Le 16/06/2011 00:53, Mark S. Miller a écrit :
On Wed, Jun 15, 2011 at 2:10 PM, David Bruant <[email protected]
<mailto:[email protected]>> 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.
David
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