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Asynchronous cancellation of a workflow

The snippet implements Async.StartCancellable method that can be used to start a given workflow and then cancel it. The cancellation of the workflow is done asynchronously, which means that the caller will wait until the workflow is actually cancelled.

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open System

/// Helper that can be used for writing CPS-style code that resumes
/// on the same thread where the operation was started.
let internal synchronize f = 
  let ctx = System.Threading.SynchronizationContext.Current 
  f (fun g ->
    let nctx = System.Threading.SynchronizationContext.Current 
    if ctx <> null && ctx <> nctx then ctx.Post((fun _ -> g()), null)
    else g() )

type Microsoft.FSharp.Control.Async with 

  /// Behaves like AwaitObservable, but calls the specified guarding function
  /// after a subscriber is registered with the observable.
  static member GuardedAwaitObservable (ev1:IObservable<'T1>) guardFunction =
    synchronize (fun f ->
      Async.FromContinuations((fun (cont,econt,ccont) -> 
        let rec finish cont value = 
          remover.Dispose()
          f (fun () -> cont value)
        and remover : IDisposable = 
          ev1.Subscribe
            ({ new IObserver<_> with
                  member x.OnNext(v) = finish cont v
                  member x.OnError(e) = finish econt e
                  member x.OnCompleted() = 
                    let msg = "Cancelling the workflow, because the Observable awaited using AwaitObservable has completed."
                    finish ccont (new System.OperationCanceledException(msg)) }) 
        guardFunction() )))

let loop = async {
  for i in 0 .. 9999 do
    printfn "Starting: %d" i
    do! Async.Sleep(1000)
    printfn "Done: %d" i }

open System.Threading

module Async = 
  let StartCancellable work = async {
    let cts = new CancellationTokenSource()
    let evt = new Event<_>()
    Async.Start(Async.TryCancelled(work, ignore >> evt.Trigger), cts.Token)
    return Async.GuardedAwaitObservable evt.Publish cts.Cancel }
  
async { let! cancelToken = Async.StartCancellable(loop)
        printfn "started"
        do! Async.Sleep(5500)
        printfn "cancelling"
        do! cancelToken
        printfn "done" }
|> Async.Start
namespace System
val internal synchronize : f:(((unit -> unit) -> unit) -> 'a) -> 'a

Full name: Script.synchronize


 Helper that can be used for writing CPS-style code that resumes
 on the same thread where the operation was started.
val f : (((unit -> unit) -> unit) -> 'a)
val ctx : Threading.SynchronizationContext
namespace System.Threading
Multiple items
type SynchronizationContext =
  new : unit -> SynchronizationContext
  member CreateCopy : unit -> SynchronizationContext
  member IsWaitNotificationRequired : unit -> bool
  member OperationCompleted : unit -> unit
  member OperationStarted : unit -> unit
  member Post : d:SendOrPostCallback * state:obj -> unit
  member Send : d:SendOrPostCallback * state:obj -> unit
  member Wait : waitHandles:nativeint[] * waitAll:bool * millisecondsTimeout:int -> int
  static member Current : SynchronizationContext
  static member SetSynchronizationContext : syncContext:SynchronizationContext -> unit

Full name: System.Threading.SynchronizationContext

--------------------
Threading.SynchronizationContext() : unit
property Threading.SynchronizationContext.Current: Threading.SynchronizationContext
val g : (unit -> unit)
val nctx : Threading.SynchronizationContext
Threading.SynchronizationContext.Post(d: Threading.SendOrPostCallback, state: obj) : unit
namespace Microsoft
namespace Microsoft.FSharp
namespace Microsoft.FSharp.Control
Multiple items
type Async<'T>

