F# Async Timeout

Async.Timeout.fs

let timeoutNone (timeoutMs:int) (a:Async<'a>) : Async<'a option> = async {
  let! ct = Async.CancellationToken
  let res = TaskCompletionSource<_>()
  use cts = CancellationTokenSource.CreateLinkedTokenSource ct
  res.Task.ContinueWith (fun _ -> cts.Cancel ()) |> ignore
  use timer = new Timer((fun _ -> res.TrySetResult None |> ignore), null, timeoutMs, Timeout.Infinite)
  Async.StartThreadPoolWithContinuations (
    a,
    (fun a -> res.TrySetResult (Some a) |> ignore),
    (fun e -> res.TrySetException e |> ignore),
    (fun _ -> res.TrySetResult None |> ignore),
     cts.Token)
  return! res.Task |> Async.AwaitTask }