Introduction

This is a reference manual for the Go programming language. For more information and other documents, see the Go home page.

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 existing implementations use a traditional compile/link model to generate executable binaries.

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 symbols are enclosed in double quotes "" (the double quote symbol is written as '"').

The form "a ... b" represents the set of characters from a through b as alternatives.

Where possible, recursive productions are used to express evaluation order and operator precedence syntactically.

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 term character to refer to a Unicode code point.

Each code point is distinct; for instance, upper and lower case letters are different characters.

Characters

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

• unicode_char an arbitrary Unicode code point
• unicode_letter a Unicode code point classified as "Letter"
• capital_letter a Unicode code point classified as "Letter, uppercase"
• unicode_digit a Unicode code point classified as "Digit"

Letters and digits

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

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

Lexical elements

Comments

There are two forms of comments. The first starts at the character sequence // and continues through the next newline. The second starts at the character sequence /* and continues through the character sequence */. Comments do not nest.

Tokens

Tokens form the vocabulary of the Go language. There are four classes: identifiers, keywords, operators and delimiters, and literals. White space, formed from blanks, tabs, and newlines, is ignored except as it separates tokens that would otherwise combine into a single token. Comments behave as white space. While breaking the input into tokens, the next token is the longest sequence of characters that form a valid token.

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

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 Delimiters

The following character sequences represent operators, delimiters, and other special tokens:

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

Integer literals

An integer literal is a sequence of one or more digits in the corresponding base, which may be 8, 10, or 16. 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.

int_lit       = decimal_lit | octal_lit | hex_lit .
decimal_lit   = ( "1" ... "9" ) { decimal_digit } .
octal_lit     = "0" { octal_digit } .
hex_lit       = "0" ( "x" | "X" ) hex_digit { hex_digit } .

42
0600
0xBadFace
170141183460469231731687303715884105727

Floating-point literals

A floating-point literal is a decimal representation of a floating-point number. 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.

float_lit    = decimals "." [ decimals ] [ exponent ] |
               decimals exponent |
               "." decimals [ exponent ] .
decimals = decimal_digit { decimal_digit } .
exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .

0.
2.71828
1.e+0
6.67428e-11
1E6
.25
.12345E+5

Ideal numbers

Integer literals represent values of arbitrary precision, or ideal integers. Similarly, floating-point literals represent values of arbitrary precision, or ideal floats. These ideal numbers have no size or type and cannot overflow. However, when (used in an expression) assigned to a variable or typed constant, the destination must be able to represent the assigned value.

Implementation restriction: A compiler may implement ideal numbers by choosing an internal representation with at least twice the precision of any machine type.

Character literals

A character literal represents an integer value, typically a Unicode code point, as one or more characters enclosed in single quotes. Within the quotes, any character may appear except single quote and newline. A single quoted character represents 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 represented 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 `Unicode' 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 character literals)
\"   U+0022 double quote  (valid escape only within string literals)

All other sequences are illegal inside character literals.

char_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'

The value of a character literal is an ideal integer, just as with integer literals.

String literals

String literals represent constant values of type string. There are two forms: raw string literals and interpreted string literals.

Raw string literals are character sequences between back quotes ``. Within the quotes, any character is legal except newline and back quote. The value of a raw string literal is the string composed of the uninterpreted bytes between the quotes; in particular, backslashes have no special meaning.

Interpreted string literals are character sequences between double quotes "". The text between the quotes forms the value of the literal, with backslash escapes interpreted as they are in character literals (except that \' is illegal and \" is legal). The three-digit octal (\000) and two-digit hexadecimal (\x00) 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 } "`" .
interpreted_string_lit = """ { unicode_value | byte_value } """ .

`abc`
`\n`
"hello, world\n"
"\n"
""
"Hello, world!\n"
"日本語"
"\u65e5本\U00008a9e"
"\xff\u00FF"

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 character literal (it is not a single code point), and will appear as two code points if placed in a string literal.

Types

A type determines the set of values and operations specific to values of that type. A type may be specified by a (possibly qualified (§Qualified identifiers)) type name (§Type declarations) or a type literal, which composes a new type in terms of previously declared types.

Type      = TypeName | TypeLit | "(" Type ")" .
TypeName  = QualifiedIdent.
TypeLit   = ArrayType | StructType | PointerType | FunctionType | InterfaceType |
	    SliceType | MapType | ChannelType .

Basic types such as int are predeclared (§Predeclared identifiers). Other types may be constructed from these, recursively, including arrays, structs, pointers, functions, interfaces, slices, maps, and channels.

At any point in the source code, a type may be complete or incomplete. An incomplete type is one whose size is not yet known, such as a struct whose fields are not yet fully defined or a forward declared type (§Forward declarations). Most types are always complete; for instance, a pointer type is always complete even if it points to an incomplete type because the size of the pointer itself is always known.

The interface of a type is the set of methods bound to it (§Method declarations); for pointer types, it is the interface of the pointer base type (§Pointer types). All types have an interface; if they have no methods, it is the empty interface.

The static type (or just type) of a variable is the type defined by its declaration. Variables of interface type (§Interface types) also have a distinct dynamic type, which is the actual type of the value stored in the variable at run-time. The dynamic type may vary during execution but is always compatible with the static type of the interface variable. For non-interfaces types, the dynamic type is always the static type.

Basic types

Basic types include traditional arithmetic types, booleans, and strings. All are predeclared.

Arithmetic types

The 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 valid IEEE-754 32-bit floating point numbers
float64  the set of all valid IEEE-754 64-bit floating point numbers

byte     familiar alias for uint8

Integer types are represented in the usual binary format; the value of an n-bit integer is n bits wide. A negative signed integer is represented as the two's complement of its absolute value.

There is also a set of architecture-independent basic numeric types whose size depends on the architecture:

uint     at least 32 bits, at most the size of the largest uint type
int      at least 32 bits, at most the size of the largest int type
float    at least 32 bits, at most the size of the largest float type
uintptr  smallest uint type large enough to store the uninterpreted
		 bits of a pointer value

Except for byte, which is an alias for uint8, to avoid portability issues all numeric types are distinct. 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.

Booleans

Strings

The string type represents the set of textual string values. Strings behave like arrays of bytes but are immutable: once created, it is impossible to change the contents of a string.

The elements of strings have type byte and may be accessed using the usual indexing operations (§Indexes). It is illegal to take the address of such an element, that is, even if s[i] is the i^th byte of a string, &s[i] is invalid. The length of a string can be computed by the function len(s1).

A sequence of string literals is concatenated into a single string.

StringLit   = string_lit { string_lit } .

"Alea iacta est."
"Alea " /* The die */ `iacta est` /* is cast */ "."

Struct types

A struct is a sequence of named elements, called fields, with various types. A struct type declares an identifier and type for each field. Within a struct field identifiers must be unique and field types must be complete (§Types).