Full name: Microsoft.FSharp.Control.Async<_>

--------------------
type Async
static member AsBeginEnd : computation:('Arg -> Async<'T>) -> ('Arg * AsyncCallback * obj -> IAsyncResult) * (IAsyncResult -> 'T) * (IAsyncResult -> unit)
static member AwaitEvent : event:IEvent<'Del,'T> * ?cancelAction:(unit -> unit) -> Async<'T> (requires delegate and 'Del :> Delegate)
static member AwaitIAsyncResult : iar:IAsyncResult * ?millisecondsTimeout:int -> Async<bool>
static member AwaitTask : task:Task<'T> -> Async<'T>
static member AwaitWaitHandle : waitHandle:WaitHandle * ?millisecondsTimeout:int -> Async<bool>
static member CancelDefaultToken : unit -> unit
static member Catch : computation:Async<'T> -> Async<Choice<'T,exn>>
static member FromBeginEnd : beginAction:(AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg:'Arg1 * beginAction:('Arg1 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * beginAction:('Arg1 * 'Arg2 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * arg3:'Arg3 * beginAction:('Arg1 * 'Arg2 * 'Arg3 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromContinuations : callback:(('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T>
static member Ignore : computation:Async<'T> -> Async<unit>
static member OnCancel : interruption:(unit -> unit) -> Async<IDisposable>
static member Parallel : computations:seq<Async<'T>> -> Async<'T []>
static member RunSynchronously : computation:Async<'T> * ?timeout:int * ?cancellationToken:CancellationToken -> 'T
static member Sleep : millisecondsDueTime:int -> Async<unit>
static member Start : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions * ?cancellationToken:CancellationToken -> Task<'T>
static member StartChild : computation:Async<'T> * ?millisecondsTimeout:int -> Async<Async<'T>>
static member StartChildAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions -> Async<Task<'T>>
static member StartImmediate : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartWithContinuations : computation:Async<'T> * continuation:('T -> unit) * exceptionContinuation:(exn -> unit) * cancellationContinuation:(OperationCanceledException -> unit) * ?cancellationToken:CancellationToken -> unit
static member SwitchToContext : syncContext:SynchronizationContext -> Async<unit>
static member SwitchToNewThread : unit -> Async<unit>
static member SwitchToThreadPool : unit -> Async<unit>
static member TryCancelled : computation:Async<'T> * compensation:(OperationCanceledException -> unit) -> Async<'T>
static member CancellationToken : Async<CancellationToken>
static member DefaultCancellationToken : CancellationToken

Full name: Microsoft.FSharp.Control.Async
static member Async.GuardedAwaitObservable : ev1:IObservable<'T1> -> guardFunction:(unit -> unit) -> Async<'T1>

Full name: Script.GuardedAwaitObservable


 Behaves like AwaitObservable, but calls the specified guarding function
 after a subscriber is registered with the observable.
val ev1 : IObservable<'T1>
type IObservable<'T> =
  member Subscribe : observer:IObserver<'T> -> IDisposable

Full name: System.IObservable<_>
val guardFunction : (unit -> unit)
val f : ((unit -> unit) -> unit)
Multiple items
type Async
static member AsBeginEnd : computation:('Arg -> Async<'T>) -> ('Arg * AsyncCallback * obj -> IAsyncResult) * (IAsyncResult -> 'T) * (IAsyncResult -> unit)
static member AwaitEvent : event:IEvent<'Del,'T> * ?cancelAction:(unit -> unit) -> Async<'T> (requires delegate and 'Del :> Delegate)
static member AwaitIAsyncResult : iar:IAsyncResult * ?millisecondsTimeout:int -> Async<bool>
static member AwaitTask : task:Task<'T> -> Async<'T>
static member AwaitWaitHandle : waitHandle:WaitHandle * ?millisecondsTimeout:int -> Async<bool>
static member CancelDefaultToken : unit -> unit
static member Catch : computation:Async<'T> -> Async<Choice<'T,exn>>
static member FromBeginEnd : beginAction:(AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg:'Arg1 * beginAction:('Arg1 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * beginAction:('Arg1 * 'Arg2 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * arg3:'Arg3 * beginAction:('Arg1 * 'Arg2 * 'Arg3 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromContinuations : callback:(('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T>
static member Ignore : computation:Async<'T> -> Async<unit>
static member OnCancel : interruption:(unit -> unit) -> Async<IDisposable>
static member Parallel : computations:seq<Async<'T>> -> Async<'T []>
static member RunSynchronously : computation:Async<'T> * ?timeout:int * ?cancellationToken:CancellationToken -> 'T
static member Sleep : millisecondsDueTime:int -> Async<unit>
static member Start : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions * ?cancellationToken:CancellationToken -> Task<'T>
static member StartChild : computation:Async<'T> * ?millisecondsTimeout:int -> Async<Async<'T>>
static member StartChildAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions -> Async<Task<'T>>
static member StartImmediate : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartWithContinuations : computation:Async<'T> * continuation:('T -> unit) * exceptionContinuation:(exn -> unit) * cancellationContinuation:(OperationCanceledException -> unit) * ?cancellationToken:CancellationToken -> unit
static member SwitchToContext : syncContext:SynchronizationContext -> Async<unit>
static member SwitchToNewThread : unit -> Async<unit>
static member SwitchToThreadPool : unit -> Async<unit>
static member TryCancelled : computation:Async<'T> * compensation:(OperationCanceledException -> unit) -> Async<'T>
static member CancellationToken : Async<CancellationToken>
static member DefaultCancellationToken : CancellationToken

