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?

Answer 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.

• Jarak: I'm reasonably certain that simply putting Async.Parallel |> before the Async.RunSynchronously would work fine also.
• ajuch: 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: Ah, yes, you're right. Somehow I had misread the code and thought that there were multiple asyncs joined up.