received in the order sent.

Properties of types and values

Underlying types

Each type T has an underlying type: If Tis one of the predeclared boolean, numeric, or string types, or a type literal, the corresponding underlying type is T itself. Otherwise, T's underlying type is the underlying type of the type to which T refers in its declaration. For a type parameter that is the underlying type of itstype constraint, which is always an interface.

type ( A1 = string A2 = A1 )

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

func f[P any](x P) { … }

The underlying type of string, A1, A2, B1, and B2 is string. The underlying type of []B1, B3, and B4 is []B1. The underlying type of P is interface{}.

Core types

Each non-interface type T has a core type, which is the same as theunderlying type of T.

An interface T has a core type if one of the following conditions is satisfied:

There is a single type U which is the underlying typeof all types in the type set of T; or
the type set of T contains only channel typeswith identical element type E, and all directional channels have the same direction.

No other interfaces have a core type.

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

the type U; or
the type chan E if T contains only bidirectional channels, or the type chan<- E or <-chan Edepending on the direction of the directional channels present.

By definition, a core type is never a defined type,type parameter, orinterface 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

Some operations (slice expressions,append and copy) rely on a slightly more loose form of core types which accept byte slices and strings. Specifically, if there are exactly two types, []byte and string, which are the underlying types of all types in the type set of interface T, the core type of T is called bytestring.

Examples of interfaces with bytestring core types:

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

Note that bytestring is not a real type; it cannot be used to declare variables or compose other types. It exists solely to describe the behavior of some operations that read from a sequence of bytes, which may be a byte slice or a string.

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

B0 and B1 are different because they are new types created by distinct type definitions;func(int, float64) *B0 and func(x int, y float64) *[]stringare different because B0 is different from []string; and P1 and P2 are different because they are different type parameters.D0[int, string] and struct{ x int; y string } are different because the former is an instantiateddefined type while the latter is a type literal (but they are still assignable).

Assignability

A value x of type V is assignable to a variable of type T("x is assignable to T") if one of the following conditions applies:

V and T are identical.
V and T have identicalunderlying typesbut are not type parameters and at least one of Vor T is not a named type.
V and T are channel types with identical element types, V is a bidirectional channel, and at least one of V or T is not a named type.
T is an interface type, but not a type parameter, andx implements T.
x is the predeclared identifier nil and Tis a pointer, function, slice, map, channel, or interface type, but not a type parameter.
x is an untyped constant representableby a value of type T.

Additionally, if x's type V or T are type parameters, xis assignable to a variable of type T if one of the following conditions applies:

x is the predeclared identifier nil, T is a type parameter, and x is assignable to each type inT's type set.
V is not a named type, T is a type parameter, and x is assignable to each type inT's type set.
V is a type parameter and T is not a named type, and values of each type in V's type set are assignable to T.

Representability

A constant x is _representable_by a value of type T, where T is not a type parameter, if one of the following conditions applies:

x is in the set of values determined by T.
T is a floating-point type and x can be rounded to T's precision without overflow. Rounding uses IEEE 754 round-to-even rules but with an IEEE negative zero further simplified to an unsigned zero. Note that constant values never result in an IEEE negative zero, NaN, or infinity.
T is a complex type, and x'scomponents real(x) and imag(x)are representable by values of T's component type (float32 orfloat64).

If T is a type parameter,x is representable by a value of type T if x is representable by a value of each type in T's type set.

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 becalled on an operand of that type. Every type has a (possibly empty) method set associated with it:

The method set of a defined type T consists of allmethods declared with receiver type T.
The method set of a pointer to a defined type T(where T is neither a pointer nor an interface) is the set of all methods declared with receiver *T or T.
The method set of an interface type is the intersection of the method sets of each type in the interface's type set(the resulting method set is usually just the set of declared methods in the interface).

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 auniquenon-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:

The universe block encompasses all Go source text.
Each package has a package block containing all Go source text for that package.
Each file has a file block containing all Go source text in that file.
Each "if","for", and"switch" statement is considered to be in its own implicit block.
Each clause in a "switch" or "select" statement acts as an implicit block.

Blocks nest and influence scoping.

Declarations and scope

A declaration binds a non-blank identifier to aconstant,type,type parameter,variable,function,label, orpackage. 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.

The blank identifier may be used like any other identifier in a declaration, but it does not introduce a binding and thus is not declared. In the package block, the identifier init may only be used forinit function declarations, and like the blank identifier it does not introduce a new binding.

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:

The scope of a predeclared identifier is the universe block.
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.
The scope of the package name of an imported package is the file block of the file containing the import declaration.
The scope of an identifier denoting a method receiver, function parameter, or result variable is the function body.
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.
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.
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.
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

The blank identifier is represented by the underscore character _. It serves as an anonymous placeholder instead of a regular (non-blank) identifier and has special meaning in declarations, as an operand, and in assignment statements.

Predeclared identifiers

The following identifiers are implicitly declared in theuniverse 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 clear close complex copy delete imag len make max min 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:

the first character of the identifier's name is a Unicode uppercase letter (Unicode character category Lu); and
the identifier is declared in the package block or it is a field name ormethod name.

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 notexported. 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 _n_th identifier on the left is bound to the value of the _n_th 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

Within a parenthesized const declaration list the expression list may be omitted from any but the first ConstSpec. Such an empty list is equivalent to the textual substitution of the first preceding non-empty expression list and its type if any. Omitting the list of expressions is therefore equivalent to repeating the previous list. The number of identifiers must be equal to the number of expressions in the previous list. Together with the iota constant generatorthis mechanism permits light-weight declaration of sequential values:

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

Iota

Within a constant declaration, the predeclared identifieriota represents successive untyped integer constants. Its value is the index of the respective ConstSpecin that constant declaration, starting at zero. It can be used to construct a set of related constants:

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

By definition, multiple uses of iota in the same ConstSpec all have the same value:

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 repetitionof 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 sameunderlying 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 setof 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 fT 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 listexcept 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 typethat acts as a placeholder for an (as of yet) unknown type in the declaration. The type parameter is replaced with a type argument uponinstantiation 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 itstype constraint.

A parsing ambiguity arises when the type parameter list for a generic type declares a single type parameter P with a constraint Csuch that the text P C forms a valid expression:

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 aninterface 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.

Within a type parameter list of a generic type T, a type constraint may not (directly, or indirectly through the type parameter list of another generic type) refer to T.

type T1[P T1[P]] … // illegal: T1 refers to itself type T2[P interface{ T2[int] }] … // illegal: T2 refers to itself type T3[P interface{ m(T3[int])}] … // illegal: T3 refers to itself type T4[P T5[P]] … // illegal: T4 refers to T5 and type T5[P T4[P]] … // T5 refers to T4

type T6[P int] struct{ f *T6[P] } // ok: reference to T6 is not in type parameter list

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.

TypeConstraint = TypeElem .

If the constraint is an interface literal of the form interface{E} whereE is an embedded type element (not a method), in a type parameter list the enclosing interface{ … } may be omitted for convenience:

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

The predeclared interface type comparabledenotes the set of all non-interface types that arestrictly comparable.

Even though interfaces that are not type parameters are comparable, they are not strictly comparable and therefore they do not implement comparable. However, they satisfy comparable.

int // implements comparable (int is strictly comparable) []byte // does not implement comparable (slices cannot be compared) interface{} // does not implement comparable (see above) interface{ ~int | ~string } // type parameter only: implements comparable (int, string types are strictly comparable) interface{ comparable } // type parameter only: implements comparable (comparable implements itself) interface{ ~int | ~[]byte } // type parameter only: does not implement comparable (slices are not comparable) interface{ ~struct{ any } } // type parameter only: does not implement comparable (field any is not strictly comparable)

The comparable interface and interfaces that (directly or indirectly) embedcomparable may only be used as type constraints. They cannot be the types of values or variables, or components of other, non-interface types.

Satisfying a type constraint

A type argument T satisfies a type constraint Cif T is an element of the type set defined by C; i.e., if T implements C. As an exception, a strictly comparabletype constraint may also be satisfied by a comparable(not necessarily strictly comparable) type argument. More precisely:

A type T satisfies a constraint C if

T implements C; or
C can be written in the form interface{ comparable; E }, where E is a basic interface andT is comparable and implements E.

type argument type constraint // constraint satisfaction

int interface{ ~int } // satisfied: int implements interface{ ~int } string comparable // satisfied: string implements comparable (string is strictly comparable) []byte comparable // not satisfied: slices are not comparable any interface{ comparable; int } // not satisfied: any does not implement interface{ int } any comparable // satisfied: any is comparable and implements the basic interface any struct{f any} comparable // satisfied: struct{f any} is comparable and implements the basic interface any any interface{ comparable; m() } // not satisfied: any does not implement the basic interface interface{ m() } interface{ m() } interface{ comparable; m() } // satisfied: interface{ m() } is comparable and implements the basic interface interface{ m() }

Because of the exception in the constraint satisfaction rule, comparing operands of type parameter type may panic at run-time (even though comparable type parameters are always strictly comparable).

