// Copyright (c) Microsoft Corporation.  All Rights Reserved.  See License.txt in the project root for license information.
//
// Implementation is taken from IcedTasks (https://github.com/TheAngryByrd/IcedTasks/blob/72638c8719014ae963f2662449c99f87090041d1/LICENSE.md?plain=1#L1-L21), under MIT license
// Which was originally written in 2016 by Robert Peele (humbobst@gmail.com)
// New operator-based overload resolution for F# 4.0 compatibility by Gustavo Leon in 2018.
// Revised for insertion into FSharp.Core by Microsoft, 2019.
// Revised to implement CancellationToken semantics
//
// Original notice:
// To the extent possible under law, the author(s) have dedicated all copyright and related and neighboring rights
// to this software to the public domain worldwide. This software is distributed without any warranty.
// IcedTasks MIT notice (https://github.com/TheAngryByrd/IcedTasks/blob/72638c8719014ae963f2662449c99f87090041d1/LICENSE.md?plain=1#L1-L21):
// MIT License

// Copyright (c) [year] [fullname]

// Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

// The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 
namespace Microsoft.VisualStudio.FSharp.Editor

// Don't warn about the resumable code invocation
#nowarn "3513"

module CancellableTasks =

    open System
    open System.Runtime.CompilerServices
    open System.Threading
    open System.Threading.Tasks
    open Microsoft.FSharp.Core
    open Microsoft.FSharp.Core.CompilerServices
    open Microsoft.FSharp.Core.CompilerServices.StateMachineHelpers
    open Microsoft.FSharp.Core.LanguagePrimitives.IntrinsicOperators
    open Microsoft.FSharp.Collections

    [<NoComparison; NoEquality>]
    type VolatileBarrier() =

        [<VolatileField>]
        let mutable isStopped = false

        member _.Proceed = not isStopped
        member _.Stop() = isStopped <- true

    /// A type that looks like an Awaiter
    type Awaiter<'Awaiter, 'TResult
        when 'Awaiter :> ICriticalNotifyCompletion
        and 'Awaiter: (member IsCompleted: bool)
        and 'Awaiter: (member GetResult: unit -> 'TResult)> = 'Awaiter

    /// A type that looks like an Awaitable
    type Awaitable<'Awaitable, 'Awaiter, 'TResult
        when 'Awaitable: (member GetAwaiter: unit -> Awaiter<'Awaiter, 'TResult>)> = 'Awaitable

    /// Functions for Awaiters
    module Awaiter =
        /// Gets a value that indicates whether the asynchronous task has completed
        let inline isCompleted<'Awaiter, 'TResult when Awaiter<'Awaiter, 'TResult>> (x: 'Awaiter) =
            x.IsCompleted

        /// Ends the wait for the completion of the asynchronous task.
        let inline getResult<'Awaiter, 'TResult when Awaiter<'Awaiter, 'TResult>> (x: 'Awaiter) =
            x.GetResult()

    /// Functions for Awaitables
    module Awaitable =
        /// Creates an awaiter for this value.
        let inline getAwaiter<'Awaitable, 'Awaiter, 'TResult
            when Awaitable<'Awaitable, 'Awaiter, 'TResult>>
            (x: 'Awaitable)
            =
            x.GetAwaiter()

    /// CancellationToken -> Task<'T>
    type CancellableTask<'T> = CancellationToken -> Task<'T>

    /// CancellationToken -> Task
    type CancellableTask = CancellationToken -> Task

    /// The extra data stored in ResumableStateMachine for tasks
    [<Struct; NoComparison; NoEquality>]
    type CancellableTaskStateMachineData<'T> =
        [<DefaultValue(false)>]
        val mutable CancellationToken: CancellationToken

        [<DefaultValue(false)>]
        val mutable Result: 'T

        [<DefaultValue(false)>]
        val mutable MethodBuilder: AsyncTaskMethodBuilder<'T>

        member inline this.ThrowIfCancellationRequested() =
            this.CancellationToken.ThrowIfCancellationRequested()

    /// This is used by the compiler as a template for creating state machine structs
    and CancellableTaskStateMachine<'TOverall> =
        ResumableStateMachine<CancellableTaskStateMachineData<'TOverall>>

    /// Represents the runtime continuation of a CancellableTask state machine created dynamically
    and CancellableTaskResumptionFunc<'TOverall> =
        ResumptionFunc<CancellableTaskStateMachineData<'TOverall>>

    /// Represents the runtime continuation of a CancellableTask state machine created dynamically
    and CancellableTaskResumptionDynamicInfo<'TOverall> =
        ResumptionDynamicInfo<CancellableTaskStateMachineData<'TOverall>>

    /// A special compiler-recognised delegate type for specifying blocks of CancellableTask code with access to the state machine
    and CancellableTaskCode<'TOverall, 'T> =
        ResumableCode<CancellableTaskStateMachineData<'TOverall>, 'T>


    /// Contains methods to build CancellableTasks using the F# computation expression syntax
    [<NoComparison; NoEquality>]
    type CancellableTaskBuilderBase() =

        /// <summary>Creates a CancellableTask that runs generator</summary>
        /// <param name="generator">The function to run</param>
        /// <returns>A cancellableTask that runs generator</returns>
        member inline _.Delay
            ([<InlineIfLambda>] generator: unit -> CancellableTaskCode<'TOverall, 'T>)
            : CancellableTaskCode<'TOverall, 'T> =
            ResumableCode.Delay(fun () ->
                CancellableTaskCode(fun sm ->
                    sm.Data.ThrowIfCancellationRequested()
                    (generator ()).Invoke(&sm)
                )
            )


