The XGo Full Specification

January 24, 2026 · View on GitHub

TODO

Composite literals construct new composite values each time they are evaluated. They consist of the type of the literal followed by a brace-bound list of elements. Each element may optionally be preceded by a corresponding key.

CompositeLit  = LiteralType LiteralValue .
LiteralType   = TypeName [ TypeArgs ] .
LiteralValue  = "{" [ ElementList [ "," ] ] "}" .
ElementList   = KeyedElement { "," KeyedElement } .
KeyedElement  = [ Key ":" ] Element .
Key           = FieldName | Expression | LiteralValue .
FieldName     = identifier .
Element       = Expression | LiteralValue .

The LiteralType's underlying type T must be a struct or a classfile type. The types of the elements and keys must be assignable to the respective field; there is no additional conversion. It is an error to specify multiple elements with the same field name.

A key must be a field name declared in the struct type.
An element list that does not contain any keys must list an element for each struct field in the order in which the fields are declared.
If any element has a key, every element must have a key.
An element list that contains keys does not need to have an element for each struct field. Omitted fields get the zero value for that field.
A literal may omit the element list; such a literal evaluates to the zero value for its type.
It is an error to specify an element for a non-exported field of a struct belonging to a different package.

Given the declarations

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

one may write

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

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

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

A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears as an operand between the keyword and the opening brace of the block of an "if", "for", or "switch" statement, and the composite literal is not enclosed in parentheses, square brackets, or curly braces. In this rare case, the opening brace of the literal is erroneously parsed as the one introducing the block of statements. To resolve the ambiguity, the composite literal must appear within parentheses.

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

C style string literals

TODO

c"Hello, world!\n"

Python string literals

TODO

py"Hello, world!\n"

Types

C style string types

import "c"

*c.Char  // alias for *int8

Python string types

import "py"

*py.Object  // TODO: *py.String?

Array types

See Array types.

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

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

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

Pointer types

See Pointer types.

Slice types

See Slice types.

Map types

See Map types.

Struct types

A struct is a sequence of named elements, called fields, each of which has a name and a type. Field names may be specified explicitly (IdentifierList) or implicitly (EmbeddedField). Within a struct, non-blank field names must be unique.

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

// An empty struct.
struct {}

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

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

// A struct with four embedded fields of types T1, *T2, P.T3 and *P.T4
struct {
	T1        // field name is T1
	*T2       // field name is T2
	P.T3      // field name is T3
	*P.T4     // field name is T4
	x, y int  // field names are x and y
}

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

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

A field f or method 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 named type T, promoted methods are included in the method set of the struct as follows:

If S contains an embedded field T, the method sets of S and *S both include promoted methods with 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 T or *T.

A field declaration may be followed by an optional string literal tag, which becomes an attribute for all the fields in the corresponding field declaration. An empty tag string is equivalent to an absent tag. The tags are made visible through a reflection 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"`
}

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

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

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

Tuple types

See Tuple types.

Brace-Style Construction

In addition to function-style construction, tuple supports brace-based initialization using : for field assignment:

type Point (x int, y int)

p1 := Point{x: 10, y: 20}
p2 := Point{10, 20}

Anonymous Tuple Literals with Braces

It allows using tuple literals within brace-based composite literals:

// Using tuples in struct fields
type Record struct {
	coords (int, int)
	data   (string, bool)
}

r := Record{
	coords: (10, 20),
	data:   ("test", true),
}

Type Compatibility and Reflection

At runtime, tuples are implemented as structs with ordinal field names X_0, X_1, X_2, etc.:

type Point (x int, y int)

// At runtime, Point is equivalent to:
// struct {
//     X_0 int  // accessible as .x or .0 at compile time, .X_0 at runtime
//     X_1 int  // accessible as .y or .1 at compile time, .X_1 at runtime
// }

Tuple types with the same element types (in the same order) have identical underlying structures but are distinct named types:

type Point (x int, y int)
type Coord (a int, b int)

// Point and Coord have identical underlying types but are different types
// Conversion is required: c := Coord(p)

Function types

See Function types.

Interface types

TODO

Builtin interfaces

See Builtin interfaces.

The comparable interface

The predeclared interface type comparable denotes the set of all non-interface types that are strictly comparable. A type is strictly comparable if values of that type can be compared using the == and != operators.

The comparable interface is primarily used as a type constraint in generic code and cannot be used as the type of a variable or struct field:

// Example: using comparable as a type constraint
func Find[T comparable](slice []T, value T) int {
	for i, v := range slice {
		if v == value {
			return i
		}
	}
	return -1
}

Types that are strictly comparable include:

Boolean, numeric, and string types
Pointer types
Channel types
Array types (if their element type is strictly comparable)
Struct types (if all their field types are strictly comparable)

Slice, map, and function types are not comparable and cannot be used with comparable.

Channel types are comparable. Two channel values are equal if they were created by the same call to make or if both have value nil.
Struct types are comparable if all their field types are comparable. Two struct values are equal if their corresponding non-blank field values are equal. The fields are compared in source order, and comparison stops as soon as two field values differ (or all fields have been compared).
Type parameters are comparable if they are strictly comparable (see below).

const c = 3 < 4            // c is the untyped boolean constant true

type MyBool bool
var x, y int
var (
	// The result of a comparison is an untyped boolean.
	// The usual assignment rules apply.
	b3        = x == y // b3 has type bool
	b4 bool   = x == y // b4 has type bool
	b5 MyBool = x == y // b5 has type MyBool
)

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

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