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.

If a type is present, each variable is given that type. Otherwise, each variable is given the type of the corresponding initialization value in the assignment. If that value is an untyped constant, it is first implicitlyconverted to its default type; if it is an untyped boolean value, it is first implicitly converted to type bool. The predeclared value nil cannot be used to initialize a variable with no explicit type.

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 declarationwith 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

Unlike regular variable declarations, a short variable declaration may _redeclare_variables provided they were originally declared earlier in the same block (or the parameter lists if the block is the function body) with the same type, and at least one of the non-blank variables is new. As a consequence, redeclaration can only appear in a multi-variable short declaration. Redeclaration does not introduce a new variable; it just assigns a new value to the original. The non-blank variable names on the left side of :=must be unique.

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 .

The receiver is specified via an extra parameter section preceding the method name. That parameter section must declare a single non-variadic parameter, the receiver. Its type must be a defined type T or a pointer to a defined type T, possibly followed by a list of type parameter names [P1, P2, …] enclosed in square brackets.T is called the receiver base type. A receiver base type cannot be a pointer or interface type and it must be defined in the same package as the method. The method is said to be bound to its receiver base type and the method name is visible only within selectors for type Tor *T.

A non-blank receiver identifier must beunique 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.

Given defined type Point the declarations

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 }

bind the methods Length and Scale, with receiver type *Point, to the base type Point.

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 aninstantiation 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 aconstant,variable, orfunction, or a parenthesized expression.

An operand name denoting a generic functionmay 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 emptytype 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 beblank.

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 .

The LiteralType's core type Tmust be a struct, array, slice, or map type (the syntax enforces this constraint except when the type is given as a TypeName). The types of the elements and keys must be assignableto the respective field, element, and key types of type T; there is no additional conversion. The key is interpreted as a field name for struct literals, an index for array and slice literals, and a key for map literals. For map literals, all elements must have a key. It is an error to specify multiple elements with the same field name or constant key value. For non-constant map keys, see the section onevaluation order.

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:

Each element has an associated integer index marking its position in the array.
An element with a key uses the key as its index. The key must be a non-negative constantrepresentable by a value of type int; and if it is typed it must be of integer type.
An element without a key uses the previous element's index plus one. If the first element has no key, its index is zero.

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 withnew.

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

The length of an array literal is the length specified in the literal type. If fewer elements than the length are provided in the literal, the missing elements are set to the zero value for the array element type. It is an error to provide elements with index values outside the index range of the array. The notation ... specifies an array length equal to the maximum element index plus one.

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]

Within a composite literal of array, slice, or map type T, elements or map keys that are themselves composite literals may elide the respective literal type if it is identical to the element or key type of T. Similarly, elements or keys that are addresses of composite literals may elide the &T when the element or key type is *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 thekeyword 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.

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

For a primary expression xthat is not a package name, the_selector expression_

x.f

denotes the field or method f of the value x(or sometimes *x; see below). The identifier f is called the (field or method) selector; it must not be the blank identifier. The type of the selector expression is the type of f. If x is a package name, see the section onqualified identifiers.

A selector f may denote a field or method f of a type T, or it may refer to a field or method f of a nestedembedded field of T. The number of embedded fields traversed to reach f is called its depth in T. The depth of a field or method fdeclared in T is zero. The depth of a field or method f declared in an embedded field A in T is the depth of f in A plus one.

The following rules apply to selectors:

For a value x of type T or *Twhere T is not a pointer or interface type,x.f denotes the field or method at the shallowest depth in T where there is such an f. If there is not exactly one fwith shallowest depth, the selector expression is illegal.
For a value x of type I where Iis an interface type, x.f denotes the actual method with namef of the dynamic value of x. If there is no method with name f in themethod set of I, the selector expression is illegal.
As an exception, if the type of x is a definedpointer type and (*x).f is a valid selector expression denoting a field (but not a method), x.f is shorthand for (*x).f.
In all other cases, x.f is illegal.
If x is of pointer type and has the valuenil and x.f denotes a struct field, assigning to or evaluating x.fcauses a run-time panic.
If x is of interface type and has the valuenil, calling orevaluating the method x.fcauses a run-time panic.

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

If M is in the method set of type T,T.M is a function that is callable as a regular function with the same arguments as M prefixed by an additional argument that is the receiver of the method.

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

Consider a struct type T with two methods,Mv, whose receiver is of type T, andMp, whose receiver is of type *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

yields a function equivalent to Mv but with an explicit receiver as its first argument; it has signature

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

yields a function value representing Mp with signature

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

yields a function value representing Mv with signature

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.

Function values derived from methods are called with function call syntax; the receiver is provided as the first argument to the call. That is, given f := T.Mv, f is invoked as f(t, 7) not t.f(7). To construct a function that binds the receiver, use afunction literal ormethod value.

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

If the expression x has static type T andM is in the method set of type T,x.M is called a method value. The method value x.M is a function value that is callable with the same arguments as a method call of x.M. The expression x is evaluated and saved during the evaluation of the method value; the saved copy is then used as the receiver in any calls, which may be executed later.

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

The type T may be an interface or non-interface type.

As in the discussion of method expressions above, consider a struct type T with two methods,Mv, whose receiver is of type T, andMp, whose receiver is of type *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

As with selectors, a reference to a non-interface method with a value receiver using a pointer will automatically dereference that pointer: pt.Mv is equivalent to (*pt).Mv.

As with method calls, a reference to a non-interface method with a pointer receiver using an addressable value will automatically take the address of that value: t.Mp is equivalent to (&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]

denotes the element of the array, pointer to array, slice, string or map a indexed by x. The value x is called the index or map key, respectively. The following rules apply:

If a is neither a map nor a type parameter:

the index x must be an untyped constant or itscore type must be an integer
a constant index must be non-negative andrepresentable by a value of type int
a constant index that is untyped is given type int
the index x is in range if 0 <= x < len(a), otherwise it is out of range

For a of array type A:

a constant index must be in range
if x is out of range at run time, a run-time panic occurs
a[x] is the array element at index x and the type ofa[x] is the element type of A

For a of pointer to array type:

a[x] is shorthand for (*a)[x]

For a of slice type S:

if x is out of range at run time, a run-time panic occurs
a[x] is the slice element at index x and the type ofa[x] is the element type of S

For a of string type:

a constant index must be in range if the string a is also constant
if x is out of range at run time, a run-time panic occurs
a[x] is the non-constant byte value at index x and the type ofa[x] is byte
a[x] may not be assigned to

For a of map type M:

x's type must beassignable to the key type of M
if the map contains an entry with key x,a[x] is the map element with key x and the type of a[x] is the element type of M
if the map is nil or does not contain such an entry,a[x] is the zero value for the element type of M

For a of type parameter type P:

The index expression a[x] must be valid for values of all types in P's type set.
The element types of all types in P's type set must be identical. In this context, the element type of a string type is byte.
If there is a map type in the type set of P, all types in that type set must be map types, and the respective key types must be all identical.
a[x] is the array, slice, or string element at index x, or the map element with key x of the type argument that P is instantiated with, and the type of a[x] is the type of the (identical) element types.
a[x] may not be assigned to if P's type set includes string types.

Otherwise a[x] is illegal.

An index expression on a map a of type map[K]Vused in an assignment statement or initialization of the special form

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

yields an additional untyped boolean value. The value of ok istrue if the key x is present in the map, andfalse otherwise.

Assigning to an element of a nil map causes arun-time panic.

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]

constructs a substring or slice. The core type ofa must be a string, array, pointer to array, slice, or abytestring. The indices low andhigh select which elements of operand a appear in the result. The result has indices starting at 0 and length equal tohigh - low. After slicing the array a

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

the slice s has type []int, length 3, capacity 4, and elements

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

For convenience, any of the indices may be omitted. A missing lowindex defaults to zero; a missing high index defaults to the length of the sliced operand:

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

If a is a pointer to an array, a[low : high] is shorthand for(*a)[low : high].

For arrays or strings, the indices are in range if0 <= low <= high <= len(a), otherwise they are out of range. For slices, the upper index bound is the slice capacity cap(a) rather than the length. A constant index must be non-negative andrepresentable by a value of typeint; for arrays or constant strings, constant indices must also be in range. If both indices are constant, they must satisfy low <= high. If the indices are out of range at run time, a run-time panic occurs.

Except for untyped strings, if the sliced operand is a string or slice, the result of the slice operation is a non-constant value of the same type as the operand. For untyped string operands the result is a non-constant value of type string. If the sliced operand is an array, it must be addressableand the result of the slice operation is a slice with the same element type as the array.

