async work

async_work.fsx

open System.Threading
open System

let random = Random(int DateTime.UtcNow.Ticks)

let asyncMockup s = async {
    let sleepFor = int (random.NextDouble() * 3000.)
    do! Async.Sleep sleepFor
    return sprintf "resulted %s" s
}

let inline (>>=) x f = async.Bind(x, f)
let inline (>>-) x f = async.Bind(x, f >> async.Return)

let requestMasterAsync limit urls = 
    let results = Array.zeroCreate (List.length urls)
    let mutable running =  0
    let mutable order   = -1
    let completed = Event<_>()

    let rec loop = function
        | [] when running = 0 -> results |> async.Return
        | [] -> 
            Async.AwaitEvent completed.Publish
            >>= fun _ -> loop []
        | work::left when running < limit ->
            Interlocked.Increment &running |> ignore
            let orderId = Interlocked.Increment &order
            work |> asyncMockup
            >>- fun result ->
                results.[orderId] <- result
                Interlocked.Decrement &running |> ignore
                completed.Trigger()
            |> Async.Start
            loop left
        | left ->
            Async.AwaitEvent completed.Publish
            >>= fun _ -> loop left
    loop urls

let results =
    List.init 20 string
    |> requestMasterAsync 3
    |> Async.RunSynchronously