Minor type confusion in proxies proposal?

Tom Van Cutsem tomvc.be at gmail.com
Fri Jul 8 07:52:28 PDT 2011


2011/7/8 Andreas Rossberg <rossberg at google.com>

> It seems like we need to make a change to Object.defineProperties, too
> (regardless of the other issue). With the current wording, it will not
> forward user attributes to the defineProperty trap. The most modular
> fix (that seems compatible with your proposal below) probably is to
> change the spec of defineProperties to go through defineProperty,
> instead of calling the internal [[DefineOwnProperty]] directly.
>

I agree. Noted.


>
> /Andreas
>
>
> On 6 July 2011 14:55, Andreas Rossberg <rossberg at google.com> wrote:
> > On 3 July 2011 13:29, Tom Van Cutsem <tomvc.be at gmail.com> wrote:
> >> Comments?
> >
> > Looks good to me. I agree with Mark's comment that it should do a
> > (shallow) copy of the attributes object, though. I guess the obvious
> > point would be in Object.defineProperty, before passing it to
> > DefineProxyProperty.
> >
> > /Andreas
> >
> >
> >> 2011/7/2 Andreas Rossberg <rossberg at google.com>
> >>>
> >>> Hi Tom.
> >>>
> >>> On 2 July 2011 13:50, Tom Van Cutsem <tomvc.be at gmail.com> wrote:
> >>> > Hi Andreas,
> >>> > First, you're right about the typing issue:
> >>> > In ES5, for values of type Object, the signature for
> >>> > [[DefineOwnProperty]]
> >>> > would be:
> >>> > [[DefineOwnProperty]](P: a property name, Desc: an internal property
> >>> > descriptor, Throw: a boolean)
> >>> > On trapping proxies, that signature would need to change to:
> >>> > [[DefineOwnProperty]](P: a property name, Desc: an Object, Throw: a
> >>> > boolean)
> >>>
> >>> I don't think such a change is consistent. [[DefineOwnProperty]] is
> >>> invoked in a number of places in the spec, and I think in many of them
> >>> the type of the receiver is not distinguished and may well be a proxy,
> >>> so a proxy may then receive both kinds of descriptors. Moreover, I
> >>> think it would be a mistake to make the appropriate case distinction
> >>> everywhere -- you really want the internal method to have the same
> >>> signature in all cases.
> >>>
> >>> > With that, I believe the strawman is otherwise internally consistent.
> In
> >>> > [[DefineOwnProperty]] step 5, what will be passed to the user-defined
> >>> > "defineProperty" trap is a proper Object, not an internal descriptor.
> >>> > I did clarify the note you referred to, to be more explicit in this
> >>> > regard.
> >>> > I don't see an alternative to changing the signature of
> >>> > [[DefineOwnProperty]]. It can't just receive an internal descriptor,
> as
> >>> > it
> >>> > doesn't preserve any non-standard attributes.
> >>>
> >>> How about simply bypassing [[DefineOwnProperty]] in
> >>> Object.defineProperty for proxies, as I suggested in my reply to
> >>> David? That seems to be the only place where additional objects can
> >>> occur, or am I wrong?
> >>>
> >>> Cheers,
> >>> /Andreas
> >>>
> >>>
> >>> > 2011/7/1 Andreas Rossberg <rossberg at google.com>
> >>> >>
> >>> >> On 1 July 2011 12:12, Andreas Rossberg <rossberg at google.com> wrote:
> >>> >> > I believe there is some "type" confusion in the proxy proposal
> spec
> >>> >> > wrt property descriptors and their reification into attributes
> >>> >> > objects.
> >>> >> >
> >>> >> > 1. In a note on the def of [[DefineOwnProperty]] for proxies, the
> >>> >> > proposal says:
> >>> >> >
> >>> >> > "The Desc argument to this trap is a property descriptor object
> >>> >> > validated by ToPropertyDescriptor, except that it also retains any
> >>> >> > non-standard attributes present in the original property
> descriptor
> >>> >> > passed to Object.defineProperty. See the semantics of the modified
> >>> >> > Object.defineProperty built-in, below."
> >>> >> >
> >>> >> > That seems fishy, since according to ES5 8.10:
> >>> >> >
> >>> >> > "Values of the Property Descriptor type are records composed of
> named
> >>> >> > fields where each field‘s name is an attribute name and its value
> is
> >>> >> > a
> >>> >> > corresponding attribute value as specified in 8.6.1."
> >>> >> >
> >>> >> > In particular, I take this to mean that property descriptors are
> not
> >>> >> > objects (but abstract records), and that there cannot be any
> fields
> >>> >> > whose name is not an attribute name. (In fact, in V8 we currently
> >>> >> > encode property descriptors using objects, but the encoding is
> >>> >> > different from the reified attributes object representation, and
> not
> >>> >> > quite compatible with the idea of adding arbitrary other fields.)
> >>> >>
> >>> >> I forgot to say: step 5 of the definition invokes the defineProperty
> >>> >> trap of the handler passing Desc as the second argument. But the
> >>> >> handler expects a reified attributes object.
> >>> >>
> >>> >> > 2. In the modified definition of Object.defineProperty, the
> proposal
> >>> >> > says in step 4.c:
> >>> >> >
> >>> >> > "Call the [[DefineOwnProperty]] internal method of O with
> arguments
> >>> >> > name, descObj, and true."
> >>> >> >
> >>> >> > This is passing descObj, which in fact is _not_ a descriptor, but
> its
> >>> >> > reification as an attributes object.
> >>> >> >
> >>> >> > /Andreas
> >>> >> >
> >>> >> _______________________________________________
> >>> >> es-discuss mailing list
> >>> >> es-discuss at mozilla.org
> >>> >> https://mail.mozilla.org/listinfo/es-discuss
> >>> >
> >>> >
> >>
> >>
> >
>
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