Destructuring syntax infelicity viz. identifier vs. property name

Brendan Eich brendan at mozilla.com
Thu Oct 21 23:20:00 PDT 2010


On Oct 21, 2010, at 11:18 PM, Dominic Cooney wrote:

> Is it helpful to keep picking at these nits?

Sure, much appreciated. These have to get fixed sooner or later, sooner's fine. Thanks again,

/be

> 
> Dominic
> 
> On Fri, Oct 22, 2010 at 3:15 PM, Brendan Eich <brendan at mozilla.com> wrote:
> 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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