It would help to have a concrete motivating example where the Promise subclass were usefully distinct from the Promise subclass' species.
On Tue, Jun 9, 2015 at 7:30 AM, C. Scott Ananian <[email protected]> wrote: > `Promise.resolve` doesn't use the species pattern any more: > https://esdiscuss.org/topic/fixing-promise-resolve > The rationale was that `resolve` is more like a constructor than a mutator. > > I don't have a strong opinion about `Promise.all`, but I think perhaps it > would be better if none of the static methods of `Promise` (Promise.all, > Promise.race, Promise.reject) used the species pattern, which is more > appropriate for instance methods (like `Promise.prototype.then`, where it > is a natural fit). > > However, consider another way of looking at it. Let's make the > fundamental implementation `Promise.prototype.all` (defined by some utility > libraries, including bluebird and prfun, and would be a good candidate for > ES7) which operates on a promise resolving to an array. It seems natural > for that to use a species pattern, since it's an instance transformation > method, like `then` and `catch`. The natural implementation of > `Promise.all` would then be: > ``` > Promise.all = function(a) { return this.resolve(a).all(); }; > ``` > This doesn't use the species pattern for the initial `resolve`, but then > does use it to implement the instance method `all`. So the result of > `Promise.all` effectively uses the species pattern. > > So: on one hand, static methods don't use @species. On the other, > `Promise.prototype.all` would naturally use the species pattern, and that > suggests a natural implementation of `Promise.all` would as well. On the > gripping hand, given a non-species-using `Promise.all`, it is easy to > define `Promise.prototype.all` (just do the initial `resolve` using the > species), but the converse isn't true. So perhaps a non-species-using > `Promise.all` would be a more useful building block. > > I'd appreciate further opinions here. My bias is against trying to make > late changes to the ES6 spec, so perhaps I'm rationalizing away the > potential problems with `Promise.all` and `Promise.race`. > --scott > > > On Tue, Jun 9, 2015 at 6:46 AM, Axel Rauschmayer <[email protected]> wrote: > >> >> https://people.mozilla.org/~jorendorff/es6-draft.html#sec-performpromiseall >> >> I’m wondering: Is it OK that PerformPromiseAll invokes `resolve()` via >> `C.resolve()` (versus `this.resolve()`) with `C` determined via the species >> pattern? >> >> Rationale: `resolve()` uses the species pattern, too (which is why >> `this.resolve()` works well). Therefore, the species pattern is used twice, >> which may lead to unexpected effects: You define the species of `this` to >> be X, but the species of X is Y. Then `Promise.all()` creates an array with >> instances of Y, not X. >> >> -- >> Dr. Axel Rauschmayer >> [email protected] >> rauschma.de >> >> >> >> >> _______________________________________________ >> 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 > > -- Cheers, --MarkM
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