If the sliced operand of a valid slice expression is a nil slice, the result is a nil slice. Otherwise, if the result is a slice, it shares its underlying array with the operand.

var a [10]int s1 := a[3:7] // underlying array of s1 is array a; &s1[2] == &a[5] s2 := s1[1:4] // underlying array of s2 is underlying array of s1 which is array a; &s2[1] == &a[5] s2[1] = 42 // s2[1] == s1[2] == a[5] == 42; they all refer to the same underlying array element

var s []int s3 := s[:0] // s3 == nil

Full slice expressions

The primary expression

a[low : high : max]

constructs a slice of the same type, and with the same length and elements as the simple slice expression a[low : high]. Additionally, it controls the resulting slice's capacity by setting it to max - low. Only the first index may be omitted; it defaults to 0. The core type of a must be an array, pointer to array, or slice (but not a string). After slicing the array a

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

the slice t has type []int, length 2, capacity 4, and elements

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

As for simple slice expressions, if a is a pointer to an array,a[low : high : max] is shorthand for (*a)[low : high : max]. If the sliced operand is an array, it must be addressable.

The indices are in range if 0 <= low <= high <= max <= cap(a), otherwise they are out of range. A constant index must be non-negative andrepresentable by a value of typeint; for arrays, constant indices must also be in range. If multiple indices are constant, the constants that are present must be in range relative to each other. If the indices are out of range at run time, a run-time panic occurs.

Type assertions

For an expression x of interface type, but not a type parameter, and a type T, the primary expression

x.(T)

asserts that x is not niland that the value stored in x is of type T. The notation x.(T) is called a type assertion.

More precisely, if T is not an interface type, x.(T) asserts that the dynamic type of x is identicalto the type T. In this case, T must implement the (interface) type of x; otherwise the type assertion is invalid since it is not possible for xto store a value of type T. If T is an interface type, x.(T) asserts that the dynamic type of x implements the interface T.

If the type assertion holds, the value of the expression is the value stored in x and its type is T. If the type assertion is false, a run-time panic occurs. In other words, even though the dynamic type of xis known only at run time, the type of x.(T) is known to be T in a correct program.

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

yields an additional untyped boolean value. The value of ok is trueif the assertion holds. Otherwise it is false and the value of v is the zero value for type T. No run-time panic occurs in this case.

Calls

Given an expression f with a core type F of function type,

f(a1, a2, … an)

calls f with arguments a1, a2, … an. Except for one special case, arguments must be single-valued expressionsassignable to the parameter types ofF and are evaluated before the function is called. The type of the expression is the result type of F. A method invocation is similar but the method itself is specified as a selector upon a value of the receiver type for the method.

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

If f denotes a generic function, it must beinstantiated before it can be called or used as a function value.

In a function call, the function value and arguments are evaluated inthe 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.

Calling a nil function value causes a run-time panic.

As a special case, if the return values of a function or methodg are equal in number and individually assignable to the parameters of another function or methodf, then the call f(g(_parametersofg_))will invoke f after binding the return values ofg to the parameters of f in order. The call of f must contain no parameters other than the call of g, and g must have at least one return value. If f has a final ... parameter, it is assigned the return values of g that remain after assignment of regular parameters.

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") }

A method call x.m() is valid if the method setof (the type of) x contains m and the argument list can be assigned to the parameter list of m. If x is addressable and &x's method set contains m, x.m() is shorthand for (&x).m():

var p Point p.Scale(3.5)

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

Passing arguments to `...` parameters

If f is variadic with a final parameter p of type ...T, then within fthe type of p is equivalent to type []T. If f is invoked with no actual arguments for p, the value passed to p is nil. Otherwise, the value passed is a new slice of type []T with a new underlying array whose successive elements are the actual arguments, which all must be assignableto T. The length and capacity of the slice is therefore the number of arguments bound to p and may differ for each call site.

Given the function and calls

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

within Greeting, who will have the valuenil in the first call, and[]string{"Joe", "Anna", "Eileen"} in the second.

If the final argument is assignable to a slice type []T and is followed by ..., it is passed unchanged as the value for a ...T parameter. In this case no new slice is created.

Given the slice s and call

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

within Greeting, who will have the same value as swith the same underlying array.

Instantiations

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

Each type argument is substituted for its corresponding type parameter in the generic declaration. This substitution happens across the entire function or type declaration, including the type parameter list itself and any types in that list.
After substitution, each type argument must satisfythe constraint (instantiated, if necessary) of the corresponding type parameter. Otherwise instantiation fails.

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 satisfies any [S ~[]E, E any] []int, int []int satisfies ~[]int, int satisfies any [P io.Writer] string illegal: string doesn't satisfy io.Writer [P comparable] any any satisfies (but does not implement) comparable

When using a generic function, type arguments may be provided explicitly, or they may be partially or completely inferredfrom the context in which the function is used. Provided that they can be inferred, type argument lists may be omitted entirely if the function is:

called with ordinary arguments,
assigned to a variable with a known type
passed as an argument to another function, or
returned as a result.

In all other cases, a (possibly partial) type argument list must be present. If a type argument list is absent or partial, all missing type arguments must be inferrable from the context in which the function is used.

// sum returns the sum (concatenation, for strings) of its arguments. func sum[T ~int | ~float64 | ~string](x... T) T { … }

x := sum // illegal: the type of x is unknown intSum := sum[int] // intSum has type func(x... int) int a := intSum(2, 3) // a has value 5 of type int b := sum[float64](2.0, 3) // b has value 5.0 of type float64 c := sum(b, -1) // c has value 4.0 of type float64

type sumFunc func(x... string) string var f sumFunc = sum // same as var f sumFunc = sum[string] f = sum // same as f = sum[string]

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 func(E) E) S { … }

f0 := apply[] // illegal: type argument list cannot be empty f1 := apply[[]int] // type argument for S explicitly provided, type argument for E inferred f2 := apply[[]string, string] // both type arguments explicitly provided

var bytes []byte r := apply(bytes, func(byte) byte { … }) // both type arguments inferred from the function arguments

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

Type inference

A use of a generic function may omit some or all type arguments if they can be_inferred_ from the context within which the function is used, including the constraints of the function's type parameters. Type inference succeeds if it can infer the missing type arguments and instantiation succeeds with the inferred type arguments. Otherwise, type inference fails and the program is invalid.

Type inference uses the type relationships between pairs of types for inference: For instance, a function argument must be assignableto its respective function parameter; this establishes a relationship between the type of the argument and the type of the parameter. If either of these two types contains type parameters, type inference looks for the type arguments to substitute the type parameters with such that the assignability relationship is satisfied. Similarly, type inference uses the fact that a type argument mustsatisfy the constraint of its respective type parameter.

Each such pair of matched types corresponds to a type equation containing one or multiple type parameters, from one or possibly multiple generic functions. Inferring the missing type arguments means solving the resulting set of type equations for the respective type parameters.

For example, given

// dedup returns a copy of the argument slice with any duplicate entries removed. func dedupS ~[]E, E comparable S { … }

type Slice []int var s Slice s = dedup(s) // same as s = dedupSlice, int

the variable s of type Slice must be assignable to the function parameter type S for the program to be valid. To reduce complexity, type inference ignores the directionality of assignments, so the type relationship between Slice and S can be expressed via the (symmetric) type equation Slice ≡A S(or S ≡A Slice for that matter), where the A in ≡Aindicates that the LHS and RHS types must match per assignability rules (see the section on type unification for details). Similarly, the type parameter S must satisfy its constraint~[]E. This can be expressed as S ≡C ~[]Ewhere X ≡C Y stands for "X satisfies constraint Y". These observations lead to a set of two equations

Slice ≡A S      (1)
S     ≡C ~[]E   (2)

which now can be solved for the type parameters S and E. From (1) a compiler can infer that the type argument for S is Slice. Similarly, because the underlying type of Slice is []intand []int must match []E of the constraint, a compiler can infer that E must be int. Thus, for these two equations, type inference infers

S ➞ Slice
E ➞ int

Given a set of type equations, the type parameters to solve for are the type parameters of the functions that need to be instantiated and for which no explicit type arguments is provided. These type parameters are called bound type parameters. For instance, in the dedup example above, the type parametersP and E are bound to dedup. An argument to a generic function call may be a generic function itself. The type parameters of that function are included in the set of bound type parameters. The types of function arguments may contain type parameters from other functions (such as a generic function enclosing a function call). Those type parameters may also appear in type equations but they are not bound in that context. Type equations are always solved for the bound type parameters only.

Type inference supports calls of generic functions and assignments of generic functions to (explicitly function-typed) variables. This includes passing generic functions as arguments to other (possibly also generic) functions, and returning generic functions as results. Type inference operates on a set of equations specific to each of these cases. The equations are as follows (type argument lists are omitted for clarity):

For a function call f(a0, a1, …) wheref or a function argument ai is a generic function:
Each pair (ai, pi) of corresponding function arguments and parameters where ai is not anuntyped constant yields an equationtypeof(pi) ≡A typeof(ai).
If ai is an untyped constant cj, and typeof(pi) is a bound type parameter Pk, the pair (cj, Pk) is collected separately from the type equations.
For an assignment v = f of a generic function f to a (non-generic) variable v of function type:
typeof(v) ≡A typeof(f).
For a return statement return …, f, … where f is a generic function returned as a result to a (non-generic) result variabler of function type:
typeof(r) ≡A typeof(f).