        /// <summary>Creates an CancellableTask that just returns ().</summary>
        /// <remarks>
        /// The existence of this method permits the use of empty else branches in the
        /// cancellableTask { ... } computation expression syntax.
        /// </remarks>
        /// <returns>An CancellableTask that returns ().</returns>
        [<DefaultValue>]
        member inline _.Zero() : CancellableTaskCode<'TOverall, unit> = ResumableCode.Zero()

        /// <summary>Creates an computation that returns the result v.</summary>
        ///
        /// <remarks>A cancellation check is performed when the computation is executed.
        ///
        /// The existence of this method permits the use of return in the
        /// cancellableTask { ... } computation expression syntax.</remarks>
        ///
        /// <param name="value">The value to return from the computation.</param>
        ///
        /// <returns>An CancellableTask that returns value when executed.</returns>
        member inline _.Return(value: 'T) : CancellableTaskCode<'T, 'T> =
            CancellableTaskCode<'T, _>(fun sm ->
                sm.Data.ThrowIfCancellationRequested()
                sm.Data.Result <- value
                true
            )

        /// <summary>Creates an CancellableTask that first runs task1
        /// and then runs computation2, returning the result of computation2.</summary>
        ///
        /// <remarks>
        ///
        /// The existence of this method permits the use of expression sequencing in the
        /// cancellableTask { ... } computation expression syntax.</remarks>
        ///
        /// <param name="task1">The first part of the sequenced computation.</param>
        /// <param name="task2">The second part of the sequenced computation.</param>
        ///
        /// <returns>An CancellableTask that runs both of the computations sequentially.</returns>
        member inline _.Combine
            (
                task1: CancellableTaskCode<'TOverall, unit>,
                task2: CancellableTaskCode<'TOverall, 'T>
            ) : CancellableTaskCode<'TOverall, 'T> =
            ResumableCode.Combine(
                CancellableTaskCode(fun sm ->
                    sm.Data.ThrowIfCancellationRequested()
                    task1.Invoke(&sm)
                ),

                CancellableTaskCode(fun sm ->
                    sm.Data.ThrowIfCancellationRequested()
                    task2.Invoke(&sm)
                )
            )

        /// <summary>Creates an CancellableTask that runs computation repeatedly
        /// until guard() becomes false.</summary>
        ///
        /// <remarks>
        ///
        /// The existence of this method permits the use of while in the
        /// cancellableTask { ... } computation expression syntax.</remarks>
        ///
        /// <param name="guard">The function to determine when to stop executing computation.</param>
        /// <param name="computation">The function to be executed.  Equivalent to the body
        /// of a while expression.</param>
        ///
        /// <returns>An CancellableTask that behaves similarly to a while loop when run.</returns>
        member inline _.While
            (
                [<InlineIfLambda>] guard: unit -> bool,
                computation: CancellableTaskCode<'TOverall, unit>
            ) : CancellableTaskCode<'TOverall, unit> =
            ResumableCode.While(
                guard,
                CancellableTaskCode(fun sm ->
                    sm.Data.ThrowIfCancellationRequested()
                    computation.Invoke(&sm)
                )
            )

        /// <summary>Creates an CancellableTask that runs computation and returns its result.
        /// If an exception happens then catchHandler(exn) is called and the resulting computation executed instead.</summary>
        ///
        /// <remarks>
        ///
        /// The existence of this method permits the use of try/with in the
        /// cancellableTask { ... } computation expression syntax.</remarks>
        ///
        /// <param name="computation">The input computation.</param>
        /// <param name="catchHandler">The function to run when computation throws an exception.</param>
        ///
        /// <returns>An CancellableTask that executes computation and calls catchHandler if an
        /// exception is thrown.</returns>
        member inline _.TryWith
            (
                computation: CancellableTaskCode<'TOverall, 'T>,
                [<InlineIfLambda>] catchHandler: exn -> CancellableTaskCode<'TOverall, 'T>
            ) : CancellableTaskCode<'TOverall, 'T> =
            ResumableCode.TryWith(
                CancellableTaskCode(fun sm ->
                    sm.Data.ThrowIfCancellationRequested()
                    computation.Invoke(&sm)
                ),
                catchHandler
            )

        /// <summary>Creates an CancellableTask that runs computation. The action compensation is executed
        /// after computation completes, whether computation exits normally or by an exception. If compensation raises an exception itself
        /// the original exception is discarded and the new exception becomes the overall result of the computation.</summary>
        ///
        /// <remarks>
        ///
        /// The existence of this method permits the use of try/finally in the
        /// cancellableTask { ... } computation expression syntax.</remarks>
        ///
        /// <param name="computation">The input computation.</param>
        /// <param name="compensation">The action to be run after computation completes or raises an
        /// exception (including cancellation).</param>
        ///
        /// <returns>An CancellableTask that executes computation and compensation afterwards or
        /// when an exception is raised.</returns>
        member inline _.TryFinally
            (
                computation: CancellableTaskCode<'TOverall, 'T>,
                [<InlineIfLambda>] compensation: unit -> unit
            ) : CancellableTaskCode<'TOverall, 'T> =
            ResumableCode.TryFinally(

                CancellableTaskCode(fun sm ->
                    sm.Data.ThrowIfCancellationRequested()
                    computation.Invoke(&sm)
                ),
                ResumableCode<_, _>(fun _ ->
                    compensation ()
                    true
                )
            )

