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Async.FromContinuations<'T> Method (F#)

Creates an asynchronous computation that captures the current success, exception and cancellation continuations. The callback must eventually call exactly one of the given continuations.

Namespace/Module Path: Microsoft.FSharp.Control

Assembly: FSharp.Core (in FSharp.Core.dll)

// Signature:
static member FromContinuations : (('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T>

// Usage:
Async.FromContinuations (callback)

callback

Type: ('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit

The function that accepts the current success, exception, and cancellation continuations.

An asynchronous computation that provides the callback with the current continuations.

The following example illustrates how to use Async.FromContinuations to wrap an event-based asynchronous computation as an F# async.


open System
open System.ComponentModel
open System.Windows.Forms

type BackgroundWorker with
        member this.AsyncRunWorker (computation, argument : 'T, progressChangedHandler) : Async<'U> =
            let workerAsync =
                Async.FromContinuations (fun (cont, econt, ccont) ->
                            let handler = new RunWorkerCompletedEventHandler (fun sender args ->          
                                if args.Cancelled then
                                    ccont (new OperationCanceledException()) 
                                elif args.Error <> null then
                                    econt args.Error
                                else
                                    cont (args.Result :?> 'U))
                            this.WorkerSupportsCancellation <- true;
                            this.WorkerReportsProgress <- true
                            this.DoWork.AddHandler(new DoWorkEventHandler(fun sender args ->
                                args.Result <- computation(argument, this, args)))
                            this.ProgressChanged.AddHandler(progressChangedHandler)
                            this.RunWorkerCompleted.AddHandler(handler)
                            this.RunWorkerAsync(argument)
                        )

            async { 
                use! holder = Async.OnCancel(fun _ -> this.CancelAsync())
                return! workerAsync
             }

let factorial number =
    let rec fact number =
        match number with
        | value when value < 0I ->
            raise (InvalidOperationException(sprintf "Cannot compute the factorial of a negative number: %s." (value.ToString())))
        | value when value > 2000I ->
            raise (InvalidOperationException(sprintf "Input too large: %s" (value.ToString())))
        | value when value = 0I -> 1I
        | value when value = 1I -> 1I
        | number -> number * fact (number - 1I)
    fact number

// Recursive isprime function.
let isprime number =
    let rec check count =
        count > number/2 || (number % count <> 0 && check (count + 1))
    check 2

let isprimeBigInt number =
    let rec check count =
        count > number/2I || (number % count <> 0I && check (count + 1I))
    check 2I

let computeNthPrime (number, worker: BackgroundWorker, eventArgs: DoWorkEventArgs) =
     if (number < 1) then
         invalidOp <| sprintf "Invalid input for nth prime: %s." (number.ToString())
     let mutable count = 0
     let mutable num = 1I
     let isDone = false
     while (count < number && not eventArgs.Cancel ) do
         if (worker.CancellationPending) then
             eventArgs.Cancel <- true
         else
             let percentComplete = int ((float count) / (float number) * 100.0)
             worker.ReportProgress(percentComplete, num.ToString())
         num <- num + 1I
         if (num < bigint System.Int32.MaxValue) then
             while (not (isprime (int num))) do
                 num <- num + 1I
         else
             while (not (isprimeBigInt num)) do
                 num <- num + 1I
         count <- count + 1
     num

let async1 (progressBar:ProgressBar) (label:Label) value =
     let worker = new BackgroundWorker()
     label.Text <- "Computing..."
     let computation value = worker.AsyncRunWorker(computeNthPrime, value,
                                                   (fun sender eventArgs ->
                                                       label.Text <- "Scanning ... " + eventArgs.UserState.ToString()
                                                       progressBar.Value <- eventArgs.ProgressPercentage ))
     Async.StartWithContinuations(
         computation value,
         (fun result -> label.Text <- sprintf "Result: %s" (result.ToString())),
         (fun exn -> label.Text <- "Operation failed with error:" + exn.Message),
         (fun _ -> label.Text <- "Operation canceled."))

let form = new Form(Text = "Test Form", Width = 400, Height = 400)
let panel1 = new Panel(Dock = DockStyle.Fill)
panel1.DockPadding.All <- 10
let spacing = 5
let button1 = new Button(Text = "Start")
let button2 = new Button(Text = "Start Invalid", Top = button1.Height + spacing)
let button3 = new Button(Text = "Cancel", Top = 2 * (button1.Height + spacing))
let updown1 = new System.Windows.Forms.NumericUpDown(Top = 3 * (button1.Height + spacing), 
                                                     Value = 20m,
                                                     Minimum = 0m,
                                                     Maximum = 1000000m)
let label1 = new Label (Text = "", Top = 4 * (button1.Height + spacing),
                        Width = 300, Height = 2 * button1.Height)
let progressBar = new ProgressBar(Top = 6 * (button1.Height + spacing),
                                  Width = 300)
panel1.Controls.AddRange [| button1; button2; button3; updown1; label1; progressBar; |]
form.Controls.Add(panel1)
button1.Click.Add(fun args -> async1 progressBar label1 (int updown1.Value))
button2.Click.Add(fun args -> async1 progressBar label1 (int (-updown1.Value)))
button3.Click.Add(fun args -> Async.CancelDefaultToken())
Application.Run(form)


Windows 7, Windows Vista SP2, Windows XP SP3, Windows XP x64 SP2, Windows Server 2008 R2, Windows Server 2008 SP2, Windows Server 2003 SP2

F# Runtime

Supported in: 2.0, 4.0

Silverlight

Supported in: 3

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