-CompositeLit = LiteralType "{" [ ( ExpressionList | KeyValueList ) [ "," ] ] "}" .
+CompositeLit = LiteralType "{" [ ElementList ] "}" .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName .
-KeyValueList = KeyValueExpr { "," KeyValueExpr } .
-KeyValueExpr = Expression ":" Expression .
+ElementList = Element { "," Element } [ "," ] .
+Element = [ Key ":" ] Value .
+Key = Expression .
+Value = Expression .
-The LiteralType must be a struct, array, slice, or map type. -(The grammar enforces this constraint except when the type is given -as a TypeName.) +The LiteralType must be a struct, array, slice, or map type +(the grammar enforces this constraint except when the type is given +as a TypeName). The types of the expressions must be assignment compatible to the respective field, element, and key types of the LiteralType; there is no additional conversion. +The key is interpreted as a field name for struct literals, +an index for array and slice literals, and a key for map literals. +For map literals, all elements must have a key. It is an error +to specify multiple elements with the same field name or +constant key value.
++For struct literals the following rules apply: +
+Given the declarations +
-type Rat struct { num, den int }
-type Num struct { r Rat; f float; s string }
+type Point struct { x, y, z float }
+type Line struct { p, q Point }
@@ -1836,36 +1868,51 @@ one may write
-pi := Num{Rat{22, 7}, 3.14159, "pi"}
+origin := Point{}; // zero value for Point
+line := Line{origin, Point{y: -4, z: 12.3}}; // zero value for line.q.x
+For array and slice literals the following rules apply: +
Taking the address of a composite literal (§Address operators) generates a unique pointer to an instance of the literal's value.
-var pi_ptr *Rat = &Rat{22, 7}
+var pointer *Point = &Point{y: 1000};
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.
+It is an error to provide elements with index values outside the index range
+of the array. The notation ... specifies an array length equal
+to the maximum element index plus one.
buffer := [10]string{}; // len(buffer) == 10
-primes := [6]int{2, 3, 5, 7, 9, 11}; // len(primes) == 6
+intSet := [6]int{1, 2, 3, 5}; // len(intSet) == 6
days := [...]string{"Sat", "Sun"}; // len(days) == 2
A slice literal describes the entire underlying array literal. -Thus, the length and capacity of a slice literal is the number of elements -(of the array) provided in the literal. A slice literal has the form +Thus, the length and capacity of a slice literal is the maximum +element index plus one. A slice literal has the form
@@ -1880,15 +1927,6 @@ 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};
-
-
A parsing ambiguity arises when a composite literal using the TypeName form of the LiteralType appears in the condition of an @@ -1904,6 +1942,28 @@ if x == (T{a,b,c}[i]) { ... } if (x == T{a,b,c}[i]) { ... } +
+Examples of valid array, slice, and map literals: +
+ +
+// list of prime numbers
+primes := []int{2, 3, 5, 7, 9, 11, 13, 17, 19, 991};
+
+// vowels[ch] is true if ch is a vowel
+vowels := [128]bool{'a': true, 'e': true, 'i': true, 'o': true, 'u': true, 'y': true};
+
+// the array [10]float{-1, 0, 0, 0, -0.1, -0.1, 0, 0, 0, -1};
+filter := [10]float{-1, 4: -0.1, -0.1, 9: -1};
+
+// frequencies in Hz for equal-tempered scale (A4 = 440Hz)
+noteFrequency := map[string]float{
+ "C0": 16.35, "D0": 18.35, "E0": 20.60, "F0": 21.83,
+ "G0": 24.50, "A0": 27.50, "B0": 30.87,
+}
+
+
+