2010/10/20 Dmitry A. Soshnikov <[email protected]> > (I answer both -- Brendan's and Mark's letters for convenience in here). > > > On 20.10.2010 21:06, Brendan Eich wrote: > > On Oct 20, 2010, at 9:16 AM, Mark S. Miller wrote: > > On Wed, Oct 20, 2010 at 7:10 AM, Dmitry A. Soshnikov < > [email protected]> wrote: > >> OK, I'd like nevertheless to continue the discussion with possible >> decisions. >> >> Here is a description of how [[Get]] method can work of a trapping proxy >> object (I took the basis from the proxy semantics page -- >> http://wiki.ecmascript.org/doku.php?id=harmony:proxies_semantics#semantics_of_proxies >> ): >> >> [[Get]] (P) >> >> When the [[Get]] internal method of a trapping proxy O is called with >> property name P, the following steps are taken: >> >> 1. Let handler be the value of the [[Handler]] internal property of O. >> 2. Let get be the result of calling the [[Get]] internal method of >> handler with argument “get”. >> 3. If get is undefined, return the result of performing the default >> behavior, by consulting handler’s “getPropertyDescriptor” trap. >> 4. If IsCallable(get) is false, throw a TypeError exception. >> 5. Let getResult be the result of calling the [[Call]] internal method >> of get providing handler as the this value, O as the first argument and P as >> the second argument. >> 6. if getResult is undefined and [[HasProperty]](P) >> > > [[HasProperty]] takes O as its receiver (this) parameter, so doesn't this > break double-lifting? ( > http://wiki.ecmascript.org/doku.php?id=harmony:proxies#a_simple_membrane) > > > > Hm.. Actually, I don't completely understand what is "double lifting" (a > proxied proxy?) as well as the whole membrane example (to many > wrap-wrap-wrap and a every time a creating of a new function), but I think I > should carefully read Mark's "markm-thesis.pdf" paper. Though, your example > from slides is simpler than on the strawman page (but there too -- wrap > calls wrapFunction, which in turn calls wrap again -- not so easy to get > from the first glance, but I'll look closer). > > However, if someone can clarify it (the main and basic idea in "two words" > and regarding "double lifting" too) I'll be very thankful, 'cause it's > interesting. > > Anyway, is there an alternative how [[HasProperty]] can work correctly? I > just though, that it's called by the "has" hook of a handler, which in turn > can "lie" of course (i.e. return true when a property does not exist -- in > such a case, noSuchMethod won't be called). >
I think Dmitry is right: calling [[HasProperty]] on a proxy object should trigger its "has()" trap in the normal way, which should work with double lifting. In short, double lifting is a trick where the handler object of a proxy is itself a handler. See section 4.5 of the paper about proxies that MarkM and I presented at the Dynamic Languages Symposium last week (here's a link to the submitted, non-final, version: < http://soft.vub.ac.be/~tvcutsem/proxies/assets/proxies.pdf>) In the paper, we use the term "meta-level shifting" instead of "double lifting", but it's the same concept. Cheers, Tom
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