x.fs

open System.Threading.Tasks
open System.Threading
open System

let criteria _ b = b = 10
let callback (i: int) = 
  Thread.Sleep(50)
  printfn "Callback %d" i

let rec compute (task: Task) state =
    async{
      do! Async.Sleep 20
      let newState = state + 1
      printfn "Loop %d" newState
      let task' = task.ContinueWith(fun _ -> callback(newState))
      if criteria state newState then return newState
      else return! compute task' newState
    }

[<EntryPoint>]
let main _ = 
  async {
    let task = Task.Factory.StartNew(fun()->printfn "Start")
    let! x = compute task 0
    printfn "Done %d" x
  } |> Async.Start
  Console.ReadLine() |> ignore
  0


// Output:

// Start
// Loop 1
// Loop 2
// Callback 1
// Loop 3
// Loop 4
// Callback 2
// Loop 5
// Loop 6
// Callback 3
// Loop 7
// Callback 4
// Loop 8
// Loop 9
// Callback 5
// Loop 10
// Done 10
// Callback 6
// Callback 7
// Callback 8
// Callback 9
// Callback 10