Fixing `Promise.resolve()`

C. Scott Ananian ecmascript at cscott.net
Wed Jun 10 15:39:26 UTC 2015


@Domenic: please see the previous discussion at
https://esdiscuss.org/topic/subclassing-es6-objects-with-es5-syntax#content-50
since there is much more discussion back-and-forth there.

And you are correct about Promise.reject; Allen is proposing to remove the
@@species from that method as well.

The changes to `Promise.resolve` are needed to make other use cases work
properly (see that other thread).  It seems that your particular use case
could be better handled by overriding `LabelledPromise.resolve` to take an
additional "label" argument.

Your understanding of @@species seems to be different from the use cases
envisioned by the smalltalk concept named species, although it's an
interesting idea in its own right.  You should probably call it something
like "custom constructors", though, to differentiate it from the smalltalk
"species", which is used in the smalltalk world to describe two related
things (1) the "generic type" of the object, for type tests, and (2) the
"derived type" which instance-transformation methods should yield.[*]  In
smalltalk (as I understand it) custom constructors are not part of the
design.
  --scott

[*] ""Answer the preferred class for reconstructing the receiver. For
example,  collections create new collections whenever enumeration messages
such as collect: or select: are invoked. The new kind of collection is
determined by the species of the original collection. Species and class are
not always the  same. For example, the species of Interval is Array."
http://forum.world.st/species-td3374278.html

On Wed, Jun 10, 2015 at 11:22 AM, Domenic Denicola <d at domenic.me> wrote:

> Allen, that change seems wrong. I thought we were only changing the
> IsPromise steps. The actual construction should still go through species.
> If nothing else, it should do so for consistency with reject.
>
> The motivation of @@species, as I understood it, was to allow alternate
> subclass constructor signatures (originally for Array and TypedArray, but
> Promise can benefit as well). It’s understandable to not involve @@species
> when doing weak type-tests. But any time you construct a promise instance,
> you should go through @@species, instead of the constructor directly.
>
> Some example usages:
>
> - Creating a LabeledPromise subclass (for better debugging, like RSVP's
> promises + Ember promise inspector) whose constructor signature is `new
> LabeledPromise(label, executor)`
> - Creating a SaneArray subclass whose constructor signature is `new
> SaneArray(...elements)` without the special-case for a single argument.
> - A more complicated usage in a proposed
> Element/HTMLElement/MyCustomElement hierarchy [1], to allow custom elements
> to have custom constructor signatures but still work well with various
> element-creating parts of the platform.
>
> The LabeledPromise case will, as currently specified, work great.
> LabeledPromise has a custom `LabeledPromise[Symbol.species](executor)`
> which basically does `return new this("<derived promise>", executor)`. Then
> it doesn't have to override `.resolve`, `.reject`, `.all`, or `.then`.
> However, with your change, `.resolve` will no longer work correctly, even
> though `.reject` will.
>
> However, the SaneArray case actually will only work for the instance
> methods, which use ArraySpeciesCreate. In contrast, Array.of and Array.from
> use Construct(C, <<len>>). That seems like a bug in the spec?
>
> [1]:
> https://github.com/domenic/element-constructors/blob/cdfe5a1d865e25d265074418df7918fda959e403/element-constructors.js#L101-L107
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