Proposal: `await.all {...}` for parallelism

manuelbarzi manuelbarzi at gmail.com
Thu Nov 21 09:50:59 UTC 2019


AFAIK `await` can only accept an `expression` as a `Promise`, not other
thing:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await

On Thu, Nov 21, 2019 at 10:46 AM Jacob Bloom <mr.jacob.bloom at gmail.com>
wrote:

> >why not just `await` as already is, but supporting an
> >iterable / array of promises, as `Promise.all` already does
>
> `await` can already accept a non-promise, so I believe that'd be
> breaking syntax if `Array.prototype.then` is set. It also requires
> collecting the promises in an array, which is what the proposed syntax
> is trying to avoid.
>
> On Thu, Nov 21, 2019 at 2:41 AM Jacob Bloom <mr.jacob.bloom at gmail.com>
> wrote:
> >
> > >Just FYI, I previously suggested a couple things substantially more
> > >flexible than this
> >
> > Ah, thank you for bringing those proposals to my attention. I looked
> > through the archives for relevant discussions but I must've missed
> > them.
> >
> > It seems like we converged on a similar syntax for what you called
> > "merging," and the idea that there ought to be a separate syntax for
> > iteration. I don't know whether that means that this is the right
> > solution or just the most obvious one, but either way it's encouraging
> > to know that other people have the same difficulties with the current
> > syntax and are thinking about the problem.
> >
> > >from my experience, committee members are in general
> > >very hesitant to add syntax for anything that doesn't pay for
> > >itself well
> >
> > Yeah, I figured the bar would be high for new syntax. I've run into
> > the awkwardness of dealing with distinct parallel tasks several times,
> > and a few of the people I discussed it with were in the same boat, so
> > I wrote up this proposal thinking it might have a wide appeal. The
> > proposed syntax desugars via a relatively simple transformation but
> > encourages developers to reason about the problem in a completely
> > different way that I'd argue is more intuitive. Whether the committee
> > agrees and thinks it justifies a new syntax remains to be seen, but
> > either way I'm excited to see where this discussion goes (whether it
> > leads to the proposed syntax, to some other syntax, or to somewhere
> > else entirely).
> >
> > As a side note: thank you to everyone for the thoughtful questions and
> > responses, I had no idea what to expect from this thread and it's gone
> > better than I could've hoped for. Thank you for not immediately
> > shooting down a proposal that looks similar to other proposals before
> > it.
> >
> > On Wed, Nov 20, 2019 at 7:36 PM Isiah Meadows <contact at isiahmeadows.com>
> wrote:
> > >
> > > Just FYI, I previously suggested a couple things substantially more
> > > flexible than this [1] [2] (originated from this [3]), and it mostly
> > > fell flat due to being highly premature. Anything exclusive to
> > > promises is unlikely to win as library methods exist for basically all
> > > use cases and from my experience, committee members are in general
> > > very hesitant to add syntax for anything that doesn't pay for itself
> > > well. Similar questions have come up a few times in the past, too, and
> > > I've commented on two of them. [4] [5]
> > >
> > > If anything, I don't feel we know the problem space well enough, and
> > > the language lacks the primitives needed to really dig into it. (This
> > > is why I came up with my generator forking strawman. [6])
> > >
> > > [1]: https://github.com/isiahmeadows/non-linear-proposal
> > > [2]: https://github.com/isiahmeadows/lifted-pipeline-strawman
> > > [3]:
> https://esdiscuss.org/topic/observable-promise-parallel-control-flow-proposal
> > > [4]: https://esdiscuss.org/topic/stream-async-await
> > > [5]:
> https://esdiscuss.org/topic/improved-syntax-for-observable-mapping-and-subscribing
> > > [6]: https://github.com/isiahmeadows/proposal-generator-fork
> > >