        /// <summary>Creates an CancellableTask that enumerates the sequence seq
        /// on demand and runs body for each element.</summary>
        ///
        /// <remarks>A cancellation check is performed on each iteration of the loop.
        ///
        /// The existence of this method permits the use of for in the
        /// cancellableTask { ... } computation expression syntax.</remarks>
        ///
        /// <param name="sequence">The sequence to enumerate.</param>
        /// <param name="body">A function to take an item from the sequence and create
        /// an CancellableTask.  Can be seen as the body of the for expression.</param>
        ///
        /// <returns>An CancellableTask that will enumerate the sequence and run body
        /// for each element.</returns>
        member inline _.For
            (
                sequence: seq<'T>,
                [<InlineIfLambda>] body: 'T -> CancellableTaskCode<'TOverall, unit>
            ) : CancellableTaskCode<'TOverall, unit> =
            ResumableCode.For(
                sequence,
                fun item ->
                    CancellableTaskCode(fun sm ->
                        sm.Data.ThrowIfCancellationRequested()
                        (body item).Invoke(&sm)
                    )
            )

    /// Contains methods to build CancellableTasks using the F# computation expression syntax
    [<Sealed; NoComparison; NoEquality>]
    type CancellableTaskBuilder(runOnBackground: bool) =

        inherit CancellableTaskBuilderBase()

        member val IsBackground = runOnBackground

        // This is the dynamic implementation - this is not used
        // for statically compiled tasks.  An executor (resumptionFuncExecutor) is
        // registered with the state machine, plus the initial resumption.
        // The executor stays constant throughout the execution, it wraps each step
        // of the execution in a try/with.  The resumption is changed at each step
        // to represent the continuation of the computation.
        /// <summary>
        /// The entry point for the dynamic implementation of the corresponding operation. Do not use directly, only used when executing quotations that involve tasks or other reflective execution of F# code.
        /// </summary>
        static member inline RunDynamicAux(code: CancellableTaskCode<'T, 'T>) : CancellableTask<'T> =

            let mutable sm = CancellableTaskStateMachine<'T>()

            let initialResumptionFunc =
                CancellableTaskResumptionFunc<'T>(fun sm -> code.Invoke(&sm))

            let resumptionInfo =
                { new CancellableTaskResumptionDynamicInfo<'T>(initialResumptionFunc) with
                    member info.MoveNext(sm) =
                        let mutable savedExn = null

                        try
                            sm.ResumptionDynamicInfo.ResumptionData <- null
                            let step = info.ResumptionFunc.Invoke(&sm)

                            if step then
                                sm.Data.MethodBuilder.SetResult(sm.Data.Result)
                            else
                                let mutable awaiter =
                                    sm.ResumptionDynamicInfo.ResumptionData
                                    :?> ICriticalNotifyCompletion

                                assert not (isNull awaiter)
                                sm.Data.MethodBuilder.AwaitUnsafeOnCompleted(&awaiter, &sm)

                        with exn ->
                            savedExn <- exn
                        // Run SetException outside the stack unwind, see https://github.com/dotnet/roslyn/issues/26567
                        match savedExn with
                        | null -> ()
                        | exn -> sm.Data.MethodBuilder.SetException exn

                    member _.SetStateMachine(sm, state) =
                        sm.Data.MethodBuilder.SetStateMachine(state)
                }

            fun (ct) ->
                if ct.IsCancellationRequested then
                    Task.FromCanceled<_>(ct)
                else
                    sm.Data.CancellationToken <- ct
                    sm.ResumptionDynamicInfo <- resumptionInfo
                    sm.Data.MethodBuilder <- AsyncTaskMethodBuilder<'T>.Create()
                    sm.Data.MethodBuilder.Start(&sm)
                    sm.Data.MethodBuilder.Task

        /// <summary>
        /// The entry point for the dynamic implementation of the corresponding operation. Do not use directly, only used when executing quotations that involve tasks or other reflective execution of F# code.
        /// </summary>
        static member inline RunDynamic(code: CancellableTaskCode<'T, 'T>, runOnBackground: bool) : CancellableTask<'T> =
            // When runOnBackground is true, task escapes to a background thread where necessary
            // See spec of ConfigureAwait(false) at https://devblogs.microsoft.com/dotnet/configureawait-faq/

            if runOnBackground
                && not (isNull SynchronizationContext.Current
                        && obj.ReferenceEquals(TaskScheduler.Current, TaskScheduler.Default))
            then
                fun (ct) ->
                    Task.Run<'T>((fun () -> CancellableTaskBuilder.RunDynamicAux (code) (ct)), ct)
            else
                CancellableTaskBuilder.RunDynamicAux(code)


        /// Hosts the task code in a state machine and starts the task.
        member inline this.Run(code: CancellableTaskCode<'T, 'T>) : CancellableTask<'T> =
            if __useResumableCode then
                __stateMachine<CancellableTaskStateMachineData<'T>, CancellableTask<'T>>
                    (MoveNextMethodImpl<_>(fun sm ->
                        //-- RESUMABLE CODE START
                        __resumeAt sm.ResumptionPoint
                        let mutable __stack_exn: Exception = null

                        try
                            let __stack_code_fin = code.Invoke(&sm)

                            if __stack_code_fin then
                                sm.Data.MethodBuilder.SetResult(sm.Data.Result)
                        with exn ->
                            __stack_exn <- exn
                        // Run SetException outside the stack unwind, see https://github.com/dotnet/roslyn/issues/26567
                        match __stack_exn with
                        | null -> ()
                        | exn -> sm.Data.MethodBuilder.SetException exn
                    //-- RESUMABLE CODE END
                    ))
                    (SetStateMachineMethodImpl<_>(fun sm state ->
                        sm.Data.MethodBuilder.SetStateMachine(state)
                    ))
                    (AfterCode<_, _>(fun sm ->
                        if this.IsBackground
                            && not (isNull SynchronizationContext.Current
                                    && obj.ReferenceEquals(TaskScheduler.Current, TaskScheduler.Default))
                        then

