An implementation of Async.Choice

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open System.Threading

type Async with
    static member Choice(tasks : Async<'T option> seq) : Async<'T option> =
        Async.FromContinuations(fun (cont,econt,_) ->
            let tasks = Seq.toArray tasks
            if tasks.Length = 0 then cont None else

            let cts = new CancellationTokenSource()
            let count = ref tasks.Length

            let wrap task =
                async {
                    try
                        let! res = task
                        match res with
                        | None -> 
                            lock count (fun () ->
                                decr count
                                if !count = 0 then cont None)
                        | Some r ->
                            lock count (fun () ->
                                cont (Some r)
                                cts.Cancel())
                    with e -> 
                        lock count (fun () ->
                            econt e
                            cts.Cancel())
                }

            for task in tasks do
                Async.Start(wrap task, cts.Token)
        )

// example 1    

let task delay result =
    async {
        do! Async.Sleep delay
        do! async.Zero () // force ct check here
        do printfn "returning %A after %d ms" result delay
        return result
    }

Async.Choice
    [ 
        task 100 None 
        task 200 (Some 1) 
        task 500 (Some 2) 
        task 1000 (Some 3)
    ] |> Async.RunSynchronously

// example 2

/// parallel existential combinator
let exists (f : 'T -> Async<bool>) inputs =
    let wrapper t =
        async {
            let! res = f t
            if res then return Some ()
            else return None
        }

    async {
        let! res = 
                inputs
                |> Seq.map wrapper
                |> Async.Choice

        return res.IsSome
    }

namespace System

namespace System.Threading

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> = System.Collections.Generic.IEnumerable<'T>

Full name: Microsoft.FSharp.Collections.seq<_>

static member Async.FromContinuations : callback:(('T -> unit) * (exn -> unit) * (System.OperationCanceledException -> unit) -> unit) -> Async<'T>

val cont : ('T option -> unit)

val econt : (exn -> unit)

val tasks : Async<'T option> []

module Seq

from Microsoft.FSharp.Collections

val toArray : source:seq<'T> -> 'T []

Full name: Microsoft.FSharp.Collections.Seq.toArray

property System.Array.Length: int

union case Option.None: Option<'T>

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 count : 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 wrap : (Async<'T option> -> Async<unit>)

val task : Async<'T option>

val async : AsyncBuilder

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

val res : 'T option

val lock : lockObject:'Lock -> action:(unit -> 'T) -> 'T (requires reference type)

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

val decr : cell:int ref -> unit

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

union case Option.Some: Value: 'T -> Option<'T>

val r : 'T

CancellationTokenSource.Cancel() : unit
CancellationTokenSource.Cancel(throwOnFirstException: bool) : unit

val e : exn

static member Async.Start : computation:Async<unit> * ?cancellationToken:CancellationToken -> unit

property CancellationTokenSource.Token: CancellationToken

val task : delay:int -> result:'a -> Async<'a>

Full name: Script.task

val delay : int

val result : 'a

static member Async.Sleep : millisecondsDueTime:int -> Async<unit>

member AsyncBuilder.Zero : unit -> 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>) -> inputs:seq<'T> -> Async<bool>

Full name: Script.exists


 parallel existential combinator

val f : ('T -> Async<bool>)

type bool = System.Boolean

Full name: Microsoft.FSharp.Core.bool

val inputs : seq<'T>

val wrapper : ('T -> Async<unit option>)

val t : 'T

val res : bool

val res : unit option

val map : mapping:('T -> 'U) -> source:seq<'T> -> seq<'U>

Full name: Microsoft.FSharp.Collections.Seq.map

property Option.IsSome: bool