StructType = "struct" [ "{" [ FieldDeclList ] "}" ] .
FieldDeclList = FieldDecl { ";" FieldDecl } [ ";" ] .
FieldDecl = (IdentifierList CompleteType | [ "*" ] TypeName) [ Tag ] .
Tag = StringLit .

// An empty struct.
struct {}

// A struct with 5 fields.
struct {
	x, y int;
	u float;
	A *[]int;
	F func();
}

A field declared with a type but no field identifier is an anonymous field. Such a field type must be specified as a type name T or as a pointer to a type name *T and T itself may not be a pointer or interface type. The unqualified type name acts as the field identifier.

// A struct with four anonymous fields of type T1, *T2, P.T3 and *P.T4
struct {
	T1;        // the field name is T1
	*T2;       // the field name is T2
	P.T3;      // the field name is the unqualified type name T3
	*P.T4;     // the field name is the unqualified type name T4
	x, y int;  
}

The unqualified type name of an anonymous field must not conflict with the field identifier (or unqualified type name for an anonymous field) of any other field within the struct. The following declaration is illegal:

struct {
	T;         // conflicts with anonymous field *T and *P.T
	*T;        // conflicts with anonymous field T and *P.T
	*P.T;      // conflicts with anonymous field T and *T
}

Fields and methods (§Method declarations) of an anonymous field are promoted to be ordinary fields and methods of the struct (§Selectors).

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the identifiers in the corresponding field declaration. The tags are made visible through a reflection library (TODO: reference?) but are otherwise ignored.

// A struct corresponding to the EventIdMessage protocol buffer.
// The tag strings define the protocol buffer field numbers.
struct {
	time_usec uint64 "field 1";
	server_ip uint32 "field 2";
	process_id uint32 "field 3";
}

Array types

An array is a numbered sequence of elements of a single type, called the element type, which must be complete (§Types). The number of elements is called the length and is never negative.

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

The length is part of the array's type and must must be a constant expression (§Constant expressions) that evaluates to a non-negative integer value. The length of array a can be discovered using the built-in function len(a), which is a compile-time constant. The elements can be indexed by integer indices 0 through the len(a)-1 (§Indexes).

[32]byte
[2*N] struct { x, y int32 }
[1000]*float64

Pointer types

A pointer type denotes the set of all pointers to variables of a given type, called the ``base type'' of the pointer. A pointer value may be nil.

PointerType = "*" BaseType .
BaseType = Type .

*int
*map[string] *chan int

Function types

A function type denotes the set of all functions with the same parameter and result types. A function value may be nil.

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

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; 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 that is not a function type it may writen as an unparenthesized type. The types of parameters and results must be complete. (TODO: is completeness necessary?)

For the last parameter only, instead of a type one may write ... to indicate that the function may be invoked with an arbitrary number (including zero) of additional arguments of any type. If parameters of such a function are named, the final identifier list must be a single name, that of the ... parameter.

func ()
func (x int)
func () int
func (string, float, ...)
func (a, b int, z float) bool
func (a, b int, z float) (bool)
func (a, b int, z float, opt ...) (success bool)
func (int, int, float) (float, *[]int)
func (n int) (func (p* T))

Interface types

An interface type specifies an unordered set of methods. A variable of interface type can store, dynamically, any value that implements at least that set of methods. An interface value may be nil.

InterfaceType      = "interface" [ "{" [ MethodSpecList ] "}" ] .
MethodSpecList     = MethodSpec { ";" MethodSpec } [ ";" ] .
MethodSpec         = IdentifierList Signature | InterfaceTypeName .
InterfaceTypeName  = TypeName .

// A simple File interface
interface {
	Read, Write	(b Buffer) bool;
	Close		();
}

Any type (including interface types) whose interface includes, possibly as a subset, the complete set of methods of an interface I is said to implement interface I. For instance, if two types S1 and S2 have the methods

func (p T) Read(b Buffer) bool { return ... }
func (p T) Write(b Buffer) bool { return ... }
func (p T) Close() { ... }

(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 (§Type declarations) to define an interface called Lock:

type Lock interface {
	Lock, Unlock	();
}

If S1 and S2 also implement

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

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

An interface may contain an interface type name T in place of a method specification. In this notation, T must denote a different, complete interface type and the effect is equivalent to enumerating the methods of T explicitly in the interface.

type ReadWrite interface {
	Read, Write	(b Buffer) bool;
}

type File interface {
	ReadWrite;  // same as enumerating the methods in ReadWrite
	Lock;       // same as enumerating the methods in Lock
	Close();
}

Slice types

A slice is a reference to a contiguous segment of an array and contains a numbered sequence of elements from that array. A slice type denotes the set of all slices of arrays of its element type. A slice value may be nil.

SliceType = "[" "]" ElementType .

Like arrays, slices are indexable and have a length. The length of a slice s can be discovered by the built-in function len(s); unlike with arrays it may change during execution. The elements can be addressed by integer indices 0 through len(s)-1 (§Indexes). 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 therfore 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 (§Slices). The capacity of a slice a can be discovered using the built-in function

cap(s)

and the relationship between len() and cap() is:

0 <= len(a) <= cap(a)

The value of an uninitialized slice is nil, and its length and capacity are 0. 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:

make([]T, length)
make([]T, length, capacity)

The make() call allocates a new, hidden array to which the returned slice value refers. That is, calling make

make([]T, length, capacity)

produces the same slice as allocating an array and slicing it:

make([capacity]T)[0 : length]

Map types

A map is an unordered group of elements of one type, called the value type, indexed by a set of unique keys of another type, called the key type. Both key and value types must be complete. (§Types). (TODO: is completeness necessary here?) A map value may be nil.

MapType     = "map" "[" KeyType "]" ValueType .
KeyType     = CompleteType .
ValueType   = CompleteType .

The comparison operators == and != (§Comparison operators) must be fully defined for operands of the key type; thus the key type must be a basic, pointer, interface, map, or channel type. 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 error.

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

The number of elements is called the length and is never negative. The length of a map m can be discovered using the built-in function len(m) and may change during execution. The value of an uninitialized map is nil

Upon creation, a map is empty. Values may be added and removed during execution using special forms of assignment (§Assignments). A new, empty map value is made using the built-in function make, which takes the map type and an optional capacity, an allocation hint, as arguments:

make(map[string] int, 100);

Channel types

A channel provides a mechanism for two concurrently executing functions to synchronize execution and communicate by passing a value of a specified element type. The element type must be complete (§Types). (TODO: is completeness necessary here?) A channel value may be nil.

ChannelType   = Channel | SendChannel | RecvChannel .
Channel       = "chan" ValueType .
SendChannel   = "chan" "<-" ValueType .
RecvChannel   = "<-" "chan" ValueType .

