parallel_seq.fsx

open System

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

let asyncMockup s = async {
    let sleepFor = int (random.NextDouble() * 3000.)
    //let sleepFor = 2000
    do! Async.Sleep sleepFor
    printfn "resulted %s" s
    return sprintf "resulted %s" s
}

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

let requestMasterAsync limit urls =
    let results = Array.zeroCreate (List.length urls)
    let chunks =
        urls
        |> Seq.chunkBySize limit
        |> Seq.indexed
    async.For (chunks, fun (i, chunk) -> 
        chunk 
        |>  Seq.map asyncMockup 
        |>  Async.Parallel
        >>- Seq.iteri (fun j r -> results.[i*limit+j]<-r))
    >>- fun _ -> results

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