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An implementation of Async.Choice

This is an implementation of the Async.Choice combinator that composes a sequence of workflows into one whose result is that of the first workflow that returns a *valid* result. You can think of it as a nondeterministic version of (Seq.tryPick id). This implementation draws ideas from Tomas Petricek's choice implementation in http://fssnip.net/6D. UPDATE (11/2015): Following the discussion in https://fslang.uservoice.com/forums/245727-f-language/suggestions/10575069-add-async-choice-to-fsharp-core I have updated the snippet.

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

type Async with
    static member Choice(tasks : Async<'T option> seq) : Async<'T option> = async {
        match Seq.toArray tasks with
        | [||] -> return None
        | [|t|] -> return! t
        | tasks ->

        let! t = Async.CancellationToken
        return! Async.FromContinuations <|
            fun (sc,ec,cc) ->
                let noneCount = ref 0
                let exnCount = ref 0
                let innerCts = CancellationTokenSource.CreateLinkedTokenSource t

                let scont (result : 'T option) =
                    match result with
                    | Some _ when Interlocked.Increment exnCount = 1 -> innerCts.Cancel() ; sc result
                    | None when Interlocked.Increment noneCount = tasks.Length -> sc None
                    | _ -> ()

                let econt (exn : exn) =
                    if Interlocked.Increment exnCount = 1 then 
                        innerCts.Cancel() ; ec exn

                let ccont (exn : OperationCanceledException) =
                    if Interlocked.Increment exnCount = 1 then
                        innerCts.Cancel(); cc exn

                for task in tasks do
                    ignore <| Task.Factory.StartNew(fun () -> Async.StartWithContinuations(task, scont, econt, ccont, innerCts.Token))
    }

// example 1

let delay interval result =
    async {
        do! Async.Sleep interval
        return! async {
            printfn "returning %A after %d ms." result interval
            return result }
    }

[ delay 100 None ; delay 1000 (Some 1) ; delay 500 (Some 2) ] |> Async.Choice |> Async.RunSynchronously
                    
// example 2

/// parallel existential combinator
let exists (f : 'T -> Async<bool>) (ts : seq<'T>) : Async<bool> =
    let wrapper t = async { let! r = f t in return if r then Some () else None }
                
    async {
        let! r = ts |> Seq.map wrapper |> Async.Choice
        return r.IsSome
    }

#time
[1..500] |> Seq.exists (fun i -> Threading.Thread.Sleep 10; i = 500)
[1..500] |> exists (fun i -> async { let! _ = Async.Sleep 10 in return i = 500 }) |> Async.RunSynchronously
namespace System
namespace System.Threading
namespace System.Threading.Tasks
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<_>
Multiple items
static member Async.Choice : tasks:seq<Async<'T option>> -> Async<'T option>

Full name: Script.Choice

--------------------
type Choice<'T1,'T2> =
  | Choice1Of2 of 'T1
  | Choice2Of2 of 'T2

Full name: Microsoft.FSharp.Core.Choice<_,_>

--------------------
type Choice<'T1,'T2,'T3> =
  | Choice1Of3 of 'T1
  | Choice2Of3 of 'T2
  | Choice3Of3 of 'T3

Full name: Microsoft.FSharp.Core.Choice<_,_,_>

--------------------
type Choice<'T1,'T2,'T3,'T4> =
  | Choice1Of4 of 'T1
  | Choice2Of4 of 'T2
  | Choice3Of4 of 'T3
  | Choice4Of4 of 'T4

Full name: Microsoft.FSharp.Core.Choice<_,_,_,_>

--------------------
type Choice<'T1,'T2,'T3,'T4,'T5> =
  | Choice1Of5 of 'T1
  | Choice2Of5 of 'T2
  | Choice3Of5 of 'T3
  | Choice4Of5 of 'T4
  | Choice5Of5 of 'T5

Full name: Microsoft.FSharp.Core.Choice<_,_,_,_,_>

--------------------
type Choice<'T1,'T2,'T3,'T4,'T5,'T6> =
  | Choice1Of6 of 'T1
  | Choice2Of6 of 'T2
  | Choice3Of6 of 'T3
  | Choice4Of6 of 'T4
  | Choice5Of6 of 'T5
  | Choice6Of6 of 'T6

Full name: Microsoft.FSharp.Core.Choice<_,_,_,_,_,_>

--------------------
type Choice<'T1,'T2,'T3,'T4,'T5,'T6,'T7> =
  | Choice1Of7 of 'T1
  | Choice2Of7 of 'T2
  | Choice3Of7 of 'T3
  | Choice4Of7 of 'T4
  | Choice5Of7 of 'T5
  | Choice6Of7 of 'T6
  | Choice7Of7 of 'T7

Full name: Microsoft.FSharp.Core.Choice<_,_,_,_,_,_,_>
val tasks : seq<Async<'T option>>
type 'T option = Option<'T>

Full name: Microsoft.FSharp.Core.option<_>
Multiple items
val seq : sequence:seq<'T> -> seq<'T>