Upon creation, a channel can be used both to send and to receive values. By conversion or assignment, a channel may be constrained only to send or to receive. This constraint is called a channel's direction; either send, receive, or bi-directional (unconstrained).

chan T         // can send and receive values of type T
chan <- float  // can only be used to send floats
<-chan int     // can only receive ints

The value of an uninitialized channel is nil. A new, initialized channel value is made using the built-in function make, 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 greater than zero, the channel is asynchronous and, provided the buffer is not full, sends can succeed without blocking. If the capacity is zero or absent, the communication succeeds only when both a sender and receiver are ready.

General properties of types and values

Types may be different, structurally equal (or just equal), or identical. Go is type safe: different types cannot be mixed in binary operations and values cannot be assigned to variables of different types. They can be assigned to variables of equal type.

Type equality and identity

Two type names denote equal types if the types in the corresponding declarations are equal (§Declarations and Scope). Two type literals specify equal types if they have the same literal structure and corresponding components have equal types. In detail:

• Two pointer types are equal if they have equal base types.
• Two array types are equal if they have equal element types and the same array length.
• Two struct types are equal if they have the same sequence of fields, with the same names and equal types. Two anonymous fields are considered to have the same name.
• Two function types are equal if they have the same number of parameters and result values and if corresponding parameter and result types are the same. All "..." parameters have equal type. Parameter and result names are not required to match.
• Two slice types are equal if they have equal element types.
• Two channel types are equal if they have equal value types and the same direction.
• Two map types are equal if they have equal key and value types.
• Two interface types are equal if they have the same set of methods with the same names and equal function types. The order of the methods is irrelevant.

Type identity is more stringent than type equality. It requires for type names that they originate in the same type declaration, while for equality it requires only that they originate in equal type declarations. Also, the names of parameters and results must match for function types. In all other respects, the definition of type identity is the same as for type equality listed above but with ``identical'' substitued for ``equal''.

By definition, identical types are also equal types. Two types are different if they are not equal.

Given the declarations

type (
	T0 []string;
	T1 []string
	T2 struct { a, b int };
	T3 struct { a, c int };
	T4 func (int, float) *T0
	T5 func (x int, y float) *[]string
)

these types are equal

T0 and T0
T0 and T1
T0 and []string
T2 and T3
T4 and T5
T3 and struct { a int; int }

and these types are identical

T0 and T0
[]int and []int
struct { a, b *T5 } and struct { a, b *T5 }

T0 and T1 are equal but not identical because they have distinct declarations.

Assignment compatibility

Values of any type may always be assigned to variables of equal static type. Some types and values have conditions under which they may be assigned to different types:

• The predeclared constant nil can be assigned to any pointer, function, slice, map, channel, or interface variable.
• Arrays can be assigned to slice variables with equal element type. When assigning to a slice variable, the array is not copied but a slice comprising the entire array is created.
• A value can be assigned to an interface variable if the dynamic type of the value implements the interface.
• A value of bidirectional channel type can be assigned to any channel variable of equal channel value type.

Comparison compatibility

Values of any type may be compared to other values of equal static type. Values of numeric and string type may be compared using the full range of comparison operators as described in §Comparison operators; booleans may be compared only for equality or inequality.

Values of composite type may be compared for equality or inequality using the == and != operators, with the following provisos:

• Arrays and structs may not be compared to anything.
• A slice value may only be compared explicitly against nil and is equal to nil if it has been assigned the explicit value nil or if it is a variable (or array element, field, etc.) that has not been modified since it was created uninitialized.
• Similarly, an interface value is equal to nil if it has been assigned the explicit value nil or if it is a variable (or array element, field, etc.) that has not been modified since it was created uninitialized.
• For types that can be compared to nil, two values of the same type are equal if they both equal nil, unequal if one equals nil and one does not.
• Pointer values are equal if they point to the same location.
• Function values are equal if they point to the same function.
• Channel and map values are equal if they were created by the same call of make (§Making slices, maps, and channels).
• Interface values are comparison compatible if they have the same static type and equal if they have the same dynamic type.

Declarations and Scope

A declaration binds an identifier to a language entity such as a variable or function and specifies properties such as its type. Every identifier in a program must be declared.

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

The scope of an identifier is the extent of source text within which the identifier denotes the bound entity. No identifier may be declared twice in a single scope, but inner blocks can declare a new entity with the same identifier, in which case the scope created by the outer declaration excludes that created by the inner.

There are levels of scoping in effect before each source file is compiled. In order from outermost to innermost:

1. The universe scope contains all predeclared identifiers.
2. An implicit scope contains only the package name.
3. The package-level scope surrounds all declarations at the top level of the file, that is, outside the body of any function or method. That scope is shared across all source files within the package (§Packages), allowing package-level identifiers to be shared between source files.

The scope of an identifier depends on the entity declared:

1. The scope of predeclared identifiers is the universe scope.
2. The scope of an (identifier denoting a) type, function or package extends from the point of the identifier in the declaration to the end of the innermost surrounding block.
3. The scope of a constant or variable extends textually from the end of the declaration to the end of the innermost surrounding block. If the variable is declared in the init statement of an if , for, or switch statement, the innermost surrounding block is the block associated with that statement.
4. The scope of a parameter or result is the body of the corresponding function.
5. The scope of a field or method is selectors for the corresponding type containing the field or method (§Selectors).
6. The scope of a label is a unique scope emcompassing the body of the innermost surrounding function, excluding nested functions. Labels do not conflict with non-label identifiers.

Predeclared identifiers

The following identifiers are implicitly declared in the outermost scope:

Basic types:
	bool byte float32 float64 int8 int16 int32 int64 string uint8 uint16 uint32 uint64

Architecture-specific convenience types:
	float int uint uintptr

Constants:
	true false iota nil

Functions:
	cap convert len make new panic panicln print println typeof (TODO: typeof??)

Packages:
	sys unsafe  (TODO: does sys endure?)

Exported identifiers

By default, identifiers are visible only within the package in which they are declared. Some identifiers are exported and can be referenced using qualified identifiers in other packages (§Qualified identifiers). If an identifier satisfies these two conditions:

1. the first character of the identifier's name is a Unicode upper case letter;
2. the identifier is declared at the package level or is a field or method of a type declared at the top level;

it will be exported automatically.

Const declarations

A constant declaration binds a list of identifiers (the names of the constants) to the values of a list of constant expressions (§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 value of the n^th expression on the right.

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

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

CompleteType = Type .

