July 26, 2012 TC39 Meeting Notes

Brendan Eich brendan at mozilla.org
Sat Jul 28 14:58:10 PDT 2012

Tom Van Cutsem wrote:
> Hi,
> Thanks to Rick for his notes. The discussion on proxy-related issues 
> went fast and touched upon a variety of issues. I complemented Rick's 
> notes on the proxy discussion with some notes on the proxies wiki 
> page, see: 
> <http://wiki.ecmascript.org/doku.php?id=harmony:direct_proxies#discussed_during_tc39_july_2012_meeting_microsoft_redmond>

Thanks for the wiki update, really useful in my opinion. One note on 
your first bullet there:

"* If __proto__ is specified in Annex B, consider adding a 
|getPrototypeOf| trap. This would simplify membranes. Writable __proto__ 
already destroys the invariant that the [[Prototype]] link is stable. 
Engines already need to accomodate."

We agreed to require __proto__, so put it in the main spec, at the May 

My memory and the meeting notes show that we agreed there to make 
__proto__ a magic data property. Those favoring accessor were not 
voluble, but we talked about poisoning the setter reflection if 
__proto__ ended up spec'ed as an accessor.

>     > getPrototypeOf trap result should be consistent wth target
>     object's proto
>     >
>     > MM: if the proto can be changed, the proxy should…?
>     >
>     > TVC: spec interceptable [[Prototype]]
>     > [[Prototype]] is currently an internal prop
>     > Would need to become internal accessor prop or split into
>     [[GetProto]]
>     > / [[SetProto]]
>     > [[GetProto]] / [[SetProto]] would trigger traps for proxies
>     >
>     > AWB/BE: This is good
>     >
>     > YK: Do we want an analogous setPrototypeOf trap?
>     >
>     > TVC: Yes
>     This is inconsistant with below...
> To clarify, I think we'd only need setPrototypeOf if __proto__ would 
> end up being specified as an accessor (to trap 
> Object.getOwnPD(Object.prototype,'__proto__').set.call(proxy) )

That's another reason not to reflect __proto__ as an accessor with a 
callable setter. Either magic data, or poisoned set (and get?) accessor. 
We should strive to agree on this, since SpiderMonkey went against the 
decision from May, joining JSC in reflecting __proto__ as an accessor.


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