                            let sm = sm // copy contents of state machine so we can capture it

                            fun (ct) ->
                                if ct.IsCancellationRequested then
                                    Task.FromCanceled<_>(ct)
                                else
                                    // Warning: this will always try to yield even if on thread pool already.
                                    Task.Run<'T>(
                                        (fun () ->
                                            let mutable sm = sm // host local mutable copy of contents of state machine on this thread pool thread
                                            sm.Data.CancellationToken <- ct

                                            sm.Data.MethodBuilder <-
                                                AsyncTaskMethodBuilder<'T>.Create()

                                            sm.Data.MethodBuilder.Start(&sm)
                                            sm.Data.MethodBuilder.Task
                                        ),
                                        ct
                                    )
                        else
                            let mutable sm = sm

                            fun (ct) ->
                                if ct.IsCancellationRequested then
                                    Task.FromCanceled<_>(ct)
                                else
                                    sm.Data.CancellationToken <- ct
                                    sm.Data.MethodBuilder <- AsyncTaskMethodBuilder<'T>.Create()
                                    sm.Data.MethodBuilder.Start(&sm)
                                    sm.Data.MethodBuilder.Task
                    ))
            else
                CancellableTaskBuilder.RunDynamic(code, this.IsBackground)

    /// Contains the cancellableTask computation expression builder.
    [<AutoOpen>]
    module CancellableTaskBuilder =

        /// <summary>
        /// Builds a cancellableTask using computation expression syntax.
        /// Default behaviour when binding (v)options is to return a cacnelled task.
        /// </summary>
        let foregroundCancellableTask = CancellableTaskBuilder(false)

        /// <summary>
        /// Builds a cancellableTask using computation expression syntax which switches to execute on a background thread if not already doing so.
        /// Default behaviour when binding (v)options is to return a cacnelled task.
        /// </summary>
        let cancellableTask = CancellableTaskBuilder(true)

    /// <exclude />
    [<AutoOpen>]
    module LowPriority =
        // Low priority extensions
        type CancellableTaskBuilderBase with

            /// <summary>
            /// The entry point for the dynamic implementation of the corresponding operation. Do not use directly, only used when executing quotations that involve tasks or other reflective execution of F# code.
            /// </summary>
            [<NoEagerConstraintApplication>]
            static member inline BindDynamic<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                (
                    sm: byref<ResumableStateMachine<CancellableTaskStateMachineData<'TOverall>>>,
                    [<InlineIfLambda>] getAwaiter: CancellationToken -> 'Awaiter,
                    [<InlineIfLambda>] continuation: ('TResult1 -> CancellableTaskCode<'TOverall, 'TResult2>)
                ) : bool =
                sm.Data.ThrowIfCancellationRequested()

                let mutable awaiter = getAwaiter sm.Data.CancellationToken

                let cont: CancellableTaskResumptionFunc<'TOverall> =
                    (CancellableTaskResumptionFunc<'TOverall>(fun (sm: byref<ResumableStateMachine<CancellableTaskStateMachineData<'TOverall>>>) ->
                        let result: 'TResult1 = Awaiter.getResult awaiter
                        (continuation result).Invoke(&sm)
                    ))

                // shortcut to continue immediately
                if Awaiter.isCompleted awaiter then
                    cont.Invoke(&sm)
                else
                    sm.ResumptionDynamicInfo.ResumptionData <-
                        (awaiter :> ICriticalNotifyCompletion)

                    sm.ResumptionDynamicInfo.ResumptionFunc <- cont
                    false

            /// <summary>Creates an CancellableTask that runs computation, and when
            /// computation generates a result T, runs binder res.</summary>
            ///
            /// <remarks>A cancellation check is performed when the computation is executed.
            ///
            /// The existence of this method permits the use of let! in the
            /// cancellableTask { ... } computation expression syntax.</remarks>
            ///
            /// <param name="getAwaiter">The computation to provide an unbound result.</param>
            /// <param name="continuation">The function to bind the result of computation.</param>
            ///
            /// <returns>An CancellableTask that performs a monadic bind on the result
            /// of computation.</returns>
            [<NoEagerConstraintApplication>]
            member inline _.Bind<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                (
                    [<InlineIfLambda>] getAwaiter: CancellationToken -> 'Awaiter,
                    [<InlineIfLambda>] continuation: ('TResult1 -> CancellableTaskCode<'TOverall, 'TResult2>)
                ) : CancellableTaskCode<'TOverall, 'TResult2> =

                CancellableTaskCode<'TOverall, _>(fun sm ->
                    if __useResumableCode then
                        //-- RESUMABLE CODE START
                        sm.Data.ThrowIfCancellationRequested()
                        // Get an awaiter from the Awaiter
                        let mutable awaiter = getAwaiter sm.Data.CancellationToken

                        let mutable __stack_fin = true

                        if not (Awaiter.isCompleted awaiter) then
                            // This will yield with __stack_yield_fin = false
                            // This will resume with __stack_yield_fin = true
                            let __stack_yield_fin = ResumableCode.Yield().Invoke(&sm)
                            __stack_fin <- __stack_yield_fin

                        if __stack_fin then
                            let result =
                                awaiter
                                |> Awaiter.getResult

                            (continuation result).Invoke(&sm)
                        else
                            sm.Data.MethodBuilder.AwaitUnsafeOnCompleted(&awaiter, &sm)
                            false
                    else
                        CancellableTaskBuilderBase.BindDynamic<'TResult1, 'TResult2, 'Awaiter, 'TOverall>(
                            &sm,
                            getAwaiter,
                            continuation
                        )
                //-- RESUMABLE CODE END
                )