If the type (CompleteType) is omitted, the constants take the individual types of the corresponding expressions, which may be ``ideal integer'' or ``ideal float'' (§Ideal number). If the type is present, all constants take the type specified, and the types of all the expressions must be assignment-compatible with that type.

const Pi float64 = 3.14159265358979323846
const E = 2.718281828
const (
	size int64 = 1024;
	eof = -1;
)
const a, b, c = 3, 4, "foo"  // a = 3, b = 4, c = "foo"
const u, v float = 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 declaration. Such an empty list is equivalent to the textual substitution of the first preceding non-empty expression list. 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 generator (§Iota) this 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 pseudo-constant iota represents successive integers. It is reset to 0 whenever the reserved word const appears in the source and increments with each semicolon. It can be used to construct a set of related constants:

const (            // iota is reset to 0
	c0 = iota;  // c0 == 0
	c1 = iota;  // c1 == 1
	c2 = iota   // c2 == 2
)

const (
	a = 1 << iota;  // a == 1 (iota has been reset)
	b = 1 << iota;  // b == 2
	c = 1 << iota;  // c == 4
)

const (
	u       = iota * 42;  // u == 0     (ideal integer)
	v float = iota * 42;  // v == 42.0  (float)
	w       = iota * 42;  // w == 84    (ideal integer)
)

const x = iota;  // x == 0 (iota has been reset)
const y = iota;  // y == 0 (iota has been reset)

Within an ExpressionList, the value of each iota is the same because it is only incremented at a semicolon:

const (
	bit0, mask0 = 1 << iota, 1 << iota - 1;  // bit0 == 1, mask0 == 0
	bit1, mask1;                             // bit1 == 2, mask1 == 1
	bit2, mask2;                             // bit2 == 4, mask2 == 3
)

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

Type declarations

A type declaration binds an identifier, the type name, to a new type. TODO: what exactly is a "new type"?

TypeDecl     = "type" ( TypeSpec | "(" [ TypeSpecList ] ")" ) .
TypeSpecList = TypeSpec { ";" TypeSpec } [ ";" ] .
TypeSpec     = identifier Type .

type IntArray [16] int

type (
	Point struct { x, y float };
	Polar Point
)

type TreeNode struct {
	left, right *TreeNode;
	value Point;
}

type Comparable interface {
	cmp(Comparable) int
}

Variable declarations

A variable declaration creates a variable, binds an identifier to it and gives it a type and optionally an initial value. The type must be complete (§Types).

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

var i int
var U, V, W float
var k = 0
var x, y float = -1.0, -2.0
var (
	i int;
	u, v, s = 2.0, 3.0, "bar"
)

If there are expressions, their number must be equal to the number of identifiers, and the n^th variable is initialized to the value of the n^th expression. Otherwise, each variable is initialized to the zero of the type (§The zero value). The expressions can be general expressions; they need not be constants.

Either the type or the expression list must be present. If the type is present, it sets the type of each variable and the expressions (if any) must be assignment-compatible to that type. If the type is absent, the variables take the types of the corresponding expressions.

If the type is absent and the corresponding expression is a constant expression of ideal integer or ideal float type, the type of the declared variable is int or float respectively:

var i = 0       // i has type int
var f = 3.1415  // f has type float

Short variable declarations

SimpleVarDecl = IdentifierList ":=" ExpressionList .

"var" IdentifierList = ExpressionList .

i, j := 0, 10;
f := func() int { return 7; }
ch := new(chan int);

Unlike regular variable declarations, short variable declarations can be used, by analogy with tuple assignment (§Assignments), to receive the individual elements of a multi-valued expression such as a call to a multi-valued function. In this form, the ExpressionList must be a single such multi-valued expression, the number of identifiers must equal the number of values, and the declared variables will be assigned the corresponding values.

count, error := os.Close(fd);  // os.Close()  returns two values

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 (§Statements).

Function declarations

A function declaration binds an identifier to a function (§Function types).

FunctionDecl = "func" identifier Signature [ Block ] .

func min(x int, y int) int {
	if x < y {
		return x;
	}
	return y;
}

A function must be declared or forward-declared before it can be invoked (§Forward declarations). Implementation restriction: Functions can only be declared at the package level.

Method declarations

A method declaration binds an identifier to a method, which is a function with a receiver.

MethodDecl = "func" Receiver identifier Signature [ Block ] .
Receiver = "(" [ identifier ] [ "*" ] TypeName ")" .

The receiver type must be a type name or a pointer to a type name, and that name is called the receiver base type or just base type. The base type must not be a pointer type and must be declared in the same source file as the method. The method is said to be bound to the base type and is visible only within selectors for that type (§Type declarations, §Selectors).

All methods bound to a base type must have the same receiver type, either all pointers to the base type or all the base type itself. Given type Point, the declarations

func (p *Point) Length() float {
	return Math.sqrt(p.x * p.x + p.y * p.y);
}

func (p *Point) Scale(factor float) {
	p.x = p.x * factor;
	p.y = p.y * factor;
}

bind the methods Length and Scale to the base type Point.

If the receiver's value is not referenced inside the the body of the method, its identifier may be omitted in the declaration. The same applies in general to parameters of functions and methods.

Methods can be declared only after their base type is declared or forward-declared, and invoked only after their own declaration or forward-declaration (§Forward declarations). Implementation restriction: They can only be declared at package level.

The type of a method is the type of a function with the receiver as first argument. For instance, the method Scale has type

(p *Point, factor float)

However, a function declared this way is not a method.

Forward declarations

Mutually-recursive types require that one be forward declared so that it may be named in the other. A forward declaration of a type omits the block containing the fields or methods of the type.

type List struct  // forward declaration of List
type Item struct {
	value int;
	next *List;
}
type List struct {
	head, tail *Item
}

A forward-declared type is incomplete (§Types) until it is fully declared. The full declaration must follow before the end of the block containing the forward declaration.

Functions and methods may similarly be forward-declared by omitting their body.

func F(a int) int  // forward declaration of F
func G(a, b int) int {
	return F(a) + F(b)
}
func F(a int) int {
	if a <= 0 { return 0 }
	return G(a-1, b+1)
}

Expressions

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

Operands

Operand    = Literal | QualifiedIdent | "(" Expression ")" .
Literal    = BasicLit | CompositeLit | FunctionLit .
BasicLit   = int_lit | float_lit | char_lit | StringLit .
StringLit  = string_lit { string_lit } .

Constants

An operand is called constant if it is a literal of a basic type (including the predeclared constants true and false, and values denoted by iota), the predeclared constant nil, or a parenthesized constant expression (§Constant expressions). Constants have values that are known at compile time.

Qualified identifiers

A qualified identifier is an identifier qualified by a package name prefix.

QualifiedIdent = [ LocalPackageName "." ] [ PackageName "." ] identifier .
LocalPackageName = identifier .
PackageName = identifier .

A qualified identifier accesses an identifier in a separate package. The identifier must be exported by that package, which means that it must begin with a Unicode upper case letter (§Exported identifiers).

The LocalPackageName is that of the package in which the qualified identifier appears and is only necessary to access names hidden by intervening declarations of a package-level identifier.

Math.Sin
mypackage.hiddenName
mypackage.Math.Sin  // if Math is declared in an intervening scope

Composite literals

Composite literals construct values for structs, arrays, slices, and maps and create a new value each time they are evaluated. They consist of the type of the value followed by a parenthesized list of expressions, or a list of expression pairs for map literals.

