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Async.Choose operation

Non-deterministic choice operation for F# asynchronous workflows - creates a workflow that returns the result of one of two asynchronous workflows, depending on which completes first. (The other workflow is not cancelled.)

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open System

type Microsoft.FSharp.Control.Async with
  /// Creates an asynchronous workflow that non-deterministically returns the 
  /// result of one of the two specified workflows (the one that completes
  /// first). This is similar to Task.WaitAny.
  static member Choose(a, b) : Async<'T> = 
    Async.FromContinuations(fun (cont, econt, ccont) ->
      // Results from the two 
      let result1 = ref (Choice1Of3())
      let result2 = ref (Choice1Of3())
      let handled = ref false
      let lockObj = new obj()
      let synchronized f = lock lockObj f

      // Called when one of the workflows completes
      let complete () = 
        let op =
          synchronized (fun () ->
            // If we already handled result (and called continuation)
            // then ignore. Otherwise, if the computation succeeds, then
            // run the continuation and mark state as handled.
            // Only throw if both workflows failed.
            match !handled, !result1, !result2 with 
            | true, _, _ -> ignore
            | false, (Choice2Of3 value), _ 
            | false, _, (Choice2Of3 value) -> 
                handled := true
                (fun () -> cont value)
            | false, Choice3Of3 e1, Choice3Of3 e2 -> 
                handled := true; 
                (fun () -> 
                    econt (new AggregateException
                                ("Both clauses of a choice failed.", [| e1; e2 |])))
            | false, Choice1Of3 _, Choice3Of3 _ 
            | false, Choice3Of3 _, Choice1Of3 _ 
            | false, Choice1Of3 _, Choice1Of3 _ -> ignore )
        op() 

      // Run a workflow and write result (or exception to a ref cell
      let run resCell workflow = async {
        try
          let! res = workflow
          synchronized (fun () -> resCell := Choice2Of3 res)
        with e ->
          synchronized (fun () -> resCell := Choice3Of3 e)
        complete() }

      // Start both work items in thread pool
      Async.Start(run result1 a)
      Async.Start(run result2 b) )
namespace System
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.Choose : a:Async<'T> * b:Async<'T> -> Async<'T>

Full name: Script.Choose


 Creates an asynchronous workflow that non-deterministically returns the
 result of one of the two specified workflows (the one that completes
 first). This is similar to Task.WaitAny.
val a : Async<'T>
val b : Async<'T>
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 : ('T -> unit)
val econt : (exn -> unit)
val ccont : (OperationCanceledException -> unit)
val result1 : Choice<unit,'T,exn> ref
Multiple items
val ref : value:'T -> 'T ref

Full name: Microsoft.FSharp.Core.Operators.ref

--------------------
type 'T ref = Ref<'T>

Full name: Microsoft.FSharp.Core.ref<_>
union case Choice.Choice1Of3: 'T1 -> Choice<'T1,'T2,'T3>
val result2 : Choice<unit,'T,exn> ref
val handled : bool ref
val lockObj : obj
type obj = Object

Full name: Microsoft.FSharp.Core.obj
val synchronized : ((unit -> 'a) -> 'a)
val f : (unit -> 'a)
val lock : lockObject:'Lock -> action:(unit -> 'T) -> 'T (requires reference type)

Full name: Microsoft.FSharp.Core.Operators.lock
val complete : (unit -> unit)
val op : (unit -> unit)
val ignore : value:'T -> unit

Full name: Microsoft.FSharp.Core.Operators.ignore
union case Choice.Choice2Of3: 'T2 -> Choice<'T1,'T2,'T3>
val value : 'T
union case Choice.Choice3Of3: 'T3 -> Choice<'T1,'T2,'T3>
val e1 : exn
val e2 : exn
Multiple items
type AggregateException =
  inherit Exception
  new : unit -> AggregateException + 6 overloads
  member Flatten : unit -> AggregateException
  member GetBaseException : unit -> Exception
  member GetObjectData : info:SerializationInfo * context:StreamingContext -> unit
  member Handle : predicate:Func<Exception, bool> -> unit
  member InnerExceptions : ReadOnlyCollection<Exception>
  member ToString : unit -> string

Full name: System.AggregateException

--------------------
AggregateException() : unit
AggregateException(message: string) : unit
AggregateException(innerExceptions: Collections.Generic.IEnumerable<exn>) : unit
AggregateException([<ParamArray>] innerExceptions: exn []) : unit
AggregateException(message: string, innerException: exn) : unit
AggregateException(message: string, innerExceptions: Collections.Generic.IEnumerable<exn>) : unit
AggregateException(message: string, [<ParamArray>] innerExceptions: exn []) : unit
val run : (Choice<'a,'b,exn> ref -> Async<'b> -> Async<unit>)
val resCell : Choice<'a,'b,exn> ref
val workflow : Async<'b>
val async : AsyncBuilder

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.async
val res : 'b
val e : exn
static member Async.Start : computation:Async<unit> * ?cancellationToken:Threading.CancellationToken -> unit
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More information

Link:http://fssnip.net/8x
Posted:6 years ago
Author:Tomas Petricek
Tags: async , asynchronous workflows , non-determinism , choice