On Apr 18, 2011, at 12:14 PM, Brendan Eich wrote:
> On Apr 17, 2011, at 9:09 AM, David Herman wrote:
>
>> * functions with expression bodies
>>
>> This was proposed for ES4 and implemented in SpiderMonkey. I believe there
>> are some unfortunate ambiguities in the grammar that came up, and I think
>> they've been treated as moribund. But I love them, and would love it if we
>> could find a way to bring them back in a way that didn't break the grammar.
>
> The counterexample Waldemar gave, adjusted to use a function with expression
> body instead of a let expression, is worth repeating:
>
> function f() {return "f"}
> var x = 3;
> alert(function (a) a ? f : x++(1));
> /*^^^^^^^^^^^^^^^^^^^^^^^^*/
>
> The alert should show "f". Note that the call grammar:
>
> CallExpression :
> MemberExpression Arguments
> CallExpression Arguments
>
> does not parse an unparenthesized AssignmentExpression (the function with
> expression body's body non-terminal, in ES4 and what we implemented) as the
> "callee" part of the right-hand side (underlined with ^^^ above). But it does
> parse a PrimaryExpression, which includes function expressions, which loops
> back to the AssignmentExpression at the end.
To be super-clear, in case anyone thought the x++(1) was a call expression, the
grammar forbids PostfixExpression (a RHS of UnaryExpression) being
unparenthesized as a callee in a call expression.
But by making functions with expression bodies be PrimaryExpressions ending
with AssignmentExpressions, we have created a bad loop up the grammar that
should allow the example with ^^^ under its callee part to parse.
It doesn't work in Firefox (SpiderMonkey), as Waldemar pointed out. We don't
use a bottom-up parser.
Ruby has a flatter grammar-graph with lots of loops through its big expression
nonterminal. Not so C-like languages including JS. Making bad loops at the end
of right-hand sides is bad. Hope this helps the problem "stick" in folks minds.
/be
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