On Oct 17, 2008, at 12:25 PM, Waldemar Horwat wrote:
> Here's a clearer case that Firefox gets wrong (or your grammar gets
> wrong, depending on your point of view):
>
> function f() {return "f"}
> var x = 3;
> let (a = 1) a ? f : x++();
>
> The grammar states that the last statement must evaluate to "f".
> Firefox gives a syntax error. This is incorrect because the let
> expression up to the ++ is a PrimaryExpression so can be used as the
> left operand of a function call.
Consider the simpler, let-expression-free test:
js> var a = 1, f = 2;
js> a ? f : x++();
typein:3: SyntaxError: missing ; before statement:
typein:3: a ? f : x++();
typein:3: .........^
(The error pointer is off by two, I'm filing a bug on that.)
Safari and IE seem to agree (IE blames the correct column number, the
one for the ( -- good for it).
This is an error according to ES1-3 because x++ is a postfixExpression
derived from
PostfixExpression :
LeftHandSideExpression
LeftHandSideExpression [no LineTerminator here] ++
LeftHandSideExpression [no LineTerminator here] --
while expr() meaning "call expr with zero arguments" derives from
CallExpression : MemberExpression Arguments
and MemberExpression is never an unparenthesized PostfixExpression:
MemberExpression :
PrimaryExpression
FunctionExpression
MemberExpression [ Expression ]
MemberExpression . Identifier
new MemberExpression Arguments
So your example with or without the leading let (a = 1) is not
syntactically valid.
> In an ambiguous grammar, you also need to prove the negative: no
> *other* expansion can match that source program and shadow your
> expansions. Proving the negative causes trouble because a language
> extension could turn the mismatch into a match and because
> sometimes, as in the above case, you expected some other part of the
> grammar to shadow your expansions but it didn't.
That's a good point, but it is not what is going on here. There is no
valid sentence x++() produced by ES1-3's expression grammar.
/be
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