+1 Allen
> On Oct 30, 2015, at 10:19 AM, Claude Pache <[email protected] > <mailto:[email protected]>> wrote: > > >> Le 29 oct. 2015 à 03:51, Boris Zbarsky <[email protected] >> <mailto:[email protected]>> a écrit : >> >> I was just implementing subclassing of Promise in Gecko, when I realized >> that given a Promise subclass MyPromise these two calls: >> >> MyPromise.race([]) >> MyPromise.all([]) >> >> will take MyPromise[@@species] into account when creating the return value, >> but these two calls: >> >> MyPromise.resolve() >> MyPromise.reject() >> >> will not; they will invoke MyPromise itself, not MyPromise[@@species]. >> >> This is because >> http://www.ecma-international.org/ecma-262/6.0/#sec-promise.all >> <http://www.ecma-international.org/ecma-262/6.0/#sec-promise.all> and >> http://www.ecma-international.org/ecma-262/6.0/#sec-promise.race >> <http://www.ecma-international.org/ecma-262/6.0/#sec-promise.race> do the >> whole @@species thing but >> http://www.ecma-international.org/ecma-262/6.0/#sec-promise.reject >> <http://www.ecma-international.org/ecma-262/6.0/#sec-promise.reject> and >> http://www.ecma-international.org/ecma-262/6.0/#sec-promise.resolve >> <http://www.ecma-international.org/ecma-262/6.0/#sec-promise.resolve> do not. >> >> Is this behavior intentional? If so, I'd really like to understand the >> reason for it. >> >> -Boris > > Relevant discussion: https://esdiscuss.org/topic/performpromiseall > <https://esdiscuss.org/topic/performpromiseall> > > As I understand, the specified behaviour is an accident of history. > Previously, `Promise.resolve()` and `Promise.reject()` used the species > pattern in order to determine the constructor to be used, just as > `Promise.all()` and `Promise.race()`. Then, at the last minute, it was > decided that it wasn’t the best semantics for `Promise.resolve()`, and it was > corrected. For `Promise.all()` and `Promise.race()`, the motivation wasn’t > stronger than the lack of time. > > Should it be corrected before @@species is widely implemented? I think so. > The arguments I give are: > > * Overall consistency in the language. Except for the two offending Promise > static methods, all uses of `@@species` in the ES2015 spec (15 uses) are for > the following pattern: Starting from one instance, one constructs a derived > object for that instance. (The effective lookup of the @@species property is > factored in the SpiecesConstructor and ArraySpeciesCreate abstract > operations.) > > * Also, in static methods like `Promise.all` and `Promise.race`, a > constructor is explicitly provided by the user: simply use it. Compare with > what is done for arrays: > > Array.from(someArray) // use the Array constructor explicitly provided. > Array.of(x, y, z) // ditto > > someArray.slice(0) // compute the constructor to be used through some > algorithm involving @@species. > [].concat(x, y, z) // ditto > > —Claude > > _______________________________________________ > es-discuss mailing list > [email protected] <mailto:[email protected]> > https://mail.mozilla.org/listinfo/es-discuss
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