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

--------------------
type seq<'T> = Collections.Generic.IEnumerable<'T>

Full name: Microsoft.FSharp.Collections.seq<_>
val async : AsyncBuilder

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.async
module Seq

from Microsoft.FSharp.Collections
val toArray : source:seq<'T> -> 'T []

Full name: Microsoft.FSharp.Collections.Seq.toArray
union case Option.None: Option<'T>
val t : Async<'T option>
val tasks : Async<'T option> []
val t : CancellationToken
property Async.CancellationToken: Async<CancellationToken>
static member Async.FromContinuations : callback:(('T -> unit) * (exn -> unit) * (OperationCanceledException -> unit) -> unit) -> Async<'T>
val sc : ('T option -> unit)
val ec : (exn -> unit)
val cc : (OperationCanceledException -> unit)
val noneCount : int 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<_>
val exnCount : int ref
val innerCts : 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
CancellationTokenSource.CreateLinkedTokenSource([<ParamArray>] tokens: CancellationToken []) : CancellationTokenSource
CancellationTokenSource.CreateLinkedTokenSource(token1: CancellationToken, token2: CancellationToken) : CancellationTokenSource
val scont : ('T option -> unit)
val result : 'T option
union case Option.Some: Value: 'T -> Option<'T>
type Interlocked =
  static member Add : location1:int * value:int -> int + 1 overload
  static member CompareExchange : location1:int * value:int * comparand:int -> int + 6 overloads
  static member Decrement : location:int -> int + 1 overload
  static member Exchange : location1:int * value:int -> int + 6 overloads
  static member Increment : location:int -> int + 1 overload
  static member Read : location:int64 -> int64

Full name: System.Threading.Interlocked
Interlocked.Increment(location: byref<int64>) : int64
Interlocked.Increment(location: byref<int>) : int
CancellationTokenSource.Cancel() : unit
CancellationTokenSource.Cancel(throwOnFirstException: bool) : unit
property Array.Length: int
val econt : (exn -> unit)
Multiple items
val exn : exn

--------------------
type exn = Exception

Full name: Microsoft.FSharp.Core.exn
val ccont : (OperationCanceledException -> unit)
Multiple items
val exn : OperationCanceledException

--------------------
type exn = Exception

Full name: Microsoft.FSharp.Core.exn
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: CancellationToken) : unit
OperationCanceledException(message: string, innerException: exn) : unit
OperationCanceledException(message: string, token: CancellationToken) : unit
OperationCanceledException(message: string, innerException: exn, token: CancellationToken) : unit
val task : Async<'T option>
val ignore : value:'T -> unit

Full name: Microsoft.FSharp.Core.Operators.ignore
Multiple items
type Task =
  new : action:Action -> Task + 7 overloads
  member AsyncState : obj
  member ContinueWith : continuationAction:Action<Task> -> Task + 9 overloads
  member CreationOptions : TaskCreationOptions
  member Dispose : unit -> unit
  member Exception : AggregateException
  member Id : int
  member IsCanceled : bool
  member IsCompleted : bool
  member IsFaulted : bool
  ...

Full name: System.Threading.Tasks.Task

--------------------
type Task<'TResult> =
  inherit Task
  new : function:Func<'TResult> -> Task<'TResult> + 7 overloads
  member ContinueWith : continuationAction:Action<Task<'TResult>> -> Task + 9 overloads
  member Result : 'TResult with get, set
  static member Factory : TaskFactory<'TResult>

Full name: System.Threading.Tasks.Task<_>

--------------------
Task(action: Action) : unit
Task(action: Action, cancellationToken: CancellationToken) : unit
Task(action: Action, creationOptions: TaskCreationOptions) : unit
Task(action: Action<obj>, state: obj) : unit
Task(action: Action, cancellationToken: CancellationToken, creationOptions: TaskCreationOptions) : unit
Task(action: Action<obj>, state: obj, cancellationToken: CancellationToken) : unit
Task(action: Action<obj>, state: obj, creationOptions: TaskCreationOptions) : unit
Task(action: Action<obj>, state: obj, cancellationToken: CancellationToken, creationOptions: TaskCreationOptions) : unit

--------------------
Task(function: Func<'TResult>) : unit
Task(function: Func<'TResult>, cancellationToken: CancellationToken) : unit
Task(function: Func<'TResult>, creationOptions: TaskCreationOptions) : unit
Task(function: Func<obj,'TResult>, state: obj) : unit
Task(function: Func<'TResult>, cancellationToken: CancellationToken, creationOptions: TaskCreationOptions) : unit
Task(function: Func<obj,'TResult>, state: obj, cancellationToken: CancellationToken) : unit
Task(function: Func<obj,'TResult>, state: obj, creationOptions: TaskCreationOptions) : unit
Task(function: Func<obj,'TResult>, state: obj, cancellationToken: CancellationToken, creationOptions: TaskCreationOptions) : unit
Multiple items
property Task.Factory: TaskFactory

