[Harmony Proxies] Proposal: Property fixing

David Bruant david.bruant at labri.fr
Thu Jun 16 09:09:08 PDT 2011

Le 16/06/2011 17:46, Mark S. Miller a écrit :
> On Thu, Jun 16, 2011 at 8:29 AM, David Bruant <david.bruant at labri.fr 
> <mailto:david.bruant at labri.fr>> wrote:
>     Le 16/06/2011 16:50, Tom Van Cutsem a écrit :
>>     2011/6/16 Mark S. Miller <erights at google.com
>>     <mailto:erights at google.com>>
>>         Ok good, I'll take you up on that. I propose that ES-next
>>         arrays differ from ES5 arrays in that their 'length' property
>>         appears to be a non-configurable accessor property. Clearly,
>>         that better reflects the way Array's 'length' works anyway.
>>         Alternatively, if there's some problem with that, I propose
>>         that array 'length' appears to be a configurable data
>>         property that arrays nevertheless refuse to delete. Either
>>         way, proxies would then be able to emulate them faithfully.
>>     This is also my feeling: part of the awkwardness in emulating
>>     array "length" is definitely that we're trying to mimic the
>>     behavior of an accessor property using a mutable data property.
>>     Would Mark's suggestion be too radical a change? (note: I'm not
>>     arguing for this change on the grounds of "it's too awkward to
>>     emulate using proxies". I'm arguing on the grounds of "ES5
>>     accessor properties seem to better describe the behavior of a
>>     dynamic property such as array |length|").
>     In arrays, "length" affect numerical properties, but the opposite
>     is also true. Should all numerical properties be considered as
>     accessors then? (there is a little bit of bad faith here, because
>     a valid implementation is possible with just "length" being an
>     accessor. See [1]).
>     Considering "length" as a data or accessor property is a secondary
>     question in my opinion. The "magic" behavior is not at the
>     property level but at the object level (even though it can be
>     emulated at the property level).
>     The question raised by Mark is: "should objects with noticeable
>     custom internal method (array, host objects, proxies...) be
>     allowed to prentend having data property even if some logic is
>     triggered under the hood?".
> Almost, and thanks for trying to summarize. My question is
>   "Should ... be allowed to pretend having a *non-configurable* data 
> property ...?"
> A perfectly fine answer to the array.length issue is to have length be 
> a configurable data property so long as it needs to operate in a 
> magical manner. For all such problematic magical behavior, we should 
> likewise report the property as configurable so long as it needs to 
> operate in a magical manner.
Currently, the "configurable" attributes has two meanings. At the same 
time it tells who whether or not you can delete the property and whether 
or not you can reconfigure (call to Object.defineProperty) your 
property. If I understand well, you would like it also to tell whether 
the property is magical or not.

If we are at a point where we'd break Object.defineProperty return 
values, shouldn't we add new keywords rather than adding semantics to 
current attribute keywords?

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