Additionally, each type parameter Pk and corresponding type constraintCk yields the type equationPk ≡C Ck.

Type inference gives precedence to type information obtained from typed operands before considering untyped constants. Therefore, inference proceeds in two phases:

The type equations are solved for the bound type parameters using type unification. If unification fails, type inference fails.
For each bound type parameter Pk for which no type argument has been inferred yet and for which one or more pairs(cj, Pk) with that same type parameter were collected, determine the constant kind of the constants cj in all those pairs the same way as forconstant expressions. The type argument for Pk is thedefault type for the determined constant kind. If a constant kind cannot be determined due to conflicting constant kinds, type inference fails.

If not all type arguments have been found after these two phases, type inference fails.

If the two phases are successful, type inference determined a type argument for each bound type parameter:

Pk ➞ Ak

A type argument Ak may be a composite type, containing other bound type parameters Pk as element types (or even be just another bound type parameter). In a process of repeated simplification, the bound type parameters in each type argument are substituted with the respective type arguments for those type parameters until each type argument is free of bound type parameters.

If type arguments contain cyclic references to themselves through bound type parameters, simplification and thus type inference fails. Otherwise, type inference succeeds.

Type unification

Type inference solves type equations through type unification. Type unification recursively compares the LHS and RHS types of an equation, where either or both types may be or contain bound type parameters, and looks for type arguments for those type parameters such that the LHS and RHS match (become identical or assignment-compatible, depending on context). To that effect, type inference maintains a map of bound type parameters to inferred type arguments; this map is consulted and updated during type unification. Initially, the bound type parameters are known but the map is empty. During type unification, if a new type argument A is inferred, the respective mapping P ➞ A from type parameter to argument is added to the map. Conversely, when comparing types, a known type argument (a type argument for which a map entry already exists) takes the place of its corresponding type parameter. As type inference progresses, the map is populated more and more until all equations have been considered, or until unification fails. Type inference succeeds if no unification step fails and the map has an entry for each type parameter.

For example, given the type equation with the bound type parameterP

[10]struct{ elem P, list []P } ≡A [10]struct{ elem string; list []string }

type inference starts with an empty map. Unification first compares the top-level structure of the LHS and RHS types. Both are arrays of the same length; they unify if the element types unify. Both element types are structs; they unify if they have the same number of fields with the same names and if the field types unify. The type argument for P is not known yet (there is no map entry), so unifying P with string adds the mapping P ➞ string to the map. Unifying the types of the list field requires unifying []P and []string and thus P and string. Since the type argument for P is known at this point (there is a map entry for P), its type argumentstring takes the place of P. And since string is identical to string, this unification step succeeds as well. Unification of the LHS and RHS of the equation is now finished. Type inference succeeds because there is only one type equation, no unification step failed, and the map is fully populated.

Unification uses a combination of exact and _loose_unification depending on whether two types have to beidentical,assignment-compatible, or only structurally equal. The respective type unification rulesare spelled out in detail in the Appendix.

For an equation of the form X ≡A Y, where X and Y are types involved in an assignment (including parameter passing and return statements), the top-level type structures may unify loosely but element types must unify exactly, matching the rules for assignments.

For an equation of the form P ≡C C, where P is a type parameter and Cits corresponding constraint, the unification rules are bit more complicated:

If C has a core type core(C) and P has a known type argument A,core(C) and A must unify loosely. If P does not have a known type argument and C contains exactly one type term T that is not an underlying (tilde) type, unification adds the mapping P ➞ T to the map.
If C does not have a core type and P has a known type argument A,A must have all methods of C, if any, and corresponding method types must unify exactly.

When solving type equations from type constraints, solving one equation may infer additional type arguments, which in turn may enable solving other equations that depend on those type arguments. Type inference repeats type unification as long as new type arguments are inferred.

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 identicalunless the operation involves shifts or untyped constants. For operations involving constants only, see the section onconstant expressions.

Except for shift operations, if one operand is an untyped constantand the other operand is not, the constant is implicitly convertedto the type of the other operand.

The right operand in a shift expression must have integer typeor be an untyped constant representable by a value of type uint. If the left operand of a non-constant shift expression is an untyped constant, it is first implicitly converted to the type it would assume if the shift expression were replaced by its left operand alone.

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

Unary operators have the highest precedence. As the ++ and -- operators form statements, not expressions, they fall outside the operator hierarchy. As a consequence, statement *p++ is the same as (*p)++.

There are five precedence levels for binary operators. Multiplication operators bind strongest, followed by addition operators, comparison operators, && (logical AND), and finally || (logical OR):

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

Binary operators of the same precedence associate from left to right. For instance, x / y * z is the same as (x / y) * z.

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

Arithmetic operators

Arithmetic operators apply to numeric values and yield a result of the same type as the first operand. The four standard arithmetic operators (+,-, *, /) apply tointeger, floating-point, andcomplex types; + also applies to strings. The bitwise logical and shift operators apply to integers only.

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 }

the product x * y and the addition s += x * yare computed with float32 or float64 precision, respectively, depending on the type argument for F.

Integer operators

For two integer values x and y, the integer quotientq = x / y and remainder r = x % y satisfy the following relationships:

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

with x / y truncated towards zero ("truncated division").

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

The one exception to this rule is that if the dividend x is the most negative value for the int type of x, the quotientq = x / -1 is equal to x (and r = 0) due to two's-complement integer overflow:

                     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

The shift operators shift the left operand by the shift count specified by the right operand, which must be non-negative. If the shift count is negative at run time, a run-time panic occurs. The shift operators implement arithmetic shifts if the left operand is a signed integer and logical shifts if it is an unsigned integer. There is no upper limit on the shift count. Shifts behave as if the left operand is shifted n times by 1 for a shift count of n. As a result, x << 1 is the same as x*2and x >> 1 is the same asx/2 but truncated towards negative infinity.

For integer operands, the unary operators+, -, and ^ are defined as follows:

+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

For unsigned integer values, the operations +,-, *, and << are computed modulo 2_n_, where n is the bit width of the unsigned integer's type. Loosely speaking, these unsigned integer operations discard high bits upon overflow, and programs may rely on "wrap around".

For signed integers, the operations +,-, *, /, and << may legally overflow and the resulting value exists and is deterministically defined by the signed integer representation, the operation, and its operands. Overflow does not cause a run-time panic. A compiler may not optimize code under the assumption that overflow does not occur. For instance, it may not assume that x < x + 1 is always true.

Floating-point operators

For floating-point and complex numbers,+x is the same as x, while -x is the negation of x. The result of a floating-point or complex division by zero is not specified beyond the IEEE-754 standard; whether a run-time panicoccurs is implementation-specific.

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.

For instance, some architectures provide a "fused multiply and add" (FMA) instruction that computes x*y + z without rounding the intermediate result x*y. These examples show when a Go implementation can use that instruction:

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

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

String concatenation

Strings can be concatenated using the + operator or the += assignment operator:

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 assignableto the type of the second operand, or vice versa.

The equality operators == and != apply to operands of comparable types. The ordering operators <, <=, >, and >=apply to operands of ordered types. These terms and the result of the comparisons are defined as follows:

Boolean types are comparable. Two boolean values are equal if they are either bothtrue or both false.
Integer types are comparable and ordered. Two integer values are compared in the usual way.
Floating-point types are comparable and ordered. Two floating-point values are compared as defined by the IEEE-754 standard.
Complex types are comparable. Two complex values u and v are equal if both real(u) == real(v) andimag(u) == imag(v).
String types are comparable and ordered. Two string values are compared lexically byte-wise.
Pointer types are comparable. Two pointer values are equal if they point to the same variable or if both have value nil. Pointers to distinct zero-size variables may or may not be equal.
Channel types are comparable. Two channel values are equal if they were created by the same call tomake or if both have value nil.
Interface types that are not type parameters are comparable. Two interface values are equal if they have identical dynamic types and equal dynamic values or if both have value nil.
A value x of non-interface type X and a value t of interface type T can be compared if type X is comparable andX implements T. They are equal if t's dynamic type is identical to X and t's dynamic value is equal to x.
Struct types are comparable if all their field types are comparable. Two struct values are equal if their corresponding non-blank field values are equal. The fields are compared in source order, and comparison stops as soon as two field values differ (or all fields have been compared).
Array types are comparable if their array element types are comparable. Two array values are equal if their corresponding element values are equal. The elements are compared in ascending index order, and comparison stops as soon as two element values differ (or all elements have been compared).
Type parameters are comparable if they are strictly comparable (see below).