            /// <summary>
            /// The entry point for the dynamic implementation of the corresponding operation. Do not use directly, only used when executing quotations that involve tasks or other reflective execution of F# code.
            /// </summary>
            [<NoEagerConstraintApplication>]
            static member inline BindDynamic<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                (
                    sm: byref<ResumableStateMachine<CancellableTaskStateMachineData<'TOverall>>>,
                    getAwaiter: 'Awaiter,
                    [<InlineIfLambda>] continuation: ('TResult1 -> CancellableTaskCode<'TOverall, 'TResult2>)
                ) : bool =

                    sm.Data.ThrowIfCancellationRequested()

                    let mutable awaiter = getAwaiter

                    let cont =
                        (CancellableTaskResumptionFunc<'TOverall>(fun sm ->
                            let result = Awaiter.getResult awaiter
                            (continuation result).Invoke(&sm)))

                    // shortcut to continue immediately
                    if Awaiter.isCompleted awaiter then
                        cont.Invoke(&sm)
                    else
                        sm.ResumptionDynamicInfo.ResumptionData <- (awaiter :> ICriticalNotifyCompletion)
                        sm.ResumptionDynamicInfo.ResumptionFunc <- cont
                        false

            /// <summary>Creates an CancellableTask that runs computation, and when
            /// computation generates a result T, runs binder res.</summary>
            ///
            /// <remarks>A cancellation check is performed when the computation is executed.
            ///
            /// The existence of this method permits the use of let! in the
            /// cancellableTask { ... } computation expression syntax.</remarks>
            ///
            /// <param name="getAwaiter">The computation to provide an unbound result.</param>
            /// <param name="continuation">The function to bind the result of computation.</param>
            ///
            /// <returns>An CancellableTask that performs a monadic bind on the result
            /// of computation.</returns>
            [<NoEagerConstraintApplication>]
            member inline _.Bind<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                (
                    getAwaiter: 'Awaiter,
                    [<InlineIfLambda>] continuation: ('TResult1 -> CancellableTaskCode<'TOverall, 'TResult2>)
                ) : CancellableTaskCode<'TOverall, 'TResult2> =

                CancellableTaskCode<'TOverall, _>(
                    fun sm ->
                        if __useResumableCode then
                            sm.Data.ThrowIfCancellationRequested()
                            
                            let mutable awaiter = getAwaiter
                            let mutable __stack_fin = true
                            
                            if not (Awaiter.isCompleted awaiter) then
                                let __stack_yield_fin = ResumableCode.Yield().Invoke(&sm)
                                __stack_fin <- __stack_yield_fin

                            if __stack_fin then
                                let result = Awaiter.getResult awaiter

                                (continuation result).Invoke(&sm)
                            else
                                sm.Data.MethodBuilder.AwaitUnsafeOnCompleted(&awaiter, &sm)
                                false
                        else
                            CancellableTaskBuilderBase.BindDynamic<'TResult1, 'TResult2, 'Awaiter, 'TOverall>(&sm, getAwaiter, continuation))


            /// <summary>Delegates to the input computation.</summary>
            ///
            /// <remarks>The existence of this method permits the use of return! in the
            /// cancellableTask { ... } computation expression syntax.</remarks>
            ///
            /// <param name="getAwaiter">The input computation.</param>
            ///
            /// <returns>The input computation.</returns>
            [<NoEagerConstraintApplication>]
            member inline this.ReturnFrom<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                ([<InlineIfLambda>] getAwaiter: CancellationToken -> 'Awaiter)
                : CancellableTaskCode<_, _> =
                this.Bind(getAwaiter, this.Return)

            [<NoEagerConstraintApplication>]
            member inline this.ReturnFrom<'TResult1, 'TResult2, 'Awaiter, 'TOverall when Awaiter<'Awaiter, 'TResult1>> (getAwaiter: 'Awaiter) : CancellableTaskCode<_, _> =
               this.Bind(getAwaiter, (fun v -> this.Return v))


            [<NoEagerConstraintApplication>]
            member inline this.BindReturn<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                (
                    [<InlineIfLambda>] getAwaiter: CancellationToken -> 'Awaiter,
                    [<InlineIfLambda>] mapper: 'TResult1 -> 'TResult2
                ) : CancellableTaskCode<'TResult2, 'TResult2> =
                this.Bind(getAwaiter, (fun v -> this.Return(mapper v)))


            [<NoEagerConstraintApplication>]
            member inline this.BindReturn<'TResult1, 'TResult2, 'Awaiter, 'TOverall
               when Awaiter<'Awaiter, 'TResult1>>
               (
                   getAwaiter: 'Awaiter,
                   [<InlineIfLambda>] mapper: 'TResult1 -> 'TResult2
               ) : CancellableTaskCode<'TResult2, 'TResult2> =
               this.Bind(getAwaiter, (fun v -> this.Return(mapper v)))

            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This is the identify function.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            [<NoEagerConstraintApplication>]
            member inline _.Source<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                ([<InlineIfLambda>] getAwaiter: CancellationToken -> 'Awaiter)
                : CancellationToken -> 'Awaiter =
                getAwaiter


            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This is the identify function.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            [<NoEagerConstraintApplication>]
            member inline _.Source<'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaiter<'Awaiter, 'TResult1>>
                (getAwaiter: 'Awaiter)
                : 'Awaiter =
                getAwaiter


