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Composite literal syntax.

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Robert Griesemer 2008-09-03 13:37:44 -07:00
parent e1e53e35a4
commit 0976e34d65
1 changed files with 22 additions and 10 deletions
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32
doc/go_spec.txt
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@ -4,7 +4,7 @@ The Go Programming Language Specification (DRAFT)
Robert Griesemer, Rob Pike, Ken Thompson
----
(August 29, 2008)
(September 3, 2008)
This document is a semi-formal specification of the Go systems
@ -438,11 +438,11 @@ The following identifiers are predeclared:
- the predeclared constants
true, false, nil
true, false, iota, nil
- the predeclared functions (note: this list is likely to change)
cap(), convert(), len(), new(), panic(), print(), ...
cap(), convert(), len(), new(), panic(), print(), typeof(), ...
TODO(gri) We should think hard about reducing the alias type list to:
@ -1116,16 +1116,23 @@ if omitted, the first two examples above can be abbreviated:
Composite Literals
----
CompositeLit = ...
Literals for composite data structures consist of the type of the value
followed by a parenthesized expression list. In appearance, they are a
conversion from expression list to composite value.
followed by a parenthesized expression list for array and structure literals,
or a list of expression pairs for map literals.
Structure literals follow this form directly. Given
CompositeLit = LiteralType "(" [ ( ExpressionList | ExprPairList ) [ "," ] ] ")" .
LiteralType = TypeName | ArrayType | MapType | StructType .
ExprPairList = ExprPair { "," ExprPair } .
ExprPair = Expression ":" Expression .
If LiteralType is a TypeName, the denoted type must be an array, map, or
structure. The types of the expressions must match the respective key, element,
and field types of the literal type; there is no automatic type conversion.
Given
type Rat struct { num, den int };
type Num struct { r Rat, f float, s string };
type Num struct { r Rat; f float; s string };
we can write
@ -1142,12 +1149,17 @@ if a specified size is less than the number of elements in the expression list.
Map literals are similar except the elements of the expression list are
key-value pairs separated by a colon:
m := map[string]int("good":0, "bad":1, "indifferent":7)
m := map[string]int("good": 0, "bad": 1, "indifferent": 7)
TODO: helper syntax for nested arrays etc? (avoids repeating types but
complicates the spec needlessly.)
TODO(gri): These are not conversions and we could use {} instead of () in
the syntax. This will make literals such as Foo(1, 2, 3) clearly stand
out from function calls.
Function Literals
----