A comparison of two interface values with identical dynamic types causes a run-time panic if that type is not comparable. This behavior applies not only to direct interface value comparisons but also when comparing arrays of interface values or structs with interface-valued fields.

Slice, map, and function types are not comparable. However, as a special case, a slice, map, or function value may be compared to the predeclared identifier nil. Comparison of pointer, channel, and interface values to nilis also allowed and follows from the general rules above.

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 )

A type is strictly comparable if it is comparable and not an interface type nor composed of interface types. Specifically:

Boolean, numeric, string, pointer, and channel types are strictly comparable.
Struct types are strictly comparable if all their field types are strictly comparable.
Array types are strictly comparable if their array element types are strictly comparable.
Type parameters are strictly comparable if all types in their type set are strictly comparable.

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

For an operand x of type T, the address operation&x generates a pointer of type *T to x. The operand must be addressable, that is, either a variable, pointer indirection, or slice indexing operation; or a field selector of an addressable struct operand; or an array indexing operation of an addressable array. As an exception to the addressability requirement, x may also be a (possibly parenthesized)composite literal. If the evaluation of x would cause a run-time panic, then the evaluation of &x does too.

For an operand x of pointer type *T, the pointer indirection *x denotes the variable of type T pointed to by x. If x is nil, an attempt to evaluate *xwill cause a run-time panic.

&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

For an operand ch whose core type is achannel, the value of the receive operation <-ch is the value received from the channel ch. The channel direction must permit receive operations, and the type of the receive operation is the element type of the channel. The expression blocks until a value is available. Receiving from a nil channel blocks forever. A receive operation on a closed channel can always proceed immediately, yielding the element type's zero valueafter any previously sent values have been received.

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

yields an additional untyped boolean result reporting whether the communication succeeded. The value of ok is trueif the value received was delivered by a successful send operation to the channel, or false if it is a zero value generated because the channel is closed and empty.

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.

An explicit conversion is an expression of the form T(x)where T is a type and x is an expression that can be converted to type T.

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

If the type starts with the operator * or <-, or if the type starts with the keyword funcand has no result list, it must be parenthesized when necessary to avoid ambiguity:

*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)

A constant value x can be converted to type T if x is representableby a value of T. As a special case, an integer constant x can be explicitly converted to astring type using thesame ruleas for non-constant x.

Converting a constant to a type that is not a type parameteryields 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 fP ~float32|~float64 { … P(1.1) … }

the conversion P(1.1) results in a non-constant value of type Pand the value 1.1 is represented as a float32 or a float64depending on the type argument for f. Accordingly, if f is instantiated with a float32 type, the numeric value of the expression P(1.1) + 1.2 will be computed with the same precision as the corresponding non-constant float32addition.

A non-constant value x can be converted to type Tin any of these cases:

x is assignable to T.
ignoring struct tags (see below),x's type and T are nottype parameters but haveidentical underlying types.
ignoring struct tags (see below),x's type and T are pointer types that are not named types, and their pointer base types are not type parameters but have identical underlying types.
x's type and T are both integer or floating point types.
x's type and T are both complex types.
x is an integer or a slice of bytes or runes and T is a string type.
x is a string and T is a slice of bytes or runes.
x is a slice, T is an array or a pointer to an array, and the slice and array types have identical element types.

Additionally, if T or x's type V are type parameters, xcan also be converted to type T if one of the following conditions applies:

Both V and T are type parameters and a value of each type in V's type set can be converted to each type in T's type set.
Only V is a type parameter and a value of each type in V's type set can be converted to T.
Only T is a type parameter and x can be converted to each type in T's type set.

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

Specific rules apply to (non-constant) conversions between numeric types or to and from a string type. These conversions may change the representation of xand incur a run-time cost. All other conversions only change the type but not the representation of x.

There is no linguistic mechanism to convert between pointers and integers. The package unsafeimplements this functionality under restricted circumstances.

Conversions between numeric types

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

When converting between integer types, if the value is a signed integer, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type's size. For example, if v := uint16(0x10F0), then uint32(int8(v)) == 0xFFFFFFF0. The conversion always yields a valid value; there is no indication of overflow.
When converting a floating-point number to an integer, the fraction is discarded (truncation towards zero).
When converting an integer or floating-point number to a floating-point type, or a complex number to another complex type, the result value is rounded to the precision specified by the destination type. For instance, the value of a variable x of type float32may be stored using additional precision beyond that of an IEEE-754 32-bit number, but float32(x) represents the result of rounding x's value to 32-bit precision. Similarly, x + 0.1 may use more than 32 bits of precision, but float32(x + 0.1) does not.

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

Converting a slice of bytes to a string type yields a string whose successive bytes are the elements of the slice.
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'}) // "🌍"
Converting a slice of runes to a string type yields a string that is the concatenation of the individual rune values converted to strings.
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" == "🌎"
Converting a value of a string type to a slice of bytes type yields a slice whose successive elements are the bytes of the string.
[]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'}
Converting a value of a string type to a slice of runes type yields a slice containing the individual Unicode code points of the string.
[]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}
Finally, for historical reasons, an integer value may be converted to a string type. This form of conversion yields a string containing the (possibly multi-byte) UTF-8 representation of the Unicode code point with the given integer value. Values outside the range of valid Unicode code points are converted to "\uFFFD".
string('a') // "a"
string(65) // "A"
string('\xf8') // "\u00f8" == "ø" == "\xc3\xb8"
string(-1) // "\ufffd" == "\xef\xbf\xbd"
type myString string
myString('\u65e5') // "\u65e5" == "日" == "\xe6\x97\xa5"
Note: This form of conversion may eventually be removed from the language. The go vet tool flags certain integer-to-string conversions as potential errors. Library functions such asutf8.AppendRune orutf8.EncodeRuneshould be used instead.

Conversions from slice to array or array pointer

Converting a slice to an array yields an array containing the elements of the underlying array of the slice. Similarly, converting a slice to an array pointer yields a pointer to the underlying array of the slice. In both cases, if the length of the slice is less than the length of the array, a run-time panic occurs.

s := make([]byte, 2, 4)

a0 := [0]byte(s) a1 := [1]byte(s[1:]) // a1[0] == s[1] a2 := [2]byte(s) // a2[0] == s[0] a4 := [4]byte(s) // panics: len([4]byte) > len(s)

s0 := ([0]byte)(s) // s0 != nil s1 := ([1]byte)(s[1:]) // &s1[0] == &s[1] s2 := ([2]byte)(s) // &s2[0] == &s[0] s4 := ([4]byte)(s) // panics: len([4]byte) > len(s)

var t []string t0 := [0]string(t) // ok for nil slice t t1 := ([0]string)(t) // t1 == nil t2 := ([1]string)(t) // panics: len([1]string) > len(t)

u := make([]byte, 0) u0 := (*[0]byte)(u) // u0 != nil

Constant expressions

Constant expressions may contain only constantoperands 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 constantshift 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 ofinteger 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)

Applying the built-in function complex to untyped integer, rune, or floating-point constants yields an untyped complex constant.

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 accuratelyrepresentable 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)

The mask used by the unary bitwise complement operator ^ matches the rule for non-constants: the mask is all 1s for unsigned constants and -1 for signed and untyped constants.

^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 dependenciesdetermine the evaluation order of individual initialization expressions invariable declarations. Otherwise, when evaluating the operands of an expression, assignment, orreturn 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()

the function calls and communication happen in the orderf(), h(), i(), j(),<-c, g(), and k(). However, the order of those events compared to the evaluation and indexing of x and the evaluation of y is not specified.

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

The function calls happen in the orderu(), sqr(), v(),f(), v(), and g().

Floating-point operations within a single expression are evaluated according to the associativity of the operators. Explicit parentheses affect the evaluation by overriding the default associativity. In the expression x + (y + z) the addition y + zis performed before adding x.

Statements

Statements control execution.

Terminating statements

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

A "return" or"goto" statement.
A call to the built-in functionpanic.
A block in which the statement list ends in a terminating statement.
An "if" statement in which:
- the "else" branch is present, and
- both branches are terminating statements.
A "for" statement in which:
- there are no "break" statements referring to the "for" statement, and
- the loop condition is absent, and
- the "for" statement does not use a range clause.
A "switch" statement in which:
- there are no "break" statements referring to the "switch" statement,
- there is a default case, and
- the statement lists in each case, including the default, end in a terminating statement, or a possibly labeled "fallthrough" statement.
A "select" statement in which:
- there are no "break" statements referring to the "select" statement, and
- the statement lists in each case, including the default if present, end in a terminating statement.
A labeled statement labeling a terminating statement.

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.

EmptyStmt = .

Labeled statements

A labeled statement may be the target of a goto,break or continue statement.

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

Error: log.Panic("error encountered")

Expression statements

With the exception of specific built-in functions, function and method calls andreceive operationscan 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 unsafe.SliceData unsafe.String unsafe.StringData

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 typemust be a channel, the channel direction must permit send operations, and the type of the value to be sent must be assignableto the channel's element type.