CompositeLit  = LiteralType "(" [ ( ExpressionList | ExprPairList ) [ "," ] ] ")" .
LiteralType   = StructType | ArrayType | "[" "..." "]" ElementType |
                SliceType | MapType | TypeName .
ExprPairList  = ExprPair { "," ExprPair } .
ExprPair      = Expression ":" Expression .

The LiteralType must be a struct, array, slice, or map type. TODO: then why doesn't the grammar say that? The types of the expressions must match the respective field, element, and key types of the LiteralType; there is no automatic type conversion. Given

type Rat struct { num, den int }
type Num struct { r Rat; f float; s string }

one may write

pi := Num(Rat(22, 7), 3.14159, "pi");

The length of an array literal is the length specified in the LiteralType. 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 more elements than specified in the type. The notation ... specifies an array length equal to the number of elements in the literal.

buffer := [10]string();               // len(buffer) == 10
primes := [6]int(2, 3, 5, 7, 9, 11);  // len(primes) == 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 is the number of elements (of the array) provided in the literal. A slice literal has the form

[]T(x1, x2, ... xn)

and is a shortcut for a slice operation applied to an array literal:

[n]T(x1, x2, ... xn)[0 : n]

In map literals only, the list contains key-value pairs separated by a colon:

m := map[string]int("good": 0, "bad": 1, "indifferent": 7);

Function literals

A function literal represents an anonymous function. It consists of a specification of the function type and a function body.

FunctionLit   = "func" Signature Block .
Block         = "{" [ StatementList ] "}" .

func (a, b int, z float) 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 } (reply_chan)

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

PrimaryExpr =
	Operand |
	PrimaryExpr Selector |
	PrimaryExpr Index |
	PrimaryExpr Slice |
	PrimaryExpr TypeGuard |
	PrimaryExpr Call .

Selector   = "." identifier .
Index      = "[" Expression "]" .
Slice      = "[" Expression ":" Expression "]" .
TypeGuard  = "." "(" Type ")" .
Call       = "(" [ ExpressionList ] ")" .

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

Selectors

A primary expression of the form

x.f

denotes the field or method f of the value denoted by x (or of *x if x is of pointer type). The identifier f is called the (field or method) selector. The type of the expression is the type of f.

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 nested anonymous field of T. The number of anonymous fields traversed to reach f is called its depth in T. The depth of a field or method f declared in T is zero. The depth of a field or method f declared in an anonymous field A in T is the depth of f in A plus one.

The following rules apply to selectors:

1. For a value x of type T or *T where T is not an 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 f with shallowest depth, the selector expression is illegal.
2. For a variable x of type I or *I where I is an interface type, x.f denotes the actual method with name f of the value assigned to x if there is such a method. If no value or nil was assigned to x, x.f is illegal.
3. In all other cases, x.f is illegal.

Selectors automatically dereference pointers as necessary. If x is of pointer type, x.y is shorthand for (*x).y; if y is also of pointer type, x.y.z is shorthand for (*(*x).y).z, and so on. If *x is of pointer type, dereferencing must be explicit; only one level of automatic dereferencing is provided. For an x of type T containing an anonymous field declared as *A, x.f is a shortcut for (*x.A).f.

For example, given the declarations:

type T0 struct {
	x int;
}

func (recv *T0) M0()

type T1 struct {
	y int;
}

func (recv T1) M1()

type T2 struct {
	z int;
	T1;
	*T0;
}

func (recv *T2) M2()

var p *T2;  // with p != nil and p.T1 != nil

one may write:

p.z         // (*p).z
p.y         // ((*p).T1).y
p.x         // (*(*p).T0).x

p.M2        // (*p).M2
p.M1        // ((*p).T1).M1
p.M0        // ((*p).T0).M0

Indexes

A primary expression of the form

a[x]

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

For a of type A or *A where A is an array type (§Array types):

• x must be an integer value and 0 <= x < len(a)
• a[x] is the array element at index x and the type of a[x] is the element type of A

For a of type M or *M where M is a map type (§Map types):

• x must be of the same type as the key type of M and the map must contain an entry with key x (but see special forms below)
• a[x] is the map value with key x and the type of a[x] is the value type of M

Otherwise a[x] is illegal. If the index or key is out of range evaluating an otherwise legal index expression, a run-time exception occurs.

However, if an index expression on a map a of type map[K] V is used in an assignment of one of the special forms

r, ok = a[x]
r, ok := a[x]

the result of the index expression is a pair of values with types (K, bool). If the key is present in the map, the expression returns the pair (a[x], true); otherwise it returns (Z, false) where Z is the zero value for V (§The zero value). No run-time exception occurs in this case. The index expression in this construct thus acts like a function call returning a value and a boolean indicating success. (§Assignments)

Similarly, if an assignment to a map has the special form

a[x] = r, ok

and boolean ok has the value false, the entry for key x is deleted from the map; if ok is true, the construct acts like a regular assignment to an element of the map.

Slices

Strings, arrays, and slices can be sliced to construct substrings or descriptors of subarrays. The index expressions in the slice select which elements appear in the result. The result has indexes starting at 0 and length equal to the difference in the index values in the slice. After slicing the array a

a := [4]int(1, 2, 3, 4);
s := a[1:3];

the slice s has type []int, length 2, and elements

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

The slice length must be non-negative. For arrays or strings, the index values in the slice must be in bounds for the original array or string; for slices, the index values must be between 0 and the capacity of the slice.

If the sliced operand is a string, the result of the slice operation is another, new string (§String types). If the sliced operand is an array or slice, the result of the slice operation is a slice (§Slice types).

Type guards

For an expression x and a type T, the primary expression

x.(T)

asserts that the value stored in x is of type T. The notation .(T) is called a type guard, and x.(T) is called a guarded expression. The type of x must be an interface type.

More precisely, if T is not an interface type, the type guard asserts that the dynamic type of x is identical to the type T (§Type equality and identity). If T is an interface type, the type guard asserts that the dynamic type of T implements the interface T (§Interface types). The type guard is said to succeed if the assertion holds. TODO: gri wants an error if x is already of type T.

If the type guard succeeds, the value of the guarded expression is the value stored in x and its type is T. If the type guard fails, a run-time exception occurs. In other words, even though the dynamic type of x is known only at run-time, the type of the guarded expression x.(T) is known to be T in a correct program.

If a guarded expression is used in an assignment of one of the special forms,

v, ok = x.(T)
v, ok := x.(T)

the result of the guarded expression is a pair of values with types (T, bool). If the type guard succeeds, the expression returns the pair (x.(T), true); otherwise, the expression returns (Z, false) where Z is the zero value for type T (§The zero value). No run-time exception occurs in this case. The type guard in this construct thus acts like a function call returning a value and a boolean indicating success. (§Assignments)

Calls

Given a function or a function variable f of function type F, the expression

f(a, b, c)

calls the function with arguments a, b, c. The arguments must be assignment compatible with the parameters of F 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.

