Run function in parallel in async workflow?

Question

The following code

let doWork n = async {
    [1..5]
    |> Seq.iter(fun i ->
        System.Threading.Thread.Sleep 1000 // Simulate working
        printfn "%s runs %i seconds" n i
        )
}

let f1 = async {
    do! doWork "f1"
    return 1
}

async {
    let a = f1
    do! doWork "main"
    let! i = a
} |> Async.RunSynchronously

And it prints the following result. It shows that the two doWork calls run sequentially.

main runs 1 seconds
main runs 2 seconds
main runs 3 seconds
main runs 4 seconds
main runs 5 seconds
f1 runs 1 seconds
f1 runs 2 seconds
f1 runs 3 seconds
f1 runs 4 seconds
f1 runs 5 seconds

However, I want the doWork called by f1 and the main code be running in parallel?

ajuch · Accepted Answer · 2019-01-10 10:24:18Z

1

Try it like this:

let computation () = async {
  printfn "Start work"
  do! Async.Sleep(100)
  printfn "Finished work"
}

let s = seq {
  for i in 1 .. 10 do
    let c = computation()
    yield c
}

s |> Async.Parallel |> Async.Ignore |> Async.Start

Async.Parallel converts the Async<unit> list to Async<unit []> which can be run in parallel then.

edited Jan 10, 2019 at 10:24

answered Jan 10, 2019 at 8:26

ajuch

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3 Comments

Jarak Over a year ago

I'm reasonably certain that simply putting Async.Parallel |> before the Async.RunSynchronously would work fine also.

ajuch Over a year ago

If you're refering to the original question's code, I'd say no... Async.Parallel's signature is seq<Async<'T>> -> Async<'T []>. It cannot parallelize a single async, only a sequence of asyncs.

Jarak Over a year ago

Ah, yes, you're right. Somehow I had misread the code and thought that there were multiple asyncs joined up.

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