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

Both the channel and the value expression are evaluated before communication begins. Communication blocks until the send can proceed. A send on an unbuffered channel can proceed if a receiver is ready. A send on a buffered channel can proceed if there is room in the buffer. A send on a closed channel proceeds by causing a run-time panic. A send on a nil channel blocks forever.

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

IncDec statements

The "++" and "--" statements increment or decrement their operands by the untyped constant 1. As with an assignment, the operand must be addressableor a map index expression.

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 variablewith 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 ] "=" .

Each left-hand side operand must be addressable, a map index expression, or (for = assignments only) theblank identifier. Operands may be parenthesized.

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

An assignment operation x op= y where op is a binary arithmetic operatoris equivalent to x = x op (y) but evaluates xonly once. The op= construct is a single token. In assignment operations, both the left- and right-hand expression lists must contain exactly one single-valued expression, and the left-hand expression must not be the blank identifier.

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

A tuple assignment assigns the individual elements of a multi-valued operation to a list of variables. There are two forms. In the first, the right hand operand is a single multi-valued expression such as a function call, a channel ormap operation, or a type assertion. The number of operands on the left hand side must match the number of values. For instance, iff is a function returning two values,

x, y = f()

assigns the first value to x and the second to y. In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the_n_th expression on the right is assigned to the _n_th operand on the left:

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 expressionsand pointer indirections(including implicit pointer indirections in selectors) on the left and the expressions on the right are allevaluated 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 is []int{3, 5, 3}

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

Any typed value may be assigned to the blank identifier.
If an untyped constant is assigned to a variable of interface type or the blank identifier, the constant is first implicitly converted to itsdefault type.
If an untyped boolean value is assigned to a variable of interface type or the blank identifier, it is first implicitly converted to type 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

In an expression switch, the switch expression is evaluated and the case expressions, which need not be constants, are evaluated left-to-right and top-to-bottom; the first one that equals the switch expression triggers execution of the statements of the associated case; the other cases are skipped. If no case matches and there is a "default" case, its statements are executed. There can be at most one default case and it may appear anywhere in the "switch" statement. A missing switch expression is equivalent to the boolean valuetrue.

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

If the switch expression evaluates to an untyped constant, it is first implicitlyconverted to its default type. The predeclared untyped value nil cannot be used as a switch expression. The switch expression type must be comparable.

If a case expression is untyped, it is first implicitly convertedto the type of the switch expression. For each (possibly converted) case expression x and the value tof the switch expression, x == t must be a valid comparison.

In other words, the switch expression is treated as if it were used to declare and initialize a temporary variable t without explicit type; it is that value of t against which each case expression x is tested for equality.

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

A type switch compares types rather than values. It is otherwise similar to an expression switch. It is marked by a special switch expression that has the form of a type assertionusing the keyword type rather than an actual type:

switch x.(type) { // cases }

Cases then match actual types T against the dynamic type of the expression x. As with type assertions, x must be ofinterface type, but not atype parameter, and each non-interface typeT listed in a case must implement the type of x. The types listed in the cases of a type switch must all bedifferent.

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

The TypeSwitchGuard may include ashort 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.

Instead of a type, a case may use the predeclared identifiernil; that case is selected when the expression in the TypeSwitchGuard is a nil interface value. There may be at most one nil case.

Given an expression x of type interface{}, the following type switch:

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 typemay 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 = fstring // v1 == 0 var v2 = fbyte // 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

In its simplest form, a "for" statement specifies the repeated execution of a block as long as a boolean condition evaluates to true. The condition is evaluated before each iteration. If the condition is absent, it is equivalent to the boolean valuetrue.

for a < b { a *= 2 }

For statements with `for` clause

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 ashort 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) }

If non-empty, the init statement is executed once before evaluating the condition for the first iteration; the post statement is executed after each execution of the block (and only if the block was executed). Any element of the ForClause may be empty but thesemicolons are required unless there is only a condition. If the condition is absent, it is equivalent to the boolean valuetrue.

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

For statements with `range` clause

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 permittingreceive operations. As with an assignment, if present the operands on the left must beaddressable 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.

The range expression x is evaluated once before beginning the loop, with one exception: if at most one iteration variable is present andlen(x) is constant, the range expression is not evaluated.

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

For an array, pointer to array, or slice value a, the index iteration values are produced in increasing order, starting at element index 0. If at most one iteration variable is present, the range loop produces iteration values from 0 up to len(a)-1 and does not index into the array or slice itself. For a nil slice, the number of iterations is 0.
For a string value, the "range" clause iterates over the Unicode code points in the string starting at byte index 0. On successive iterations, the index value will be the index of the first byte of successive UTF-8-encoded code points in the string, and the second value, of type rune, will be the value of the corresponding code point. If the iteration encounters an invalid UTF-8 sequence, the second value will be 0xFFFD, the Unicode replacement character, and the next iteration will advance a single byte in the string.
The iteration order over maps is not specified and is not guaranteed to be the same from one iteration to the next. If a map entry that has not yet been reached is removed during iteration, the corresponding iteration value will not be produced. If a map entry is created during iteration, that entry may be produced during the iteration or may be skipped. The choice may vary for each entry created and from one iteration to the next. If the map is nil, the number of iterations is 0.
For channels, the iteration values produced are the successive values sent on the channel until the channel is closed. If the channel is nil, the range expression blocks forever.

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

The iteration variables may be declared by the "range" clause using a form ofshort variable declaration(:=). In this case their types are set to the types of the respective iteration values and their scope is the block of the "for" statement; they are re-used in each iteration. If the iteration variables are declared outside the "for" statement, after execution their values will be those of the last iteration.

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 forexpression statements.

The function value and parameters areevaluated as usualin 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 possiblesend orreceiveoperations 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 ashort 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:

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.
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.
Unless the selected case is the default case, the respective communication operation is executed.
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.
The statement list of the selected case is executed.

Since communication on nil channels can never proceed, a select with only nil channels and no default case blocks forever.

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

A "return" statement in a function F terminates the execution of F, and optionally provides one or more result values. Any functions deferred by Fare executed before F returns to its caller.

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:

The return value or values may be explicitly listed in the "return" statement. Each expression must be single-valued and assignable to the corresponding element of the function's result type.
func simpleF() int {
return 2
}
func complexF1() (re float64, im float64) {
return -7.0, -4.0
}
The expression list in the "return" statement may be a single call to a multi-valued function. The effect is as if each value returned from that function were assigned to a temporary variable with the type of the respective value, followed by a "return" statement listing these variables, at which point the rules of the previous case apply.
func complexF2() (re float64, im float64) {
return complexF1()
}
The expression list may be empty if the function's result type specifies names for its result parameters. The result parameters act as ordinary local variables and the function may assign values to them as necessary. The "return" statement returns the values of these variables.
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 inscope 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" loopby 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 intoscope that were not already in scope at the point of the goto. For instance, this example:

goto L  // BAD
v := 3

is erroneous because the jump to label L skips the creation of v.

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-- }

is erroneous because the label L1 is inside the "for" statement's block but the goto is not.

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 forexpression statements.

Each time a "defer" statement executes, the function value and parameters to the call areevaluated as usualand saved anew but the actual function is not invoked. Instead, deferred functions are invoked immediately before the surrounding function returns, in the reverse order they were deferred. That is, if the surrounding function returns through an explicit return statement, deferred functions are executed after any result parameters are set by that return statement but before the function returns to its caller. If a deferred function value evaluates to nil, execution panicswhen the function is invoked, not when the "defer" statement is executed.

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 arepredeclared. 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.

Appending to and copying slices

The built-in functions append and copy assist in common slice operations. For both functions, the result is independent of whether the memory referenced by the arguments overlaps.

The variadic function appendappends zero or more values x to a slice sand returns the resulting slice of the same type as s. The core type of s must be a slice of type []E. The values x are passed to a parameter of type ...Eand the respective parameter passing rules apply. As a special case, if the core type of s is []byte,append also accepts a second argument with core typebytestring followed by .... This form appends the bytes of the byte slice or string.

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

If the capacity of s is not large enough to fit the additional values, append allocates a new, sufficiently large underlying array that fits both the existing slice elements and the additional values. Otherwise, append re-uses the underlying array.

s0 := []int{0, 0} s1 := append(s0, 2) // append a single element s1 is []int{0, 0, 2} s2 := append(s1, 3, 5, 7) // append multiple elements s2 is []int{0, 0, 2, 3, 5, 7} s3 := append(s2, s0...) // append a slice s3 is []int{0, 0, 2, 3, 5, 7, 0, 0} s4 := append(s3[3:6], s3[2:]...) // append overlapping slice s4 is []int{3, 5, 7, 2, 3, 5, 7, 0, 0}

var t []interface{} t = append(t, 42, 3.1415, "foo") // t is []interface{}{42, 3.1415, "foo"}

var b []byte b = append(b, "bar"...) // append string contents b is []byte{'b', 'a', 'r' }

The function copy copies slice elements from a source src to a destination dst and returns the number of elements copied. The core types of both arguments must be slices with identical element type. The number of elements copied is the minimum oflen(src) and len(dst). As a special case, if the destination's core type is []byte,copy also accepts a source argument with core typebytestring. This form copies the bytes from the byte slice or string into the byte slice.