            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a 'Awaitable into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            [<NoEagerConstraintApplication>]
            member inline _.Source<'Awaitable, 'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaitable<'Awaitable, 'Awaiter, 'TResult1>>
                (task: 'Awaitable)
                : 'Awaiter =
                    Awaitable.getAwaiter task
                

            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a CancellationToken -> 'Awaitable into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            [<NoEagerConstraintApplication>]
            member inline _.Source<'Awaitable, 'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaitable<'Awaitable, 'Awaiter, 'TResult1>>
                ([<InlineIfLambda>] task: CancellationToken -> 'Awaitable)
                : CancellationToken -> 'Awaiter =
                (fun ct -> Awaitable.getAwaiter (task ct))


            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a unit -> 'Awaitable into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            [<NoEagerConstraintApplication>]
            member inline _.Source<'Awaitable, 'TResult1, 'TResult2, 'Awaiter, 'TOverall
                when Awaitable<'Awaitable, 'Awaiter, 'TResult1>>
                ([<InlineIfLambda>] task: unit -> 'Awaitable)
                : CancellationToken -> 'Awaiter =
                (fun _ct -> Awaitable.getAwaiter (task ()))


            /// <summary>Creates an CancellableTask that runs binder(resource).
            /// The action resource.Dispose() is executed as this computation yields its result
            /// or if the CancellableTask exits by an exception or by cancellation.</summary>
            ///
            /// <remarks>
            ///
            /// The existence of this method permits the use of use and use! in the
            /// cancellableTask { ... } computation expression syntax.</remarks>
            ///
            /// <param name="resource">The resource to be used and disposed.</param>
            /// <param name="binder">The function that takes the resource and returns an asynchronous
            /// computation.</param>
            ///
            /// <returns>An CancellableTask that binds and eventually disposes resource.</returns>
            ///
            member inline _.Using<'Resource, 'TOverall, 'T when 'Resource :> IDisposable>
                (
                    resource: 'Resource,
                    [<InlineIfLambda>] binder: 'Resource -> CancellableTaskCode<'TOverall, 'T>
                ) =
                ResumableCode.Using(
                    resource,
                    fun resource ->
                        CancellableTaskCode<'TOverall, 'T>(fun sm ->
                            sm.Data.ThrowIfCancellationRequested()
                            (binder resource).Invoke(&sm)
                        )
                )

    /// <exclude />
    [<AutoOpen>]
    module HighPriority =

        let inline startAsyncImmediateAsTask computation (cancellationToken: CancellationToken) =
            // Protect against blocking the UI thread by switching to thread pool
            let computation =
                match SynchronizationContext.Current with
                | null -> computation
                | _ ->
                    async {
                        do! Async.SwitchToThreadPool()
                        return! computation
                    }

            // try not to yield if on bg thread already
            let tcs = new TaskCompletionSource<_>(TaskCreationOptions.None)
            let barrier = VolatileBarrier()

            let reg =
                cancellationToken.Register(fun _ ->
                    if barrier.Proceed then
                        tcs.TrySetCanceled(cancellationToken) |> ignore)

            let task = tcs.Task

            let disposeReg () =
                barrier.Stop()

                if not task.IsCanceled then
                    reg.Dispose()

            Async.StartWithContinuations(
                computation,
                continuation =
                    (fun result ->
                        disposeReg ()
                        tcs.TrySetResult(result) |> ignore),
                exceptionContinuation =
                    (fun exn ->
                        disposeReg ()

                        match exn with
                        | :? OperationCanceledException -> tcs.TrySetCanceled(cancellationToken) |> ignore
                        | exn -> tcs.TrySetException(exn) |> ignore),
                cancellationContinuation =
                    (fun _oce ->
                        disposeReg ()
                        tcs.TrySetCanceled(cancellationToken) |> ignore),
                cancellationToken = cancellationToken
            )

            task

        type Control.Async with

            /// <summary>Return an asynchronous computation that will wait for the given task to complete and return
            /// its result.</summary>
            static member inline AwaitCancellableTask([<InlineIfLambda>] t: CancellableTask<'T>) =
                async {
                    let! ct = Async.CancellationToken
                    return! Async.AwaitTask (t ct)
                }

            /// <summary>Return an asynchronous computation that will wait for the given task to complete and return
            /// its result.</summary>
            static member inline AwaitCancellableTask([<InlineIfLambda>] t: CancellableTask) =
                async {
                    let! ct = Async.CancellationToken

                    return! Async.AwaitTask (t ct)
                }

            /// <summary>Runs an asynchronous computation, starting on the current operating system thread.</summary>
            static member inline AsCancellableTask(computation: Async<'T>) : CancellableTask<_> =
                fun ct -> startAsyncImmediateAsTask computation ct

        // High priority extensions
        type CancellableTaskBuilderBase with

            /// <summary>
            /// Turn option into "awaitable", will return cancelled task if None
            /// </summary>
            /// <param name="s">Option instance to bind on</param>
            (*member inline _.Source(s: 'T option) =
                (fun (_ct: CancellationToken) ->
                    match s with
                    | Some x -> Task.FromResult<'T>(x).GetAwaiter()
                    | None -> Task.FromCanceled<'T>(CancellationToken(true)).GetAwaiter()
                )*)

            /// <summary>
            /// Turn a value option into "awaitable", will return cancelled task if None
            /// </summary>
            /// <param name="s">Option instance to bind on</param>
            (*member inline _.Source(s: 'T voption) =
                (fun (_ct: CancellationToken) ->
                    match s with
                    | ValueSome x -> Task.FromResult<'T>(x).GetAwaiter()
                    | ValueNone -> Task.FromCanceled<'T>(CancellationToken(true)).GetAwaiter()
                )*)

