F# Async Race

Async.Race.fs

let race (a:Async<'a>) (b:Async<'a>) : Async<'a * Async<'a>> = async {
  return!
    Async.FromContinuations <| fun (ok,err,cnc) ->
      let state = ref 0
      let iv = new TaskCompletionSource<_>()
      let ok a =
        if (Interlocked.CompareExchange(state, 1, 0) = 0) then
          ok (a, iv.Task |> Async.AwaitTask)
        else
          iv.SetResult a
      let err (ex:exn) =
        if (Interlocked.CompareExchange(state, 1, 0) = 0) then err ex
        else iv.SetException ex
      let cnc ex =
        if (Interlocked.CompareExchange(state, 1, 0) = 0) then cnc ex
        else iv.SetCanceled ()
    Async.StartThreadPoolWithContinuations (a, ok, err, cnc)
    Async.StartThreadPoolWithContinuations (b, ok, err, cnc) }