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 is []int{0, 1, 2, 3, 4, 5} n2 := copy(s, s[2:]) // n2 == 4, s is []int{2, 3, 4, 5, 4, 5} n3 := copy(b, "Hello, World!") // n3 == 5, b is []byte("Hello")

Clear

The built-in function clear takes an argument of map,slice, or type parameter type, and deletes or zeroes out all elements.

Call Argument type Result

clear(m) map[K]T deletes all entries, resulting in an empty map (len(m) == 0)

clear(s) []T sets all elements up to the length of s to the zero value of T

clear(t) type parameter see below

If the type of the argument to clear is atype parameter, all types in its type set must be maps or slices, and clearperforms the operation corresponding to the actual type argument.

If the map or slice is nil, clear is a no-op.

Close

For an argument ch with a core typethat is a channel, the built-in function closerecords that no more values will be sent on the channel. It is an error if ch is a receive-only channel. Sending to or closing a closed channel causes a run-time panic. Closing the nil channel also causes a run-time panic. After calling close, and after any previously sent values have been received, receive operations will return the zero value for the channel's type without blocking. The multi-valued receive operationreturns a received value along with an indication of whether the channel is closed.

Manipulating complex numbers

Three functions assemble and disassemble complex numbers. The built-in function complex constructs a complex value from a floating-point real and imaginary part, whilereal and imagextract the real and imaginary parts of a complex value.

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

The type of the arguments and return value correspond. For complex, the two arguments must be of the samefloating-point type and the return type is thecomplex typewith the corresponding floating-point constituents:complex64 for float32 arguments, andcomplex128 for float64 arguments. If one of the arguments evaluates to an untyped constant, it is first implicitlyconverted to the type of the other argument. If both arguments evaluate to untyped constants, they must be non-complex numbers or their imaginary parts must be zero, and the return value of the function is an untyped complex constant.

For real and imag, the argument must be of complex type, and the return type is the corresponding floating-point type: float32 for a complex64 argument, andfloat64 for a complex128 argument. If the argument evaluates to an untyped constant, it must be a number, and the return value of the function is an untyped floating-point constant.

The real and imag functions together form the inverse ofcomplex, so for a value z of a complex type Z,z == 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.

Deletion of map elements

The built-in function delete removes the element with keyk from a map m. The value k must be assignableto the key type of m.

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

If the type of m is a type parameter, all types in that type set must be maps, and they must all have identical key types.

If the map m is nil or the element m[k]does not exist, delete is a no-op.

Length and capacity

The built-in functions len and cap take arguments of various types and return a result of type int. The implementation guarantees that the result always fits into an int.

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

If the argument type is a type parameter P, the call len(e) (or cap(e) respectively) must be valid for each type in P's type set. The result is the length (or capacity, respectively) of the argument whose type corresponds to the type argument with which P wasinstantiated.

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)

The length of a nil slice, map or channel is 0. The capacity of a nil slice or channel is 0.

The expression len(s) is constant ifs is a string constant. The expressions len(s) andcap(s) are constants if the type of s is an array or pointer to an array and the expression s does not containchannel receives or (non-constant)function calls; in this case s is not evaluated. Otherwise, invocations of len and cap are not constant and s is evaluated.

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

Making slices, maps and channels

The built-in function make takes a type T, optionally followed by a type-specific list of expressions. The core type of T must be a slice, map or channel. It returns a value of type T (not *T). The memory is initialized as described in the section oninitial values.

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

Each of the size arguments n and m must be of integer type, have a type set containing only integer types, or be an untyped constant. A constant size argument must be non-negative and representableby a value of type int; if it is an untyped constant it is given type int. If both n and m are provided and are constant, thenn must be no larger than m. For slices and channels, if n is negative or larger than m at run time, a run-time panic occurs.

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

Calling make with a map type and size hint n will create a map with initial space to hold n map elements. The precise behavior is implementation-dependent.

Min and max

The built-in functions min and max compute the smallest—or largest, respectively—value of a fixed number of arguments of ordered types. There must be at least one argument.

The same type rules as for operators apply: for ordered arguments x andy, min(x, y) is valid if x + y is valid, and the type of min(x, y) is the type of x + y(and similarly for max). If all arguments are constant, the result is constant.

var x, y int m := min(x) // m == x m := min(x, y) // m is the smaller of x and y m := max(x, y, 10) // m is the larger of x and y but at least 10 c := max(1, 2.0, 10) // c == 10.0 (floating-point kind) f := max(0, float32(x)) // type of f is float32 var s []string _ = min(s...) // invalid: slice arguments are not permitted t := max("", "foo", "bar") // t == "foo" (string kind)

For numeric arguments, assuming all NaNs are equal, min and max are commutative and associative:

min(x, y) == min(y, x) min(x, y, z) == min(min(x, y), z) == min(x, min(y, z))

For floating-point arguments negative zero, NaN, and infinity the following rules apply:

x y min(x, y) max(x, y)

-0.0 0.0 -0.0 0.0 // negative zero is smaller than (non-negative) zero -Inf y -Inf y // negative infinity is smaller than any other number +Inf y y +Inf // positive infinity is larger than any other number NaN y NaN NaN // if any argument is a NaN, the result is a NaN

For string arguments the result for min is the first argument with the smallest (or for max, largest) value, compared lexically byte-wise:

min(x, y) == if x <= y then x else y min(x, y, z) == min(min(x, y), z)

Allocation

The built-in function new takes a type T, allocates storage for a variable of that type at run time, and returns a value of type *T pointing to it. The variable is initialized as described in the section oninitial values.

new(T)

For instance

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

allocates storage for a variable of type S, initializes it (a=0, b=0.0), and returns a value of type *S containing the address of the location.

Handling panics

Two built-in functions, panic and recover, assist in reporting and handling run-time panicsand program-defined error conditions.

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

While executing a function F, an explicit call to panic or a run-time panicterminates the execution of F. Any functions deferred by Fare then executed as usual. Next, any deferred functions run by F's caller are run, and so on up to any deferred by the top-level function in the executing goroutine. At that point, the program is terminated and the error condition is reported, including the value of the argument to panic. This termination sequence is called panicking.

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

The recover function allows a program to manage behavior of a panicking goroutine. Suppose a function G defers a function D that callsrecover and a panic occurs in a function on the same goroutine in which Gis executing. When the running of deferred functions reaches D, the return value of D's call to recover will be the value passed to the call of panic. If D returns normally, without starting a newpanic, the panicking sequence stops. In that case, the state of functions called between G and the call to panicis discarded, and normal execution resumes. Any functions deferred by G before D are then run and G's execution terminates by returning to its caller.

The return value of recover is nil when the goroutine is not panicking or recover was not called directly by a deferred function. Conversely, if a goroutine is panicking and recover was called directly by a deferred function, the return value of recover is guaranteed not to be nil. To ensure this, calling panic with a nil interface value (or an untyped nil) causes a run-time panic.

The protect function in the example below invokes the function argument g and protects callers from run-time panics raised by g.

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

Implementation restriction: print and println need not accept arbitrary argument types, but printing of boolean, numeric, and stringtypes must be supported.

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 beexported 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 PackageName is used in qualified identifiersto access exported identifiers of the package within the importing source file. It is declared in the file block. If the PackageName is omitted, it defaults to the identifier specified in thepackage clause of the imported package. If an explicit period (.) appears instead of a name, all the package's exported identifiers declared in that package'spackage block will be declared in the importing source file's file block and must be accessed without a qualifier.

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.

Implementation restriction: A compiler may restrict ImportPaths to non-empty strings using only characters belonging toUnicode'sL, M, N, P, and S general categories (the Graphic characters without spaces) and may also exclude the characters!"#$%&'()*,:;<=>?[\]^`{|}and the Unicode replacement character U+FFFD.

Consider a compiled a package containing the package clausepackage math, which exports function Sin, and installed the compiled package in the file identified by"lib/math". This table illustrates how Sin is accessed in files that import the package after the various types of import declaration.

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 blankidentifier 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

When storage is allocated for a variable, either through a declaration or a call of new, or when a new value is created, either through a composite literal or a call of make, and no explicit initialization is provided, the variable or value is given a default value. Each element of such a variable or value is set to the zero value for its type: false for booleans,0 for numeric types, ""for strings, and nil for pointers, functions, interfaces, slices, channels, and maps. This initialization is done recursively, so for instance each element of an array of structs will have its fields zeroed if no value is specified.

