[Harmony Proxies] Proposal: Property fixing

David Bruant david.bruant at labri.fr
Thu Jun 16 02:18:38 PDT 2011

Le 16/06/2011 00:53, Mark S. Miller a écrit :
> On Wed, Jun 15, 2011 at 2:10 PM, David Bruant <david.bruant at labri.fr 
> <mailto:david.bruant at labri.fr>> wrote:
>     In a way, the fixed properties proposal make proxies bicephal. For
>     some
>     inputs (property names), they plug their handler-provided
>     MOP-brain and
>     for some others, they plug a native object MOP-brain (ES5 - 8.12).
>     These
>     brains cannot communicate. This is why changing .length in the "native
>     object brain" has no effect in the other brain (which handles numeric
>     properties(...unless some of these are non-configurable)). And I think
>     it has been said before, but there would be no logging possible for
>     non-configurable properties in the context of the fixed properties
>     strawman since native MOP-brain doesn't allow that.
> Cute metaphor. But as Tom's code showed, the proxy can create fixed 
> (non-configurable) accessor properties whose getters and setters form 
> a corpus callosum ;).
Interactions with getter/setter is already a good thing, but I think 
it's not enough. ES5 offers a very fine-grained API to study objects. If 
we pretend to be able to emulate arrays based on proxies, we should be 
able to emulate everything including answering correctly when it comes 
to "is it a data or an accessor property descriptor?".
For instance, for a data property descriptor, even if non-configurable, 
"writable" can be changed from true to false (then it cannot be changed 
afterward). This behavior is not possible when dealing with getter/setters.
With fixed properties Tom's code, it is not possible to change an 
array's length from writable to not writable. Trying to do so would 
throw an error (because the property is non-configurable and any attempt 
to switch from accessor to data property descriptor throw an error (or 
just reject?)).

I'd like to insist on the ability for proxies to be able to emulate 
native arrays (and new ES5 Object.* API interaction in particular) 
especially because currently, SpiderMonkey has a problem with redefining 
".length" on arrays. See https://bugzilla.mozilla.org/show_bug.cgi?id=598996
I wish proxies to be able to compensate if there is such a bug (in any 
ES implementation) and I need this level of spec conformance.
This discredits the current fixed properties proposal (especially the 
getter/setter compensation code), I think.

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