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

If the receiver type of the method is declared as a pointer of type *T, the actual receiver may be a value of type T; in such cases method invocation implicitly takes the receiver's address:

var p Point;
p.Scale(3.5)

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

Passing arguments to ... parameters

When a function f has a ... parameter, it is always the last formal parameter. Within calls to f, the arguments before the ... are treated normally. After those, an arbitrary number (including zero) of trailing arguments may appear in the call and are bound to the ... parameter.

Within f, the ... parameter has static type interface{} (the empty interface). For each call, its dynamic type is a structure whose sequential fields are the trailing arguments of the call. That is, the actual arguments provided for a ... parameter are wrapped into a struct that is passed to the function instead of the actual arguments. Using the reflection library (TODO: reference), f may unpack the elements of the dynamic type to recover the actual arguments.

Given the function and call

func Fprintf(f io.Write, format string, args ...)
Fprintf(os.Stdout, "%s %d", "hello", 23);

Within Fprintf, the dynamic type of args for this call will be, schematically, struct { string; int }.

As a special case, if a function passes its own ... parameter as the argument for a ... in a call to another function with a ... parameter, the parameter is not wrapped again but passed directly. In short, a formal ... parameter is passed unchanged as an actual ... parameter.

Operators

Operators combine operands into expressions.

Expression = UnaryExpr | Expression binaryOp UnaryExpr .
UnaryExpr  = PrimaryExpr | unary_op UnaryExpr .

binary_op  = log_op | com_op | rel_op | add_op | mul_op .
log_op     = "||" | "&&" .
com_op     = "<-" .
rel_op     = "==" | "!=" | "<" | "<=" | ">" | ">=" .
add_op     = "+" | "-" | "|" | "^" .
mul_op     = "*" | "/" | "%" | "<<" | ">>" | "&" .

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

The operand types in binary operations must be equal, with the following exceptions:

• If one operand has numeric type and the other operand is an ideal number, the ideal number is converted to match the type of the other operand (§Expressions).
• If both operands are ideal numbers, the conversion is to ideal floats if one of the operands is an ideal float (relevant for / and %).
• The right operand in a shift operation must be always be of unsigned integer type or an ideal number that can be safely converted into an unsigned integer type (§Arithmetic operators).
• The operands in channel operations differ in type: one is always a channel and the other is a variable or value of the channel's element type.
• When comparing two operands of channel type, the channel value types must be equal but the channel direction is ignored.

Unary operators have the highest precedence. They are evaluated from right to left. As the ++ and -- operators form statements, not expressions, they fall outside the unary operator hierarchy and apply to the operand on the left. As a consequence, statement *p++ is the same as (*p)++.

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

Precedence    Operator
    6             *  /  %  <<  >>  &
    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.

Examples:

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

Arithmetic operators

Arithmetic operators apply to numeric types and yield a result of the same type as the first operand. The four standard arithmetic operators (+, -, *, /) apply both to integer and floating point types, while + applies also to strings; all other arithmetic operators apply to integers only.

+    sum             integers, floats, strings
-    difference      integers, floats
*    product         integers, floats
/    quotient        integers, floats
%    remainder       integers

&    bitwise and     integers
|    bitwise or      integers
^    bitwise xor     integers

<<   left shift      integer << unsigned integer
>>   right shift     integer >> unsigned integer

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.

For integer values, / and % satisfy the following relationship:

(a / b) * b + a % b == a

with (a / b) truncated towards zero. Examples:

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

If the dividend is positive and the divisor is a constant power of 2, the division may be replaced by a left 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. They implement arithmetic shifts if the left operand is a signed integer and logical shifts if it is an unsigned integer. The shift count must be 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*2 and x >> 1 is the same as x/2 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"

Integer overflow

For unsigned integer values, the operations +, -, *, and << are computed modulo 2ⁿ, where n is the bit width of the unsigned integer's type (§Arithmetic types). 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. No exception is raised as a result of overflow. 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.

Comparison operators

Comparison operators yield a boolean result. All comparison operators apply to basic types except bools. The operators == and != apply, at least in some cases, to all types except arrays and structs.

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

Numeric basic types are compared in the usual way.

Strings are compared byte-wise (lexically).

Booleans are equal if they are either both "true" or both "false".

The rules for comparison of composite types are described in the section on §Comparison compatibility.

Logical operators

Logical operators apply to boolean operands and yield a boolean result. 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

TODO: Need to talk about unary "*", clean up section below.

TODO: This text needs to be cleaned up and go elsewhere, there are no address operators involved.

Methods are a form of function, and a method ``value'' has a function type. Consider the type T with method M:

type T struct {
	a int;
}
func (tp *T) M(a int) int;
var t *T;

t.M

func (t *T, a int) int

var f func (t *T, a int) int;
f = t.M;
x := f(t, 7);

t.M(args)

(t.M)(t, args)

If T is an interface type, the expression t.M does not determine which underlying type's M is called until the point of the call itself. Thus given T1 and T2, both implementing interface I with method M, the sequence

var t1 *T1;
var t2 *T2;
var i I = t1;
m := i.M;
m(t2, 7);

func (recv I, a int) {
	recv.M(a);
}

TODO: Document implementation restriction: It is illegal to take the address of a result parameter (e.g.: func f() (x int, p *int) { return 2, &x }). (TBD: is it an implementation restriction or fact?)

Communication operators

The term channel means "variable of channel type" (§Channel types).

The send operation uses the binary operator "<-", which operates on a channel and a value (expression):

ch <- 3

The send operation sends the value on the channel. Both the channel and the expression are evaluated before communication begins. Communication blocks until the send can proceed, at which point the value is transmitted on the channel. A send can proceed if the channel is asynchronous and there is room in its buffer or the channel is synchronous and a receiver is ready.

If the send operation appears in an expression context, the value of the expression is a boolean and the operation is non-blocking. The value of the boolean reports true if the communication succeeded, false if it did not. (The channel and the expression to be sent are evaluated regardless.) These two examples are equivalent:

ok := ch <- 3;
if ok { print("sent") } else { print("not sent") }

if ch <- 3 { print("sent") } else { print("not sent") }

In other words, if the program tests the value of a send operation, the send is non-blocking and the value of the expression is the success of the operation. If the program does not test the value, the operation blocks until it succeeds.

The receive operation uses the prefix unary operator "<-". The value of the expression is the value received, whose type is the element type of the channel.

<-ch

The expression blocks until a value is available, which then can be assigned to a variable or used like any other expression. If the receive expression does not save the value, the value is discarded.

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

If a receive expression is used in a tuple assignment of the form

x, ok = <-ch;  // or: x, ok := <-ch

the receive operation becomes non-blocking. If the operation can proceeed, the boolean variable ok will be set to true and the value stored in x; otherwise ok is set to false and x is set to the zero value for its type (§The zero value).

TODO: Probably in a separate section, communication semantices need to be presented regarding send, receive, select, and goroutines.

Constant expressions

Constant expressions may contain only constants, iota, numeric literals, string literals, and some constant-valued built-in functions such as unsafe.Sizeof and len applied to an array. In practice, constant expressions are those that can be evaluated at compile time.

