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go_spec: don't allow parens around the literal type of composite literals

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Background: The current spec is imprecise with respect to the parsing ambiguity
for composite literals: It says that the ambiguity arises when the TypeName form
of the LiteralType is used. The following code:

    if (B) {} ...

is not using the TypeName form (but the parenthesized TypeName form) and thus
could be interpreted as:

    if ((B){}) ...

instead of

    if B {} ...

Both compilers and gofmt choose the latter interpretation. One could fix the
spec by making the clause regarding the parsing ambiguity more precise ("...using
the _possibly parenthesized_ TypeName form of the LiteralType..."). The alternative
(chosen here) is to simply disallow parenthesized literal types. Except for a single
test case (test/parentype.go) there appears to be no Go code under $GOROOT containing
parenthesized literal types. Furthermore, parentheses are never needed around a
literal type for correct parsing.

R=golang-dev
CC=golang-dev
https://golang.org/cl/1913041
This commit is contained in:
Robert Griesemer 2010-07-29 18:13:41 -07:00
parent bab711b184
commit 07cc6440dc
2 changed files with 6 additions and 7 deletions
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9
doc/go_spec.html
View File

@ -1,5 +1,5 @@
<!-- title The Go Programming Language Specification -->
<!-- subtitle Version of July 14, 2010 -->
<!-- subtitle Version of July 29, 2010 -->
<!--
TODO
@ -1974,7 +1974,7 @@ a single expression or a key-value pair.
<pre class="ebnf">
CompositeLit = LiteralType "{" [ ElementList [ "," ] ] "}" .
LiteralType = StructType | ArrayType | "[" "..." "]" ElementType |
SliceType | MapType | TypeName | "(" LiteralType ")" .
SliceType | MapType | TypeName .
ElementList = Element { "," Element } .
Element = [ Key ":" ] Value .
Key = FieldName | ElementIndex .
@ -2096,10 +2096,11 @@ and is a shortcut for a slice operation applied to an array literal:
<p>
A parsing ambiguity arises when a composite literal using the
TypeName form of the LiteralType appears in the condition of an
TypeName form of the LiteralType appears between the
<a href="#Keywords">keyword</a> and the opening brace of the block of an
"if", "for", or "switch" statement, because the braces surrounding
the expressions in the literal are confused with those introducing
a block of statements. To resolve the ambiguity in this rare case,
the block of statements. To resolve the ambiguity in this rare case,
the composite literal must appear within
parentheses.
</p>

4
test/parentype.go
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@ -11,9 +11,7 @@ func g() {}
func main() {
f(map[string]string{"a":"b","c":"d"});
f([...]int{1,2,3});
f(([...]int){1,2,3});
f((map[string]string){"a":"b","c":"d"});
f((map[string]func()){"a":g,"c":g});
f(map[string]func(){"a":g,"c":g});
f(make(chan(<-chan int)));
f(make(chan<-(chan int)));
}