This is a reference manual for the Go programming language. 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 grammar is compact and regular, allowing for easy analysis by automatic tools such as integrated development environments.
Notation
The syntax is specified using Extended Backus-Naur Form (EBNF):
Production = production_name "=" [ Expression ] "." .
Expression = Alternative { "|" Alternative } .
Alternative = Term { Term } .
Term = 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)
Lower-case production names are used to identify lexical 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 from a 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 in UTF-8. The text is not canonicalized, so a single accented code point is distinct from the same character constructed from combining an accent and a letter; those are treated as two code points. For simplicity, this document will use the unqualified term character to refer to a Unicode code point in the source text.
Each code point is distinct; for instance, upper and lower case 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 classes:
newline = /* the Unicode code point U+000A */ .
unicode_char = /* an arbitrary Unicode code point except newline */ .
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 letter.
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 grammar 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
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.
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
An integer literal is a sequence of digits representing an integer constant. An optional prefix sets a non-decimal base: 0 for octal, 0x or 0X for hexadecimal. In hexadecimal literals, letters a-f and A-F represent values 10 through 15.
A floating-point literal is a decimal representation of a floating-point constant. It has an integer part, a decimal point, a fractional part, and an exponent part. The integer and fractional part comprise decimal digits; the exponent part is an e or E followed by an optionally signed decimal exponent. One of the integer part or the fractional part may be elided; one of the decimal point or the exponent may be elided.
An imaginary literal is a decimal representation of the imaginary part of a complex constant. It consists of afloating-point literal or decimal integer followed by the lower-case letter i.
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 (0xc30xa4) 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 \U represent 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)
All other sequences starting with a backslash are illegal inside rune 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 0xc30xbf of the UTF-8 encoding of character U+00FF.
A constant value is represented by a rune, integer, floating-point, imaginary, or string 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 as unsafe.Sizeof applied to any value, cap or len applied to some expressions, real and imag applied to a complex constant and complex applied to numeric constants. The boolean truth values are represented by the predeclared constants true and false. The predeclared identifier iota denotes an integer constant.
In general, complex constants are a form of constant expression and are discussed in that section.
Numeric constants represent exact values of arbitrary precision and do not overflow. Consequently, there are no constants denoting the IEEE-754 negative zero, infinity, and not-a-number values.
Constants may be typed or untyped. Literal constants, true, false, iota, and certain constant expressionscontaining only untyped constant operands are untyped.
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 declaration such 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.
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 the new 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 concrete 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 assignable to 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 an expression; it is the most recent value assigned to the variable. If a variable has not yet been assigned a value, its value is the zero value for its type.
Types
A type determines a set of values together with operations and methods specific to those values. A type may be denoted by a type name, if it has one, or specified using a type literal, which composes a type from existing types.
The language predeclares certain type names. Others are introduced with type declarations. Composite types—array, struct, pointer, function, interface, slice, map, and channel types—may be constructed using type literals.
Each type T has an underlying type: If T is 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 type declaration.
type (
A1 = string
A2 = A1
)
type (
B1 string
B2 B1
B3 []B1
B4 B3
)
The underlying type of string, A1, A2, B1, and B2 is string. The underlying type of []B1, B3, and B4 is []B1.
Method sets
A type may have a method set associated with it. The method set of an interface type is its interface. The method set of any other type T consists of all methods declared with receiver type T. The method set of the corresponding pointer type*T is the set of all methods declared with receiver *T or T (that is, it also contains the method set of T). Further rules apply to structs containing embedded fields, as described in the section on struct types. Any other type has an empty method set. In a method set, each method must have a unique non-blankmethod name.
The method set of a type determines the interfaces that the type implements and the methods that can be calledusing a receiver of that type.
Boolean types
A boolean type represents the set of Boolean truth values denoted by the predeclared constants true and false. The predeclared boolean type is bool; it is a defined type.
Numeric types
A numeric type represents sets of integer or floating-point values. 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
There is also a set of predeclared numeric 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 except byte, which is an alias for uint8, and rune, which is an alias for int32. Conversions are required when different numeric types are mixed in an expression or assignment. For instance, int32 and int are 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. 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 (its size in bytes) 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 indices 0 through len(s)-1. It is illegal to take the address of such an element; if s[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 and is never negative.
The length is part of the array's type; it must evaluate to a non-negative constantrepresentable 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 indices 0 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]([2]([2]float64))
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 value of an uninitialized slice is nil.
Like arrays, slices are indexable and have a length. The length of a slice s can be discovered by the built-in function len; unlike with arrays it may change during execution. The elements can be addressed by integer indices 0 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 by slicing 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 is made using the built-in function make, 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 slicing it, so these two expressions are equivalent:
make([]int, 50, 100)
new([100]int)[0:50]
Like arrays, slices are always one-dimensional but may be composed to construct higher-dimensional objects. With arrays of arrays, the inner arrays are, by construction, always the same length; however with slices of slices (or arrays of slices), the inner lengths may vary dynamically. Moreover, the inner slices must be initialized individually.
Struct types
A struct is a sequence of named elements, called fields, each of which has a name and a type. Field names may be specified explicitly (IdentifierList) or implicitly (EmbeddedField). Within a struct, non-blank field names must be unique.
// 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 methodf of an embedded field in a struct x is called promoted if x.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 in composite literals of the struct.
Given a struct type S and a defined typeT, 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 receiver T. 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 Tor *T.
A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. An empty tag string is equivalent to an absent tag. The tags are made visible through a reflection interface and take part in type identity for structs but are otherwise ignored.
struct {
x, y float64 "" // an empty tag string is like an absent tag
name string "any string is permitted as a tag"
_ [4]byte "ceci n'est pas un champ de structure"
}
// A struct corresponding to a TimeStamp protocol buffer.
// The tag strings define the protocol buffer field numbers;
// they follow the convention outlined by the reflect package.
struct {
microsec uint64 `protobuf:"1"`
serverIP6 uint64 `protobuf:"2"`
}
Pointer types
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.
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.
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 specifies a method set called its interface. A variable of interface type can store a value of any type with a method set that is any superset of the interface. Such a type is said to implement the interface. The value of an uninitialized variable of interface type is nil.
(where T stands for either S1 or S2) then the File interface is implemented by both S1 and S2, regardless of what other methods S1 and S2 may have or share.
A type implements any interface comprising any subset of its methods and may therefore implement several distinct interfaces. For instance, all types implement the empty interface:
interface{}
Similarly, consider this interface specification, which appears within a type declaration to 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.
An interface T may use a (possibly qualified) interface type name E in place of a method specification. This is called embedding interface E in T; it adds all (exported and non-exported) methods of E to the interface T.
type ReadWriter interface {
Read(b Buffer) bool
Write(b Buffer) bool
}
type File interface {
ReadWriter // same as adding the methods of ReadWriter
Locker // same as adding the methods of Locker
Close()
}
type LockedFile interface {
Locker
File // illegal: Lock, Unlock not unique
Lock() // illegal: Lock not unique
}
An interface type T may not embed itself or any interface type that embeds T, recursively.
// illegal: Bad cannot embed itself
type Bad interface {
Bad
}
// illegal: Bad1 cannot embed itself using Bad2
type Bad1 interface {
Bad2
}
type Bad2 interface {
Bad1
}
Map types
A map is an unordered group of elements of one type, called the element type, indexed by a set of unique keysof another type, called the key type. The value of an uninitialized map is nil.
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 with index expressions; they may be removed with the delete 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.
The optional <- operator specifies the channel direction, send or receive. If no direction is given, the channel isbidirectional. A channel may be constrained only to send or only to receive by conversion or
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