On Oct 21, 2010, at 10:58 PM, Dominic Cooney wrote:
> The proposed syntax for destructuring objects at
> <http://wiki.ecmascript.org/doku.php?id=harmony:destructuring> is:
>
> Pattern ::= "{" (Field ("," Field)* ","?)? "}"
> (array destructuring elided)
> Element ::= Pattern | LValue
> Field ::= Identifier (":" Element)?
> LValue ::= <any lvalue expression allowed in a normal assignment expression>
>
> This misses the opportunity for some parallelism with object literals. For
> example, I can construct {' 0blah': x} with an object literal, but I can't
> access the ' 0blah' with object destructuring.
This is just another (still not formal enough, obviously not yet not carefully
spec'ed :-|) grammar bug, copied from the ES4 destructuring proposal page.
SpiderMonkey and Rhino get it right:
js> var {"0": x} = {"0":42}
js> x
42
js> var {"0blah": z} = {"0blah":42}
js> z
42
> Maybe Field ::= Identifier (":" Element)? could be changed to:
>
> Field ::= PropertyName (":" Element)?
>
> (using ECMA-262's PropertyName) to permit {' 0blah': x} = ...
>
> This also allows seeming nonsense like {' 0blah'}, but I'm not sure that is
> so bad since bindings in *objects* can have names that aren't identifiers, so
> why not in scopes in general, too?
No, that goes too far. There's no way to name the binding. It's not a property
of any object. Again, SpiderMonkey:
js> var {"0blah"} = {"0blah":42}
typein:1: SyntaxError: missing variable name:
typein:1: var {"0blah"} = {"0blah":42}
typein:1: .....^
> So to preserve desugarability you could spell out the alternatives of
> PropertyName in Field, to require elements with strings and numbers:
>
> Field ::= Identifier (":" Element)?
> | StringLiteral ":" Element
> | NumericLiteral ":" Element
Right, that's the way to go. I'll make sure the wiki page is updated again.
Thanks,
/be
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