使用Async.FromContinuations在f＃中定制异步原语函数

Question

I'm a bit newbie with the async workflow and I think it has somethings I am not understand so well...I'm following the book Real world functional programming and in it writes the async primitive in this way

let Sleep(time) =
  Async.FromContinuations(fun (cont, econt, ccont) ->
     let tmr = new System.Timers.Timer(time, AutoReset = false)
     tmr.Elapsed.Add(fun _ -> cont())
    tmr.Start()
  );;

I've tried write my own async primitive using a very slow factorial implementation, the code is it:

let rec factorial(n : int) (mem : bigint) =
   match n with
   | 0 | 1 -> printfn "%A" mem
   | _ ->  factorial (n - 1) (mem * bigint(n))


let BigFactorial(numero,mesaje)=
    Async.FromContinuations(fun (cont,error,cancelation) ->
                                printfn "begin number: %s" mesaje
                                factorial numero 1I |>ignore
                                printfn "End number: %s ." mesaje
                                cont())

now I wrote this

 Async.RunSynchronously(async{
     printfn "Start!!..."
     do! BigFactorial(30000,"30M")
     do! BigFactorial(10000, "10M")
     printfn "End!!..."
 })

but it isn't running in async way precisely....

Start!!...
begin number: 30M
2759537246219...(A nice long number!)
end number: 30M .
begin number: 10M .
2846259680917054518906...(other nice number)
End!!...

if I execute it in parallel then works good...

let task1 = async{
   printfn "begin"
   do! BigFactorial(30000,"30")
   printfn "end..." 
}

let task2 = async{
   printfn "begin"
   do! BigFactorial(10000,"10")
   printfn "end!!..." 
}


Async.RunSynchronously(Async.Parallel[task1;task2])

begin
begin: 30 
begin
begin: 10M

//long numbers here

end....
end.....

---

but I wish know the reason (or the several reasons!) why the first code doesn't work... thanks so much I appreciate any help...

Answer 1

As with all async bindings, do! runs on the current thread, and thus your two sequential do! s run sequentially.

If you want them to run in parallel, you must say so explicitly:

async {
   printfn "Start!!..."
   let! tokenA = BigFactorial (30000, "30M") |> Async.StartChild // on new thread
   let! tokenB = BigFactorial (10000, "10M") |> Async.StartChild // on new thread
   do! tokenA // block until `BigFactorial (30000, "30M")` finishes
   do! tokenB // block until `BigFactorial (10000, "10M")` finishes
   printfn "End!!..." }
|> Async.RunSynchronously