--------------------
property Task.Factory: TaskFactory<'TResult>
Multiple items
TaskFactory.StartNew<'TResult>(function: Func<'TResult>) : Task<'TResult>
   (+0 other overloads)
TaskFactory.StartNew(action: Action) : Task
   (+0 other overloads)
TaskFactory.StartNew<'TResult>(function: Func<obj,'TResult>, state: obj) : Task<'TResult>
   (+0 other overloads)
TaskFactory.StartNew<'TResult>(function: Func<'TResult>, creationOptions: TaskCreationOptions) : Task<'TResult>
   (+0 other overloads)
TaskFactory.StartNew<'TResult>(function: Func<'TResult>, cancellationToken: CancellationToken) : Task<'TResult>
   (+0 other overloads)
TaskFactory.StartNew(action: Action<obj>, state: obj) : Task
   (+0 other overloads)
TaskFactory.StartNew(action: Action, creationOptions: TaskCreationOptions) : Task
   (+0 other overloads)
TaskFactory.StartNew(action: Action, cancellationToken: CancellationToken) : Task
   (+0 other overloads)
TaskFactory.StartNew<'TResult>(function: Func<obj,'TResult>, state: obj, creationOptions: TaskCreationOptions) : Task<'TResult>
   (+0 other overloads)
TaskFactory.StartNew<'TResult>(function: Func<obj,'TResult>, state: obj, cancellationToken: CancellationToken) : Task<'TResult>
   (+0 other overloads)

--------------------
TaskFactory.StartNew(function: Func<'TResult>) : Task<'TResult>
TaskFactory.StartNew(function: Func<obj,'TResult>, state: obj) : Task<'TResult>
TaskFactory.StartNew(function: Func<'TResult>, creationOptions: TaskCreationOptions) : Task<'TResult>
TaskFactory.StartNew(function: Func<'TResult>, cancellationToken: CancellationToken) : Task<'TResult>
TaskFactory.StartNew(function: Func<obj,'TResult>, state: obj, creationOptions: TaskCreationOptions) : Task<'TResult>
TaskFactory.StartNew(function: Func<obj,'TResult>, state: obj, cancellationToken: CancellationToken) : Task<'TResult>
TaskFactory.StartNew(function: Func<'TResult>, cancellationToken: CancellationToken, creationOptions: TaskCreationOptions, scheduler: TaskScheduler) : Task<'TResult>
TaskFactory.StartNew(function: Func<obj,'TResult>, state: obj, cancellationToken: CancellationToken, creationOptions: TaskCreationOptions, scheduler: TaskScheduler) : Task<'TResult>
static member Async.StartWithContinuations : computation:Async<'T> * continuation:('T -> unit) * exceptionContinuation:(exn -> unit) * cancellationContinuation:(OperationCanceledException -> unit) * ?cancellationToken:CancellationToken -> unit
property CancellationTokenSource.Token: CancellationToken
val delay : interval:int -> result:'a -> Async<'a>

Full name: Script.delay
val interval : int
val result : 'a
static member Async.Sleep : millisecondsDueTime:int -> Async<unit>
val printfn : format:Printf.TextWriterFormat<'T> -> 'T

Full name: Microsoft.FSharp.Core.ExtraTopLevelOperators.printfn
static member Async.Choice : tasks:seq<Async<'T option>> -> Async<'T option>
static member Async.RunSynchronously : computation:Async<'T> * ?timeout:int * ?cancellationToken:CancellationToken -> 'T
val exists : f:('T -> Async<bool>) -> ts:seq<'T> -> Async<bool>

Full name: Script.exists


 parallel existential combinator
val f : ('T -> Async<bool>)
type bool = Boolean

Full name: Microsoft.FSharp.Core.bool
val ts : seq<'T>
val wrapper : ('T -> Async<unit option>)
val t : 'T
val r : bool
val r : unit option
val map : mapping:('T -> 'U) -> source:seq<'T> -> seq<'U>

Full name: Microsoft.FSharp.Collections.Seq.map
property Option.IsSome: bool
val exists : predicate:('T -> bool) -> source:seq<'T> -> bool

Full name: Microsoft.FSharp.Collections.Seq.exists
val i : int
Multiple items
type Thread =
  inherit CriticalFinalizerObject
  new : start:ThreadStart -> Thread + 3 overloads
  member Abort : unit -> unit + 1 overload
  member ApartmentState : ApartmentState with get, set
  member CurrentCulture : CultureInfo with get, set
  member CurrentUICulture : CultureInfo with get, set
  member DisableComObjectEagerCleanup : unit -> unit
  member ExecutionContext : ExecutionContext
  member GetApartmentState : unit -> ApartmentState
  member GetCompressedStack : unit -> CompressedStack
  member GetHashCode : unit -> int
  ...

Full name: System.Threading.Thread

--------------------
Thread(start: ThreadStart) : unit
Thread(start: ParameterizedThreadStart) : unit
Thread(start: ThreadStart, maxStackSize: int) : unit
Thread(start: ParameterizedThreadStart, maxStackSize: int) : unit
Thread.Sleep(timeout: TimeSpan) : unit
Thread.Sleep(millisecondsTimeout: int) : unit

More information

Link:http://fssnip.net/dO
Posted:12 years ago
Author:Eirik Tsarpalis
Tags: async , choice , 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.