> > > -----
> > >
> > > Isiah Meadows
> > > contact at isiahmeadows.com
> > > www.isiahmeadows.com
> > >
> > > On Wed, Nov 20, 2019 at 6:16 PM Jacob Bloom <mr.jacob.bloom at gmail.com>
> wrote:
> > > >
> > > > ...strike that, I misread the "but that still waits for the async
> > > > functions to complete" part. So what you're proposing is that
> > > > everything functions normally inside the curly braces, but execution
> > > > doesn't continue until all promises have resolved? So your example
> > > > would work essentially like this:
> > > >
> > > > ```javascript
> > > > const x = doSomethingAsync();
> > > > const y = doSomethingElseAsync();
> > > > await x, await y;
> > > > // all promises are resolved by now, but
> > > > // still need to use await to unbox the values
> > > > someFunction(await x, await y);
> > > > ```
> > > >
> > > > On Wed, Nov 20, 2019 at 3:28 PM Jacob Bloom <
> mr.jacob.bloom at gmail.com> wrote:
> > > > >
> > > > > >Maybe if you drop the "await" in your example:
> > > > > >
> > > > > >```javascript
> > > > > >await.all {
> > > > > >    const x = doSomethingAsync();
> > > > > >    //x is just the promise here
> > > > > >}
> > > > > >```
> > > > > >
> > > > > >...but that still waits for the async functions to complete, I
> think it would
> > > > > >cause fewer bugs and would seem to still satisfy the motivation?
> > > > >
> > > > > It doesn't seem like the `await.all` block is doing anything in
> that
> > > > > case. That code seems equivalent to this:
> > > > >
> > > > > ```javascript
> > > > > const x = doSomethingAsync();
> > > > > myFunction(await x)
> > > > > ```
> > > > >
> > > > > >```javascript
> > > > > >await.all {
> > > > > >  const x = await doSomethingAsync();
> > > > > >  //x is still undefined here!
> > > > > >}
> > > > > >```
> > > > >
> > > > > You bring up a good point about scoping and race conditions. It's a
> > > > > little tricky since the curly braces create a block scope but none
> of
> > > > > the parallel statements should be allowed to access each-other's
> > > > > variables, it's almost like each statement should have its own
> scope.
> > > > > Maybe it'd be better to have a syntax that ensures a set of curly
> > > > > braces for each parallel task? Async do-expressions could be a good
> > > > > solution (assuming they'd work kind of like an async IIFE):
> > > > >
> > > > > ```javascript
> > > > > async function initialize() {
> > > > >   let foo, bar, baz;
> > > > >   await Promise.all([
> > > > >     async do { foo = (await request('foo.json')).data },
> > > > >     async do { bar = (await request('bar.json')).data },
> > > > >     async do { baz = (await request('baz.json')).data },
> > > > >   ]);
> > > > >   render(foo, bar, baz);
> > > > > }
> > > > > ```
> > > > >
> > > > > (this is also a less drastic syntax change that piggybacks on an
> > > > > existing proposal)
> > > > >
> > > > > On Wed, Nov 20, 2019 at 11:50 AM Bergi <a.d.bergi at web.de> wrote:
> > > > > >
> > > > > > Hello!
> > > > > >
> > > > > > > This [current] structure is also just fundamentally different
> from working
> > > > > > > serially in async/await and it forces you to reason about the
> problem
> > > > > > > in a specific way. This doesn't appear to be a conscious
> decision to
> > > > > > > force good code practices
> > > > > >
> > > > > > Actually I'd argue that it is. Doing stuff concurrently *is*
> > > > > > fundamentally different from doing it serially, and should be
> reasoned
> > > > > > about every time you use it.
> > > > > >
> > > > > > kind regards,
> > > > > >  Bergi
> > > > > > _______________________________________________
> > > > > > es-discuss mailing list
> > > > > > es-discuss at mozilla.org
> > > > > > https://mail.mozilla.org/listinfo/es-discuss
> > > > _______________________________________________
> > > > es-discuss mailing list
> > > > es-discuss at mozilla.org
> > > > https://mail.mozilla.org/listinfo/es-discuss
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