The type of a constant expression is determined by the type of its elements. If it contains only numeric literals, its type is ideal integer or ideal float (§Ideal number). Whether it is an integer or float depends on whether the value can be represented precisely as an integer (123 vs. 1.23). The nature of the arithmetic operations within the expression depends, elementwise, on the values; for example, 3/2 is an integer division yielding 1, while 3./2. is a floating point division yielding 1.5. Thus

const x = 3./2. + 3/2;

yields a floating point constant of ideal float value 2.5 (1.5 + 1); its constituent expressions are evaluated using distinct rules for division.

Intermediate values and the constants themselves may require precision significantly larger than any concrete type in the language. The following are legal declarations:

const Huge = 1 << 100;
const Four int8 = Huge >> 98;

A constant expression may appear in any context, such as assignment to a variable of any numeric type, as long as the value of the expression can be represented accurately in that context. It is erroneous to assign a value with a non-zero fractional part to an integer, or if the assignment would overflow or underflow, or in general if the value cannot be represented by the type of the variable. For instance, 3 can be assigned to any integer variable but also to any floating point variable, while -1e12 can be assigned to a float32, float64, or even int64 but not uint64 or string.

Statements

Statement =
	Declaration | LabelDecl | EmptyStat |
	SimpleStat | GoStat | ReturnStat | BreakStat | ContinueStat | GotoStat |
	FallthroughStat | Block | IfStat | SwitchStat | SelectStat | ForStat |
	DeferStat .

SimpleStat =
	ExpressionStat | IncDecStat | Assignment | SimpleVarDecl .

StatementList = Statement { OptSemicolon Statement } .

A semicolon may be omitted immediately following:

• a closing parenthesis ")" ending a list of declarations (§Declarations and Scope)
• a closing brace "}" ending a type declaration (§Type declarations)
• a closing brace "}" ending a block (including switch and select statements)
• a label declaration (§Label declarations)

Empty statements

EmptyStat = .

Expression statements

ExpressionStat = Expression .

f(x+y)

IncDec statements

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

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

Note that increment and decrement are statements, not expressions. For instance, "x++" cannot be used as an operand in an expression.

Assignments

Assignment = ExpressionList assign_op ExpressionList .

assign_op = [ add_op | mul_op ] "=" .

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

j <<= 2

v1, v2, v3 = e1, e2, e3

a, b = b, a

x, y = f()

value, present = map_var[key]

map_var[key] = value, false

If statements

IfStat = "if" [ [ SimpleStat ] ";" ] [ Expression ] Block [ "else" Statement ] .

if x > 0 {
	return true;
}

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

Switch statements

SwitchStat = "switch" [ [ SimpleStat ] ";" ] [ Expression ] "{" { CaseClause } "}" .
CaseClause = SwitchCase ":" [ StatementList ] .
SwitchCase = "case" ExpressionList | "default" .

Each case clause effectively acts as a block for scoping purposes ($Declarations and scope rules).

The expressions do not need to be constants. They will be evaluated top to bottom until the first successful non-default case is reached. If none matches and there is a default case, the statements of the default case are executed.

switch tag {
default: s3()
case 0, 1: s1()
case 2: s2()
}

switch x := f(); true {
case x < 0: return -x
default: return x
}

switch a {
case 1:
	b();
	fallthrough
case 2:
	c();
}

switch {
case x < y: f1();
case x < z: f2();
case x == 4: f3();
}

For statements

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

for a < b {
	a *= 2
}

ForClause = [ InitStat ] ";" [ Condition ] ";" [ PostStat ] .
InitStat = SimpleStat .
PostStat = SimpleStat .

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

The init and post statement as well as the condition may be omitted; however if either the init or post statement are present, the separating semicolons must be present. If the condition is absent, it is equivalent to "true". The following statements are equivalent:

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

RangeClause = IdentifierList ( "=" | ":=" ) "range" Expression .

The iteration variables may be declared by the range clause (":="), in which case their scope ends at the end of the for statement block ($Declarations and scope rules, Rule 3). In this case their types are the array index and element, or the map key and value types, respectively.

var a [10]string;
m := map[string]int("mon":0, "tue":1, "wed":2, "thu":3, "fri":4, "sat":5, "sun":6);

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 {};  // value type of m is assignment-compatible to val
for key, value = range m {
	h(key, value)
}
// key == last map key encountered in iteration
// val == map[key]

Go statements

GoStat = "go" Expression .

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

Select statements

SelectStat = "select" "{" { CommClause } "}" .
CommClause = CommCase ":" [ StatementList ] .
CommCase = "case" ( SendExpr | RecvExpr) | "default" .
SendExpr =  Expression "<-" Expression .
RecvExpr =  [ Expression ( "=" | ":=" ) ] "<-" Expression .

For all the send and receive expressions in the select statement, the channel expression is evaluated. Any values that appear on the right hand side of send expressions are also evaluated. If any of the resulting channels can proceed, one is chosen and the corresponding communication and statements are evaluated. Otherwise, if there is a default case, that executes; if not, the statement blocks until one of the communications can complete. The channels and send expressions are not re-evaluated. A channel pointer may be nil, which is equivalent to that case not being present in the select statement.

Since all the channels and send expressions are evaluated, any side effects in that evaluation will occur for all the communications in the select.

If the channel sends or receives an interface type, its communication can proceed only if the type of the communication clause matches that of the dynamic value to be exchanged.

If multiple cases can proceed, a uniform fair choice is made regarding which single communication will execute.

The receive case may declare a new variable (via a ":=" assignment). The scope of such variables begins immediately after the variable identifier and ends at the end of the respective "select" case (that is, before the next "case", "default", or closing brace).

var c, c1, c2 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");
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:
	}
}

var ca chan interface {};
var i int;
var f float;
select {
case i = <-ca:
	print("received int ", i, " from ca\n");
case f = <-ca:
	print("received float ", f, " from ca\n");
}

Return statements

ReturnStat = "return" [ ExpressionList ] .

func simple_f() int {
	return 2;
}

func complex_f1() (re float, im float) {
	return -7.0, -4.0;
}

func complex_f2() (re float, im float) {
	re = 7.0;
	im = 4.0;
	return;
}

Break statements

BreakStat = "break" [ identifier ].

L: for i < n {
	switch i {
	case 5: break L
	}
}

Continue statements

ContinueStat = "continue" [ identifier ].

Label declarations

LabelDecl = identifier ":" .

Error:

Goto statements

GotoStat = "goto" identifier .

goto Error

goto L;  // BAD
v := 3;
L:

Fallthrough statements

FallthroughStat = "fallthrough" .

Defer statements

DeferStat = "defer" Expression .

