async.fs

let retrieveNumber = async {
    do! Async.Sleep 4000
    return 0
};;

let anotherJob = async {
    do! Async.Sleep 500
};;

// start retrieving number;
// do anotherJob meanwhile (in parallel);
// return the number.
let doWork =
  let numberPromise = retrieveNumber |> Async.StartAsTask
  anotherJob |> Async.RunSynchronously
  numberPromise |> Async.AwaitTask |> Async.RunSynchronously;;