On Nov 9, 2011, at 3:53 AM, Tom Van Cutsem wrote:
> 2011/11/8 Allen Wirfs-Brock <[email protected]>
> On Nov 8, 2011, at 7:33 AM, Andreas Rossberg wrote:
> > But I have a follow-up request. :) Regarding redundant trap calls with
> > proxies there is another, more pervasive problem with the current
> > spec: in lots of places it first calls [[HasProperty]] and then
> > [[Get]]. With proxies, this always implies two trap calls, which seems
> > wasteful. Would it be possible to refactor that, too?
> >
> > Seems more difficult, because we would need to enable [[Get]] (and
> > hence the get trap) to signal lookup failure. (Too bad that we cannot
> > reuse `undefined' for it.) But I think the current situation isn't
> > satisfactory.
>
> I agree, the current specification style is fine when we are dealing with
> side-effectly free internal operations but as soon as they are reified they
> become problematic and a performance issue.
>
> Is the situation really that problematic?
> Skimming the ES5.1 spec for [[HasProperty]], I encounter calls in the
> following places:
>
> - ToPropertyDescriptor conversion (only triggers proxy code via
> Object.defineProperty(obj, name, aProxyAsDescriptor) and
> Object.defineProperties).
>
> - lots of methods on Array.prototype (map, forEach, filter, some, reduce,
> reduceRight, reverse, splice, indexOf, lastIndexOf, every, shift, slice,
> sort, unshift): the only way in which these trigger proxy code is when |this|
> is a proxy, i.e. when they are called as
> Array.prototype.theMethod.call(aProxy).
> (calling aProxy.theMethod triggers the proxy's "get" trap)
>
> - Array.prototype.concat: [[HasProperty]] is called on a built-in Array only.
the array methods are my primary concern as the HasProperty/Get combination is
used to preserve "holes" as the algorithms iterate over "arrays". On large
arrays, there can be lots of these calls.
However, I'm not sure I understand your comments about theMethod.call(aProxy)
vs. aProxy.theMethod(). In either case when theMethod is actually executed the
this value must be aProxy and the internal calls to [[HasProperty]] and [[Get]]
need to trap.
>
> - 10.2.1.2 Object environment records uses [[HasProperty]] in a number of
> places. Isn't this needed only for |with|, so only triggered by |with
> (aProxy) { ... }|? Since ES.next builds on ES5/strict, this doesn't seem to
> carry much weight.
And for global object bindings. It still has to work, consider for example, a
proxy based DOM object that is accessed from non-Harmony code. The global
object, it self, might be a Proxy.
>
> - The in operator: only calls [[HasProperty]], not followed by a call to
> [[Get]] (so it's not an instance of the "conditional [[Get]]" pattern).
>
> None of the conditional [[Get]] patterns on proxies seem particularly common
> operations, even if proxies would be widely used. Are they worth the
> complexity of changing one of the most critical and common operations in
> Javascript?
The array functions are my primary concern.
Allen
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