            /// <summary>Allows the computation expression to turn other types into other types</summary>
            ///
            /// <remarks>This is the identify function for For binds.</remarks>
            ///
            /// <returns>IEnumerable</returns>
            member inline _.Source(s: #seq<_>) : #seq<_> = s

            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a Task&lt;'T&gt; into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            member inline _.Source(task: Task<'T>) =
                (fun (_ct: CancellationToken) -> task.GetAwaiter())

            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a ColdTask&lt;'T&gt; into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            member inline _.Source([<InlineIfLambda>] task: unit -> Task<'TResult1>) =
                (fun (_ct: CancellationToken) -> (task ()).GetAwaiter())

            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a CancellableTask&lt;'T&gt; into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            member inline _.Source([<InlineIfLambda>] task: CancellableTask<'TResult1>) =
                (fun ct -> (task ct).GetAwaiter())

            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a Async&lt;'T&gt; into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            member inline this.Source(computation: Async<'TResult1>) =
                this.Source(fun ct -> Async.AsCancellableTask(computation) ct)

            /// <summary>Allows the computation expression to turn other types into CancellationToken -> 'Awaiter</summary>
            ///
            /// <remarks>This turns a CancellableTask&lt;'T&gt; into a CancellationToken -> 'Awaiter.</remarks>
            ///
            /// <returns>CancellationToken -> 'Awaiter</returns>
            member inline _.Source(awaiter: TaskAwaiter<'TResult1>) = awaiter

    /// <summary>
    /// A set of extension methods making it possible to bind against <see cref='T:CancellableTask`1'/> in async computations.
    /// </summary>
    [<AutoOpen>]
    module AsyncExtenions =
        type Control.AsyncBuilder with

            member inline this.Bind([<InlineIfLambda>] t: CancellableTask<'T>, [<InlineIfLambda>] binder: ('T -> Async<'U>)) : Async<'U> =
                this.Bind(Async.AwaitCancellableTask t, binder)

            member inline this.ReturnFrom([<InlineIfLambda>] t: CancellableTask<'T>) : Async<'T> =
                this.ReturnFrom(Async.AwaitCancellableTask t)

            member inline this.Bind([<InlineIfLambda>] t: CancellableTask, [<InlineIfLambda>] binder: (unit -> Async<'U>)) : Async<'U> =
                this.Bind(Async.AwaitCancellableTask t, binder)

            member inline this.ReturnFrom([<InlineIfLambda>] t: CancellableTask) : Async<unit> =
                this.ReturnFrom(Async.AwaitCancellableTask t)

    /// Contains a set of standard functional helper function
    [<RequireQualifiedAccess>]
    module CancellableTask =

        /// <summary>Gets the default cancellation token for executing computations.</summary>
        ///
        /// <returns>The default CancellationToken.</returns>
        ///
        /// <category index="3">Cancellation and Exceptions</category>
        ///
        /// <example id="default-cancellation-token-1">
        /// <code lang="F#">
        /// use tokenSource = new CancellationTokenSource()
        /// let primes = [ 2; 3; 5; 7; 11 ]
        /// for i in primes do
        ///     let computation =
        ///         cancellableTask {
        ///             let! cancellationToken = CancellableTask.getCurrentCancellationToken()
        ///             do! Task.Delay(i * 1000, cancellationToken)
        ///             printfn $"{i}"
        ///         }
        ///     computation tokenSource.Token |> ignore
        /// Thread.Sleep(6000)
        /// tokenSource.Cancel()
        /// printfn "Tasks Finished"
        /// </code>
        /// This will print "2" 2 seconds from start, "3" 3 seconds from start, "5" 5 seconds from start, cease computation and then
        /// followed by "Tasks Finished".
        /// </example>
        let inline getCancellationToken () : CancellationToken -> Task<CancellationToken> =
            fun ct -> Task.FromResult ct

        /// <summary>Lifts an item to a CancellableTask.</summary>
        /// <param name="item">The item to be the result of the CancellableTask.</param>
        /// <returns>A CancellableTask with the item as the result.</returns>
        let inline singleton (item: 'item) : CancellableTask<'item> = fun _ -> Task.FromResult(item)


        /// <summary>Allows chaining of CancellableTasks.</summary>
        /// <param name="binder">The continuation.</param>
        /// <param name="cTask">The value.</param>
        /// <returns>The result of the binder.</returns>
        let inline bind
            ([<InlineIfLambda>] binder: 'input -> CancellableTask<'output>)
            ([<InlineIfLambda>] cTask: CancellableTask<'input>)
            =
            cancellableTask {
                let! cResult = cTask
                return! binder cResult
            }

        /// <summary>Allows chaining of CancellableTasks.</summary>
        /// <param name="mapper">The continuation.</param>
        /// <param name="cTask">The value.</param>
        /// <returns>The result of the mapper wrapped in a CancellableTasks.</returns>
        let inline map
            ([<InlineIfLambda>] mapper: 'input -> 'output)
            ([<InlineIfLambda>] cTask: CancellableTask<'input>)
            =
            cancellableTask {
                let! cResult = cTask
                return mapper cResult
            }

        /// <summary>Allows chaining of CancellableTasks.</summary>
        /// <param name="applicable">A function wrapped in a CancellableTasks</param>
        /// <param name="cTask">The value.</param>
        /// <returns>The result of the applicable.</returns>
        let inline apply
            ([<InlineIfLambda>] applicable: CancellableTask<'input -> 'output>)
            ([<InlineIfLambda>] cTask: CancellableTask<'input>)
            =
            cancellableTask {
                let! applier = applicable
                let! cResult = cTask
                return applier cResult
            }

