On Jun 19, 2011, at 4:04 PM, Brendan Eich wrote:
> On Jun 19, 2011, at 3:17 PM, Claus Reinke wrote:
>
>> https://github.com/clausreinke/jstr/tree/master/es-discuss
>>
>> You can find there the proposal text, with motivation, grammar changes,
>> semantics (entirely via desugaring to existing syntax), and related issues,
>> as well as a prototype implementation, using jstr's nascent grammar-based
>> ES-to-ES rewriting:
>>
>> tailnests.txt: proposal text
>
>
> Quick reply to point out that:
> FunctionExpression :
> function Identifier_opt ( FormalParameterList_opt ) { FunctionBody }
> + function Identifier_opt ( FormalParameterList_opt ) =>
> AssignmentExpression
>
> inverts precedence, as MemberExpression : FunctionExpression but now
> FunctionExpression can end with a low-precedence expression
> (AssignmentExpression).
>
> For example, the last line in
>
> function f(x) => x;
> z = a + function (b) => b ? f : x++(1);
>
> parses awkwardly, as ( z = ( a + ( function (b) => ( b ? f : ( x ++ ) ) ) (1)
> ) ).
This is not just awkward, of course -- the grammar you propose is ambiguous.
The precedence inversion means there are two ways to parse
z = a + function (b) => b ? c : d;
Either as
z = (a + (function (b) => b)) ? c : d;
or as
z = a + (function (b) => (b ? c : d));
And we've been around this block before. Ambiguous grammars are future-hostile.
/be
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