extracting namespace from a property

Brendan Eich brendan at mozilla.org
Mon Feb 5 21:26:18 PST 2007

On Feb 5, 2007, at 7:05 PM, Yuh-Ruey Chen wrote:

> Brendan Eich wrote:
>>    intrinsic static function invoke(ns : Namespace, id : string) :  
>> Name
>>      new Name(ns, id);
> invoke is called when Name is called as a function, right?


>> An enumerating for-in loop or comprehension must reflect an
>> enumerable property name as a Name instance if the property was
>> defined with an explicit or default namespace other than public.
>> Otherwise, with no or a public namespace, enumerating for-in reflects
>> the name as a string per ES1-3 and existing JS implementations.
> Although this helps, it would still be a hassle to have to check  
> whether
> an enumerated property n is a Name or String. I can't do  
> n.identifier if
> n is a String.

You have to use 'switch type' or equivalent.

> How about this: Each enumerated property n is always an instance of
> Name. If n doesn't have a qualifier, n.qualifier would be null, while
> n.identifier would be n's id. Thus, to get a property's id,  
> n.identifier
> would work regardless of whether n has a qualifier. typeof(Name) would
> be "string". Name's prototype would be a String, so name instanceof
> String is true. These two requirements should be enough to satisfy ES3
> compatibility.

That's far from clear, because ES3 mandates a primitive string, not a  
String object. This will break any for-in loop that compares the  
iterating variable to a saved value of that same variable (since ==  
on Strings and other objects is identity, not string value comparison).

You make a good point about this identity being important:

   'foo' in obj === Name(null, 'foo') in obj

The cost of a Name being cons'ed for every iteration of for-in is  
also troubling, in optimization effort if not in runtime and space if  
one argues that it could be optimized away. SpiderMonkey stores 31- 
bit int ids in tagged machine words, not as strings -- never mind as  
anything like Name instances.


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