        /// <summary>Takes two CancellableTasks, starts them serially in order of left to right, and returns a tuple of the pair.</summary>
        /// <param name="left">The left value.</param>
        /// <param name="right">The right value.</param>
        /// <returns>A tuple of the parameters passed in</returns>
        let inline zip
            ([<InlineIfLambda>] left: CancellableTask<'left>)
            ([<InlineIfLambda>] right: CancellableTask<'right>)
            =
            cancellableTask {
                let! r1 = left
                let! r2 = right
                return r1, r2
            }

        /// <summary>Takes two CancellableTask, starts them concurrently, and returns a tuple of the pair.</summary>
        /// <param name="left">The left value.</param>
        /// <param name="right">The right value.</param>
        /// <returns>A tuple of the parameters passed in.</returns>
        let inline parallelZip
            ([<InlineIfLambda>] left: CancellableTask<'left>)
            ([<InlineIfLambda>] right: CancellableTask<'right>)
            =
            cancellableTask {
                let! ct = getCancellationToken ()
                let r1 = left ct
                let r2 = right ct
                let! r1 = r1
                let! r2 = r2
                return r1, r2
            }


        /// <summary>Coverts a CancellableTask to a CancellableTask\&lt;unit\&gt;.</summary>
        /// <param name="unitCancellabletTask">The CancellableTask to convert.</param>
        /// <returns>a CancellableTask\&lt;unit\&gt;.</returns>
        let inline ofUnit ([<InlineIfLambda>] unitCancellabletTask: CancellableTask) =
            cancellableTask {
                return! unitCancellabletTask
            }

        /// <summary>Coverts a CancellableTask\&lt;_\&gt; to a CancellableTask.</summary>
        /// <param name="ctask">The CancellableTask to convert.</param>
        /// <param name="ct">A cancellation token.</param>
        /// <returns>a CancellableTask.</returns>
        let inline toUnit ([<InlineIfLambda>] ctask: CancellableTask<_>) : CancellableTask =
            fun ct -> ctask ct

        let inline getAwaiter ([<InlineIfLambda>] ctask: CancellableTask<_>) =
            fun ct -> (ctask ct).GetAwaiter()

        let inline start ct ([<InlineIfLambda>] ctask: CancellableTask<_>) = ctask ct

        let inline startTask ct ([<InlineIfLambda>] ctask: CancellableTask) = ctask ct

        let inline startWithoutCancellation ([<InlineIfLambda>] ctask: CancellableTask<_>) = start CancellationToken.None ctask

        let inline startTaskWithoutCancellation ([<InlineIfLambda>] ctask: CancellableTask) = startTask CancellationToken.None ctask

        let inline runSynchronously ct ([<InlineIfLambda>] ctask: CancellableTask<_>) =
            let task = start ct ctask
            task.GetAwaiter().GetResult()

        let inline runSynchronouslyWithoutCancellation  ([<InlineIfLambda>] ctask: CancellableTask<_>) =
            let task = startWithoutCancellation ctask
            task.GetAwaiter().GetResult()

        let inline runTaskSynchronously ct ([<InlineIfLambda>] ctask: CancellableTask) =
           let task = startTask ct ctask
           task.GetAwaiter().GetResult()

        let inline runTaskSynchronouslyWithoutCancellation ([<InlineIfLambda>] ctask: CancellableTask) =
           let task = startTaskWithoutCancellation ctask
           task.GetAwaiter().GetResult()

        let inline startAsTask ct ([<InlineIfLambda>] ctask: CancellableTask<_>) = (start ct ctask) :> Task

        let inline startAsTaskWithoutCancellation ([<InlineIfLambda>] ctask: CancellableTask<_>) = (start CancellationToken.None ctask) :> Task

        let inline runAsTaskSynchronously ct ([<InlineIfLambda>] ctask: CancellableTask<_>) =
            let task = startAsTask ct ctask
            task.GetAwaiter().GetResult()

        let inline runAsTaskSynchronouslyWithoutCancellation  ([<InlineIfLambda>] ctask: CancellableTask<_>) =
            let task = startAsTaskWithoutCancellation ctask
            task.GetAwaiter().GetResult()

        let inline whenAll (tasks: CancellableTask<'a> seq) =
            cancellableTask {
                let! ct = getCancellationToken ()
                return! Task.WhenAll (seq { for task in tasks do yield start ct task })
            }

        let inline whenAllTasks (tasks: CancellableTask seq) =
            cancellableTask {
                let! ct = getCancellationToken ()
                return! Task.WhenAll (seq { for task in tasks do yield startTask ct task })
            }

    /// <exclude />
    [<AutoOpen>]
    module MergeSourcesExtensions =

        type CancellableTaskBuilderBase with

            [<NoEagerConstraintApplication>]
            member inline _.MergeSources<'TResult1, 'TResult2, 'Awaiter1, 'Awaiter2
                when Awaiter<'Awaiter1, 'TResult1> and Awaiter<'Awaiter2, 'TResult2>>
                (
                    [<InlineIfLambda>] left: CancellationToken -> 'Awaiter1,
                    [<InlineIfLambda>] right: CancellationToken -> 'Awaiter2
                ) : CancellationToken -> TaskAwaiter<'TResult1 * 'TResult2> =

                cancellableTask {
                    let! ct = CancellableTask.getCancellationToken ()
                    let leftStarted = left ct
                    let rightStarted = right ct
                    let! leftResult = leftStarted
                    let! rightResult = rightStarted
                    return leftResult, rightResult
                }
                |> CancellableTask.getAwaiter
