Task limiter

limiter.fs

open System

type IParallelLimiter =
    abstract GetToken : unit -> Async<IDisposable>

type Message= 
    | GetToken of AsyncReplyChannel<IDisposable>
    | Release

let start count =
    let agent =
        MailboxProcessor.Start(fun inbox ->
            let newToken () =
                { new IDisposable with
                    member x.Dispose () = inbox.Post Release }

            let rec loop n = async {
                    let! msg = inbox.Scan <| function
                        | GetToken _ when n = 0 -> None
                        | msg -> async.Return msg |> Some

                    return!
                        match msg with
                        | Release ->
                            loop (n + 1)
                        | GetToken port ->
                            port.Reply <| newToken ()
                            loop (n - 1)
                }
            loop count)

    { new IParallelLimiter with
        member x.GetToken () =
            agent.PostAndAsyncReply GetToken}

let limiter = start 100;;

for _ in 0..1000 do
    async {
        use! token = limiter.GetToken ()
        Console.WriteLine "Sleeping..."
        do! Async.Sleep 3000
        Console.WriteLine "About to release..."
    } |> Async.Start