Will array of async values be possible?

Evolution Pitches
1

The async let bindings are quite handy when you have one or two, but it is not possible to have an array of them.

I wish I could to this:

let fv: [() async -> Int]  = [f, g, h]
let uv: [async Int] = [] // <--- not possible

for f in fv {
    async let u = f ()
    uv.append (u)
}

for u in uv {
    await print (u)
}

There is a workaround, but it involves explicitly spawning tasks.

let fv: [() async -> Int]   = [f, g, h]
var tv: [Task <Int, Never>] = []

for f in fv {
    let t = Task {
        await f ()
    }
    tv.append (t)
}

for t in tv {
    await print (t.value)
}

I am wondering if this will be possible one day?

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2

In theory it would be possible I suppose but the resulting array would have to be nonescapable and it might have surprising semantics since removing an element form such array cannot remove the value from the scope — we still MUST await on it. So it could be a bit weird to use in practice… implementation might be tricky but doable.

So the question is if this is solving enough problems to warrant the complexity?

Right now I think we have bigger fish to fry, in making task groups easier to use etc. but maybe someday

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3

I don't know about the standard library… but you might be able to use AsyncSyncSequence in a creative way to unblock your use-case.

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4

Isn’t this just a task group? (Potentially with some sort of wrapper that buffers values so that they come back in order)

let fv: [() async -> Int]  = [f, g, h]
await withTaskGroup(of: Int.self) { group in
    for f in fv {
        group.addTask { await f() }
    }

     for u in group {
        await print (u)
    }
}
1 Like
5

Not an answer to your question (whether it will become a language feature) but the pattern potentially can become a part of the standard library. I have a utility struct called Zip in my library that serves this purpose in case of homogenous actions, and a family of functions zip(...) for non-homogenous ones, see AsyncMux/AsyncMux/Sources/Zip.swift at main · crontab/AsyncMux · GitHub

Example use:

let fv: [() async -> Int]  = [f, g, h]

let results = await Zip(actions: fv)
    .result

results.forEach { print($0) }

(although I only have a throwing version of the above, but you get the idea)

1 Like
6

This sounds like just a parallel async map. I agree we should have an API to do that, implemented on top of task groups.

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