Note that determining desired pattern is relevant to the new DOM promises design (tracking this at https://github.com/domenic/promises-unwrapping/issues/25). Omitting a few steps not relevant to the issue under discussion, currently we do:
- Let `then` be `Get(potentialThenable, "then")`. - If `IsCallable(then)`, call `then.[[Call]](potentialThenable, (resolve, reject))`. It seems like we'd want to apply the same logic here as we do to `valueOf` etc., which is what I tried to model this after. > -----Original Message----- > From: es-discuss [mailto:[email protected]] On Behalf Of > Brendan Eich > Sent: Tuesday, September 10, 2013 07:14 > To: Tom Van Cutsem; Allen Wirfs-Brock > Cc: es-discuss > Subject: Re: [[Invoke]] and implicit method calls > > I think we should preserve double-lifting via one trap in the meta-handler. > No has-before-invoke. Allen? > > /be > > > Tom Van Cutsem <mailto:[email protected]> September 10, 2013 3:58 > AM > > +1 for refactoring to use [[Invoke]] for most of these. > > > > One reservation about the use of [[Invoke]] to call proxy traps (i.e. > > functions on handler objects): the alternative pattern of first > > probing with HasProperty destroys the "double lifting" pattern, which > > depends on the proxy only ever performing 1 type of operation (a > > property access) on its handler. > > > > (double lifting is a pattern whereby the handler object is itself a > > proxy. It allows you to write very generic proxies that only need to > > implement a single "get" trap. See > > <http://soft.vub.ac.be/~tvcutsem/invokedynamic/assets/proxies.pdf > > > <http://soft.vub.ac.be/%7Etvcutsem/invokedynamic/assets/proxies.pdf>> > > section 4.5 for details.) > > > > If we refactor to use [[HasProperty]] + [[Invoke]] to call traps, the > > double lifting pattern requires implementing both the has() trap and > > the get() trap. Not fatal, but I just want to point it out. > > > > Regards, > > Tom > > > > > > > > _______________________________________________ > > es-discuss mailing list > > [email protected] > > https://mail.mozilla.org/listinfo/es-discuss > > Jason Orendorff <mailto:[email protected]> > > September 9, 2013 4:23 PM > > > > A lot more can be found by searching for [[Call]]: > > - the call to @@toPrimitive from ToPrimitive (in 7.1.1) > > - the call to @@hasInstance from instanceofOperator (in 12.8.1) > > - the call to .toISOString from Date.prototype.toJSON (in 20.3.4.37) > > - the call to .join from Array.prototype.toString (in 22.1.3.27) > > - the call to .set from the Map constructor (in 23.1.1.1) > > - the call to .add from the Set constructor (in 23.2.1.1) > > > > and at that point I stopped looking. > > > > -j > > _______________________________________________ > > es-discuss mailing list > > [email protected] > > https://mail.mozilla.org/listinfo/es-discuss > > > > Allen Wirfs-Brock <mailto:[email protected]> > > September 9, 2013 2:50 PM > > On Sep 9, 2013, at 2:37 PM, Jason Orendorff wrote: > > > >> There are some places in the spec where we might use [[Invoke]], but > don't: > >> > >> - implicit .toString() and .valueOf() calls > >> (except Object.prototype.toLocaleString) > >> - implicit .toJSON() calls > >> - calls to Proxy handler methods > >> > >> This seems unfortunate. Proxies with default handlers will work in > >> most places, thanks to the default [[Invoke]] trap, but not in ("" + > >> proxyObject) etc. > >> > >> Is this still likely to change? > > > > I'll take a look at it. My first reaction is that this sounds like a good > > idea. > > > >> The apparent reason [[Invoke]] is not used in these places is that > >> they all do a [[Get]] first and check whether the property has a > >> callable value. > > > > Actually, I don't think even considered those uses when I added [[Invoke]]. > > > > > > Allen > > _______________________________________________ > > es-discuss mailing list > > [email protected] > > https://mail.mozilla.org/listinfo/es-discuss > > > > Jason Orendorff <mailto:[email protected]> > > September 9, 2013 2:37 PM > > There are some places in the spec where we might use [[Invoke]], but > > don't: > > > > - implicit .toString() and .valueOf() calls > > (except Object.prototype.toLocaleString) > > - implicit .toJSON() calls > > - calls to Proxy handler methods > > > > This seems unfortunate. Proxies with default handlers will work in > > most places, thanks to the default [[Invoke]] trap, but not in ("" + > > proxyObject) etc. > > > > Is this still likely to change? > > > > The apparent reason [[Invoke]] is not used in these places is that > > they all do a [[Get]] first and check whether the property has a > > callable value. But in the algorithm for yield*, in 14.4.1.2, we have > > a similar check-before-calling situation, implemented using a > > different idiom: > > > > (step 4.d.viii in the last algorithm of 14.4.1.2) > > > > Having a [[GetMethod]] hook instead of an [[Invoke]] hook might make > > it easier to use consistently everywhere. > > > > -j > > _______________________________________________ > > es-discuss mailing list > > [email protected] > > https://mail.mozilla.org/listinfo/es-discuss > > > _______________________________________________ > es-discuss mailing list > [email protected] > https://mail.mozilla.org/listinfo/es-discuss _______________________________________________ es-discuss mailing list [email protected] https://mail.mozilla.org/listinfo/es-discuss