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);
}

Predeclared functions

• cap
• convert
• len
• make
• new
• panic
• panicln
• print
• println
• typeof

Length and capacity

Call       Argument type       Result

len(s)    string, *string      string length (in bytes)
          [n]T, *[n]T          array length (== n)
          []T, *[]T            slice length
          map[K]T, *map[K]T    map length
          chan T               number of elements in channel buffer

cap(s)    []T, *[]T            capacity of s
          map[K]T, *map[K]T    capacity of s
          chan T               channel buffer capacity

The type of the result is always int and the implementation guarantees that the result always fits into an int.

The capacity of a slice or map is the number of elements for which there is space allocated in the underlying array (for a slice) or map. For a slice s, at any time the following relationship holds:

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

Conversions

T(value)

The following conversion rules apply:

1) Between integer types. If the value is a signed quantity, it is sign extended to implicit infinite precision; otherwise it is zero extended. It is then truncated to fit in the result type size. For example, uint32(int8(0xFF)) is 0xFFFFFFFF. The conversion always yields a valid value; there is no signal for overflow.

2) Between integer and floating point types, or between floating point types. To avoid overdefining the properties of the conversion, for now it is defined as a ``best effort'' conversion. The conversion always succeeds but the value may be a NaN or other problematic result. TODO: clarify?

3) Strings permit two special conversions.

3a) Converting an integer value yields a string containing the UTF-8 representation of the integer. (TODO: this one could be done just as well by a library.)

string(0x65e5)  // "\u65e5"

3b) Converting an array of uint8s yields a string whose successive bytes are those of the array. (Recall byte is a synonym for uint8.)

string([]byte('h', 'e', 'l', 'l', 'o')) // "hello"

There is no linguistic mechanism to convert between pointers and integers. A library may be provided under restricted circumstances to acccess this conversion in low-level code. TODO: Do we allow interface/ptr conversions in this form or do they have to be written as type guards? (§Type guards)

Allocation

new(T)

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

TODO Once this has become clearer, connect new() and make() (new() may be explained by make() and vice versa).

Making slices, maps, and channels

make(T [, optional list of expressions])

make(map[string] int)

s := make([]int, 10, 100);        # slice with len(s) == 10, cap(s) == 100
c := make(chan int, 10);          # channel with a buffer size of 10
m := make(map[string] int, 100);  # map with initial space for 100 elements

Packages

Package = PackageClause { ImportDecl [ ";" ] } { Declaration [ ";" ] } .

Every source file identifies the package to which it belongs. The file must begin with a package clause.

PackageClause = "package" PackageName .

package Math

ImportDecl = "import" ( ImportSpec | "(" [ ImportSpecList ] ")" ) .
ImportSpecList = ImportSpec { ";" ImportSpec } [ ";" ] .
ImportSpec = [ "." | PackageName ] PackageFileName .
PackageFileName = StringLit .

In the following discussion, assume we have a package in the file "/lib/math", called package "math", which exports the identifiers "Sin" and "Cos" denoting the respective trigonometric functions.

In the general form, with an explicit package name, the import statement declares that package name as an identifier whose contents are the exported elements of the imported package. For instance, after

import M "/lib/math"

In its simplest form, with no package name, the import statement implicitly uses the imported package name itself as the local package name. After

import "/lib/math"

Finally, if instead of a package name the import statement uses an explicit period, the contents of the imported package are added to the current package. After

import . "/lib/math"

Here is a complete example Go package that implements a concurrent prime sieve:

package main

// 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 'in' to channel 'out',
// removing those divisible by 'prime'.
func filter(in chan <- int, out *<-chan int, prime int) {
	for {
		i := <-in;  // Receive value of new variable 'i' from 'in'.
		if i % prime != 0 {
			out <- i  // Send 'i' to channel 'out'.
		}
	}
}

// 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;
		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 memory is allocated to store a value, either through a declaration or new(), and no explicit initialization is provided, the memory is given a default initialization. Each element of such a value is set to the zero value for its type: false for booleans, 0 for integers, 0.0 for floats, "" for strings, and nil for pointers and interfaces. 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 float; next *T };
t := new(T);

the following holds:

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

Program execution

A package with no imports is initialized by assigning initial values to all its global variables in declaration order and then calling any init() functions defined in its source. Since a package may contain more than one source file, there may be more than one init() function, but only one per source file.

Initialization code may contain "go" statements, but the functions they invoke do not begin execution until initialization of the entire program is complete. Therefore, all initialization code is run in a single thread of execution.

Furthermore, an "init()" function cannot be referred to from anywhere in a program. In particular, "init()" cannot be called explicitly, nor can a pointer to "init" be assigned to a function variable).

If a package has imports, the imported packages are initialized before initializing the package itself. If multiple packages import a package P, P will be initialized only once.

The importing of packages, by construction, guarantees that there can be no cyclic dependencies in initialization.

A complete program, possibly created by linking multiple packages, must have one package called main, with a function

func main() { ...  }

defined. The function main.main() takes no arguments and returns no value.

Program execution begins by initializing the main package and then invoking main.main().

When main.main() returns, the program exits.

System considerations

Package unsafe

The built-in package unsafe, known to the compiler, provides facilities for low-level programming including operations that violate the type system. A package using unsafe must be vetted manually for type safety. The package provides the following interface:

package unsafe

const Maxalign int

type Pointer *any  // "any" is shorthand for any Go type; it is not a real type.

func Alignof(variable any) int
func Offsetof(selector any) int
func Sizeof(variable any) int

Any pointer or value of type uintptr can be converted into a Pointer and vice versa.

The function Sizeof takes an expression denoting a variable of any (complete) type and returns the size of the variable in bytes.

The function Offsetof takes a selector (§Selectors) denoting a struct field of any type and returns the field offset in bytes relative to the struct's address. For a struct s with field f:

uintptr(unsafe.Pointer(&s)) + uintptr(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 Alignof takes an expression denoting a variable of any type and returns the alignment of the (type of the) variable in bytes. For a variable x:

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

The maximum alignment is given by the constant Maxalign. It usually corresponds to the value of Sizeof(x) for a variable x of the largest arithmetic type (8 for a float64), but may be smaller on systems with weaker alignment restrictions.

Calls to Alignof, Offsetof, and Sizeof are constant expressions of type int.

Size and alignment guarantees

type                      size in bytes

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

The following minimal alignment properties are guaranteed:

1. For a variable x of any type: 1 <= unsafe.Alignof(x) <= unsafe.Maxalign.
2. For a variable x of arithmetic type: unsafe.Alignof(x) is the smaller of unsafe.Sizeof(x) and unsafe.Maxalign, but at least 1.
3. 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.
4. For a variable x of array type: unsafe.Alignof(x) is the same as unsafe.Alignof(x[0]), but at least 1.

Differences between this doc and implementation - TODO

Implementation accepts only ASCII digits for digits; doc says Unicode.
Implementation does not allow p.x where p is the local package name.
Implementation does not honor the restriction on goto statements and targets (no intervening declarations).
cap() does not work on maps or chans.
len() does not work on chans.
Conversions work for any type; doc says only arithmetic types and strings.