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

manuelbarzi manuelbarzi at gmail.com
Tue Nov 26 11:21:53 UTC 2019

hi Tom

So the parallel awaits would execute before the series awaits?

not exactly. it would group the parallel awaits at the position the first
parallel await|| is sentenced.

following the demo i sent:

// NOTE async {} = asynchronous scope (it groups parallel awaits => `await||`)
// NOTE p? = function call that returns a promise (? = just and index)

async {
    const x1 = await || p1()
    const x2 = await p2(x1)
    const x3 = await || p3()
    const x10 = await p10(x2, x3)

    let x4, x5, x6

    async {
        x4 = await || p4(x1, x2)
        x5 = await || p5(x2, x3)
        x6 = await p6(x4, x5, x10)

    let x7, x8, x9

    async {
        x7 = await || p7(x4, x6)
        x8 = await p8(x6, x7)
        x9 = await || p9(x5, x6)

    await p11(x8, x9)

in the outer block, the first sentence is already parallel, ok, then all
parallel awaits in this block will group at first position, siblings of
this first one (that is p1 and p3).

in the inner blocks too (just a casualty i created the demos this way, but
could be different and still work). in the first inner block, p4 and p5,
and in the second inner block, p7 and p9.

"transpiling" it to promises, it would resolve a complex scenario like
following (the demo could be improved, of course):

// it would resolve a tree of parallel and series like following with
traditional promises

Promise.all([p1(), p3()])
    .then((x1, x3) =>
            .then(x2 =>
                p10(x2, x3)
                    .then(x10 => {
                        let x4, x5, x6

                        return Promise.all([p4(x1, x2), p5(x2, x3)])
                            .then(results => [x4, x5] = results)
                            .then(() => p6(x4, x5, x10))
                            .then(_x6 => x6 = _x6)
                            .then(() => {
                                let x7, x8, x9

                                return Promise.all([p7(x4, x6), p9(x5, x6)])
                                    .then(results => [x7, x9] = results)
                                    .then(() => p8(x6, x7))
                                    .then(_x8 => x8 = _x8)
                                    .then(() => p11(x8, x9))

What if there are series awaits before the first parallel await?

no problem, if there was one before, then in it would be chained first in
the outer promise, and following it would be the promise.all for the next
parallel group.

> What if the parallel awaits depend on the results of those series awaits?
> It reads very strangely.

that's totally valid as far as the parallel await is located after the
series awaits.
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