Full name: Microsoft.FSharp.Control.Async

--------------------
type Async<'T>

Full name: Microsoft.FSharp.Control.Async<_>
static member Async.FromContinuations : callback:(('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T>
val cont : ('T1 -> unit)
val econt : (exn -> unit)
val ccont : (OperationCanceledException -> unit)
val finish : (('a -> unit) -> 'a -> unit)
val cont : ('a -> unit)
val value : 'a
val remover : IDisposable
IDisposable.Dispose() : unit
type IDisposable =
  member Dispose : unit -> unit

Full name: System.IDisposable
member IObservable.Subscribe : callback:('T -> unit) -> IDisposable
IObservable.Subscribe(observer: IObserver<'T1>) : IDisposable
type IObserver<'T> =
  member OnCompleted : unit -> unit
  member OnError : error:Exception -> unit
  member OnNext : value:'T -> unit

Full name: System.IObserver<_>
val x : IObserver<'T1>
IObserver.OnNext(value: 'T1) : unit
val v : 'T1
IObserver.OnError(error: exn) : unit
val e : exn
IObserver.OnCompleted() : unit
val msg : string
Multiple items
type OperationCanceledException =
  inherit SystemException
  new : unit -> OperationCanceledException + 5 overloads
  member CancellationToken : CancellationToken with get, set

Full name: System.OperationCanceledException

--------------------
OperationCanceledException() : unit
OperationCanceledException(message: string) : unit
OperationCanceledException(token: Threading.CancellationToken) : unit
OperationCanceledException(message: string, innerException: exn) : unit
OperationCanceledException(message: string, token: Threading.CancellationToken) : unit
OperationCanceledException(message: string, innerException: exn, token: Threading.CancellationToken) : unit
val loop : Async<unit>

Full name: Script.loop
val async : AsyncBuilder

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.async
val i : int
val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn
static member Async.Sleep : millisecondsDueTime:int -> Async<unit>
val StartCancellable : work:Async<unit> -> Async<Async<unit>>

Full name: Script.Async.StartCancellable
val work : Async<unit>
val cts : CancellationTokenSource
Multiple items
type CancellationTokenSource =
  new : unit -> CancellationTokenSource
  member Cancel : unit -> unit + 1 overload
  member Dispose : unit -> unit
  member IsCancellationRequested : bool
  member Token : CancellationToken
  static member CreateLinkedTokenSource : [<ParamArray>] tokens:CancellationToken[] -> CancellationTokenSource + 1 overload

Full name: System.Threading.CancellationTokenSource

--------------------
CancellationTokenSource() : unit
val evt : Event<unit>
Multiple items
module Event

from Microsoft.FSharp.Control

--------------------
type Event<'T> =
  new : unit -> Event<'T>
  member Trigger : arg:'T -> unit
  member Publish : IEvent<'T>

Full name: Microsoft.FSharp.Control.Event<_>

--------------------
type Event<'Delegate,'Args (requires delegate and 'Delegate :> Delegate)> =
  new : unit -> Event<'Delegate,'Args>
  member Trigger : sender:obj * args:'Args -> unit
  member Publish : IEvent<'Delegate,'Args>