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, blankvariables 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. 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.

Dependency analysis does not rely on the actual values of the variables, only on lexical references to them in the source, analyzed transitively. For instance, if a variable x's initialization expression refers to a function whose body refers to variable y then x depends on y. Specifically:

A reference to a variable or function is an identifier denoting that variable or function.
A reference to a method m is amethod value ormethod expression of the formt.m, where the (static) type of t is not an interface type, and the method m is in themethod set of t. It is immaterial whether the resulting function valuet.m is invoked.
A variable, function, or method x depends on a variabley if x's initialization expression or body (for functions and methods) contains a reference to yor to a function or method that depends on y.

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 }

the initialization order is d, b, c, a. Note that the order of subexpressions in initialization expressions is irrelevant:a = c + b and a = b + c result in the same initialization order in this example.

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} }

the variable a will be initialized after b but whether x is initialized before b, betweenb and a, or after a, and thus also the moment at which sideEffect() is called (before or after x is initialized) is not specified.

Variables may also be initialized using functions named initdeclared in the package block, with no arguments and no result parameters.

func init() { … }

Multiple such functions may be defined per package, even within a single source file. In the package block, the init identifier can be used only to declare init functions, yet the identifier itself is not declared. Thusinit functions cannot be referred to from anywhere in a program.

The entire package is initialized by assigning initial values to all its package-level variables followed by calling all init functions in the order they appear in the source, possibly in multiple files, as presented to the compiler.

Program initialization

The packages of a complete program are initialized stepwise, one package at a time. If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package, the imported package will be initialized only once. The importing of packages, by construction, guarantees that there can be no cyclic initialization dependencies. More precisely:

Given the list of all packages, sorted by import path, in each step the first uninitialized package in the list for which all imported packages (if any) are already initialized is initialized. This step is repeated until all packages are initialized.

Package initialization—variable initialization and the invocation ofinit functions—happens in a single goroutine, sequentially, one package at a time. An init function may launch other goroutines, which can run concurrently with the initialization code. However, initialization always sequences the init functions: it will not invoke the next one until the previous one has returned.

Program execution

A complete program is created by linking a single, unimported package called the main package with all the packages it imports, transitively. The main package must have package name main and declare a function main that takes no arguments and returns no value.

func main() { … }

Program execution begins by initializing the programand then invoking the function main in package main. When that function invocation returns, the program exits. It does not wait for other (non-main) goroutines to complete.

Errors

The predeclared type error is defined as

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

Execution errors such as attempting to index an array out of bounds trigger a run-time panic equivalent to a call of the built-in function panicwith a value of the implementation-defined interface type runtime.Error. That type satisfies the predeclared interface typeerror. The exact error values that represent distinct run-time error conditions are unspecified.

package runtime

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

System considerations

Package `unsafe`

The built-in package unsafe, known to the compiler and accessible through the import path "unsafe", provides facilities for low-level programming including operations that violate the type system. A package using unsafemust be vetted manually for type safety and may not be portable. The package provides the following interface:

package unsafe

type ArbitraryType int // shorthand for an arbitrary Go type; it is not a real type type Pointer *ArbitraryType

func Alignof(variable ArbitraryType) uintptr func Offsetof(selector ArbitraryType) uintptr func Sizeof(variable ArbitraryType) uintptr

type IntegerType int // shorthand for an integer type; it is not a real type func Add(ptr Pointer, len IntegerType) Pointer func Slice(ptr *ArbitraryType, len IntegerType) []ArbitraryType func SliceData(slice []ArbitraryType) *ArbitraryType func String(ptr *byte, len IntegerType) string func StringData(str string) *byte

A Pointer is a pointer type but a Pointervalue may not be dereferenced. Any pointer or value of underlying type uintptr can beconverted to a type of underlying type Pointer and vice versa. The effect of converting between Pointer and uintptr is implementation-defined.

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

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

var p ptr = nil

The functions Alignof and Sizeof take an expression xof any type and return the alignment or size, respectively, of a hypothetical variable vas if v was declared via var v = x.

The function Offsetof takes a (possibly parenthesized) selector s.f, denoting a field f of the struct denoted by sor *s, and returns the field offset in bytes relative to the struct's address. If f is an embedded field, it must be reachable without pointer indirections through fields of the struct. For a struct s with field f:

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

Computer architectures may require memory addresses to be aligned; that is, for addresses of a variable to be a multiple of a factor, the variable's type's alignment. The function Alignoftakes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variablex:

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

A (variable of) type T has variable size if Tis a type parameter, or if it is an array or struct type containing elements or fields of variable size. Otherwise the size is constant. Calls to Alignof, Offsetof, and Sizeofare compile-time constant expressions of type uintptr if their arguments (or the struct s in the selector expression s.f for Offsetof) are types of constant size.

The function Add adds len to ptrand returns the updated pointer unsafe.Pointer(uintptr(ptr) + uintptr(len)). The len argument must be of integer type or an untyped constant. A constant len argument must be representable by a value of type int; if it is an untyped constant it is given type int. The rules for valid uses of Pointer still apply.

The function Slice returns a slice whose underlying array starts at ptrand whose length and capacity are len.Slice(ptr, len) is equivalent to

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

except that, as a special case, if ptris nil and len is zero,Slice returns nil.

The len argument must be of integer type or an untyped constant. A constant len argument must be non-negative and representable by a value of type int; if it is an untyped constant it is given type int. At run time, if len is negative, or if ptr is nil and len is not zero, a run-time panic occurs.

The function SliceData returns a pointer to the underlying array of the slice argument. If the slice's capacity cap(slice) is not zero, that pointer is &slice[:1][0]. If slice is nil, the result is nil. Otherwise it is a non-nil pointer to an unspecified memory address.

The function String returns a string value whose underlying bytes start atptr and whose length is len. The same requirements apply to the ptr and len argument as in the functionSlice. If len is zero, the result is the empty string "". Since Go strings are immutable, the bytes passed to String must not be modified afterwards.

The function StringData returns a pointer to the underlying bytes of the str argument. For an empty string the return value is unspecified, and may be nil. Since Go strings are immutable, the bytes returned by StringData must not be modified.

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:

For a variable x of any type: unsafe.Alignof(x) is at least 1.
For a variable x of struct type: unsafe.Alignof(x) is the largest of all the values unsafe.Alignof(x.f) for each field f of x, but at least 1.
For a variable x of array type: unsafe.Alignof(x) is the same as the alignment of a variable of the array's element type.

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.

Appendix

Type unification rules

The type unification rules describe if and how two types unify. The precise details are relevant for Go implementations, affect the specifics of error messages (such as whether a compiler reports a type inference or other error), and may explain why type inference fails in unusual code situations. But by and large these rules can be ignored when writing Go code: type inference is designed to mostly "work as expected", and the unification rules are fine-tuned accordingly.

Type unification is controlled by a matching mode, which may be exact or loose. As unification recursively descends a composite type structure, the matching mode used for elements of the type, the element matching mode, remains the same as the matching mode except when two types are unified forassignability (≡A): in this case, the matching mode is loose at the top level but then changes to exact for element types, reflecting the fact that types don't have to be identical to be assignable.

Two types that are not bound type parameters unify exactly if any of following conditions is true:

Both types are identical.
Both types have identical structure and their element types unify exactly.
Exactly one type is an unbound type parameter with a core type, and that core type unifies with the other type per the unification rules for ≡A (loose unification at the top level and exact unification for element types).

If both types are bound type parameters, they unify per the given matching modes if:

Both type parameters are identical.
At most one of the type parameters has a known type argument. In this case, the type parameters are joined: they both stand for the same type argument. If neither type parameter has a known type argument yet, a future type argument inferred for one the type parameters is simultaneously inferred for both of them.
Both type parameters have a known type argument and the type arguments unify per the given matching modes.

A single bound type parameter P and another type T unify per the given matching modes if:

P doesn't have a known type argument. In this case, T is inferred as the type argument for P.
P does have a known type argument A,A and T unify per the given matching modes, and one of the following conditions is true:
- Both A and T are interface types: In this case, if both A and T are also defined types, they must be identical. Otherwise, if neither of them is a defined type, they must have the same number of methods (unification of A and T already established that the methods match).
- Neither A nor T are interface types: In this case, if T is a defined type, T replaces A as the inferred type argument for P.

Finally, two types that are not bound type parameters unify loosely (and per the element matching mode) if:

Both types unify exactly.
One type is a defined type, the other type is a type literal, but not an interface, and their underlying types unify per the element matching mode.
Both types are interfaces (but not type parameters) with identical type terms, both or neither embed the predeclared typecomparable, corresponding method types unify per the element matching mode, and the method set of one of the interfaces is a subset of the method set of the other interface.
Only one type is an interface (but not a type parameter), corresponding methods of the two types unify per the element matching mode, and the method set of the interface is a subset of the method set of the other type.
Both types have the same structure and their element types unify per the element matching mode.