CODEWITHSUNDEEP
f#
ca9163d9
ca9163d9

Reputation: 29179

Async.Parallel doesn't run list |> Async.Parallel |> Async.Ignore |> Async.Start?

I have the following code to test https://github.com/fsprojects/FSharpx.Async/blob/master/src/FSharpx.Async/BlockingQueueAgent.fs

However, it prints Done 0 at the end. It seems the [ enqueue(); enqueue(); enqueue() ] is not run?

let ag = new BlockingQueueAgent<int option>(500)

let enqueue() = async { for i = 0 to 1000 do ag.Add (Some i) }

[ enqueue(); enqueue(); enqueue() ] |> Async.Parallel |> Async.Ignore |> Async.Start

ag.Add None

let mutable x = 0

let rec dequeue() =
    async {
        let! m = ag.AsyncGet()
        match m with
        | Some v ->
            x <- x + v
            return! dequeue()
        | None -> 
            printfn "Done %d" x
    }

dequeue() |> Async.RunSynchronously

Upvotes: 0

Views: 49

Answers (1)

Fyodor Soikin
Fyodor Soikin

Reputation: 80765

It seems that you have a race condition: ag.Add None happens before the enqueue copies even have a chance to run, because you're starting them on a background thread with Async.Start. In order to maintain the correct order of operations, make them all part of a single async workflow:

let enqueueAll = async {
    do! [ enqueue(); enqueue(); enqueue() ] |> Async.Parallel |> Async.Ignore
    ag.Add None
}

Async.Start enqueueAll

This way, ag.Add None won't be executed until Async.Parallel finishes, which will happen after all copies of enqueue() are done.

Upvotes: 1

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