Full name: Microsoft.FSharp.Control.Event<_,_>

--------------------
new : unit -> Event<'T>

--------------------
new : unit -> Event<'Delegate,'Args>
static member Async.Start : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member Async.TryCancelled : computation:Async<'T> * compensation:(OperationCanceledException -> unit) -> Async<'T>
val ignore : value:'T -> unit

Full name: Microsoft.FSharp.Core.Operators.ignore
member Event.Trigger : arg:'T -> unit
property CancellationTokenSource.Token: CancellationToken
static member Async.GuardedAwaitObservable : ev1:IObservable<'T1> -> guardFunction:(unit -> unit) -> Async<'T1>


 Behaves like AwaitObservable, but calls the specified guarding function
 after a subscriber is registered with the observable.
property Event.Publish: IEvent<unit>
CancellationTokenSource.Cancel() : unit
CancellationTokenSource.Cancel(throwOnFirstException: bool) : unit
val cancelToken : Async<unit>
Multiple items
module Async

from Script

--------------------
type Async
static member AsBeginEnd : computation:('Arg -> Async<'T>) -> ('Arg * AsyncCallback * obj -> IAsyncResult) * (IAsyncResult -> 'T) * (IAsyncResult -> unit)
static member AwaitEvent : event:IEvent<'Del,'T> * ?cancelAction:(unit -> unit) -> Async<'T> (requires delegate and 'Del :> Delegate)
static member AwaitIAsyncResult : iar:IAsyncResult * ?millisecondsTimeout:int -> Async<bool>
static member AwaitTask : task:Task<'T> -> Async<'T>
static member AwaitWaitHandle : waitHandle:WaitHandle * ?millisecondsTimeout:int -> Async<bool>
static member CancelDefaultToken : unit -> unit
static member Catch : computation:Async<'T> -> Async<Choice<'T,exn>>
static member FromBeginEnd : beginAction:(AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg:'Arg1 * beginAction:('Arg1 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * beginAction:('Arg1 * 'Arg2 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromBeginEnd : arg1:'Arg1 * arg2:'Arg2 * arg3:'Arg3 * beginAction:('Arg1 * 'Arg2 * 'Arg3 * AsyncCallback * obj -> IAsyncResult) * endAction:(IAsyncResult -> 'T) * ?cancelAction:(unit -> unit) -> Async<'T>
static member FromContinuations : callback:(('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T>
static member Ignore : computation:Async<'T> -> Async<unit>
static member OnCancel : interruption:(unit -> unit) -> Async<IDisposable>
static member Parallel : computations:seq<Async<'T>> -> Async<'T []>
static member RunSynchronously : computation:Async<'T> * ?timeout:int * ?cancellationToken:CancellationToken -> 'T
static member Sleep : millisecondsDueTime:int -> Async<unit>
static member Start : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions * ?cancellationToken:CancellationToken -> Task<'T>
static member StartChild : computation:Async<'T> * ?millisecondsTimeout:int -> Async<Async<'T>>
static member StartChildAsTask : computation:Async<'T> * ?taskCreationOptions:TaskCreationOptions -> Async<Task<'T>>
static member StartImmediate : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit
static member StartWithContinuations : computation:Async<'T> * continuation:('T -> unit) * exceptionContinuation:(exn -> unit) * cancellationContinuation:(OperationCanceledException -> unit) * ?cancellationToken:CancellationToken -> unit
static member SwitchToContext : syncContext:SynchronizationContext -> Async<unit>
static member SwitchToNewThread : unit -> Async<unit>
static member SwitchToThreadPool : unit -> Async<unit>
static member TryCancelled : computation:Async<'T> * compensation:(OperationCanceledException -> unit) -> Async<'T>
static member CancellationToken : Async<CancellationToken>
static member DefaultCancellationToken : CancellationToken

Full name: Microsoft.FSharp.Control.Async

--------------------
type Async<'T>

Full name: Microsoft.FSharp.Control.Async<_>
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More information

Link:http://fssnip.net/d4
Posted:11 years ago
Author:Tomas Petricek
Tags: async , workflow , cancellation

This web site is created using F# and Suave web server. It is hosted on Azure and the source code is on GitHub. Contributions are welcome!

The first version of fssnip.net has been created by @tomaspetricek back in 2010. This web site is a new, open-source and contribution-friendly version.