[RFC FS-1028] - Implement Async.StartImmediateAsTask (#2534)

Thank you for this contribution

Kevin

src/fsharp/FSharp.Core.Unittests/FSharp.Core/Microsoft.FSharp.Control/AsyncType.fs

@@ -275,6 +275,102 @@ type AsyncType() =
         Assert.IsTrue(t.IsCanceled)      
         Assert.IsTrue(!cancelled)      
 
+    [<Test>]
+    member this.CreateImmediateAsTask () =
+        let s = "Hello tasks!"
+        let a = async { return s }
+#if FSCORE_PORTABLE_NEW || coreclr
+        let t : Task<string> =
+#else
+        use t : Task<string> =
+#endif
+            Async.StartImmediateAsTask a
+        this.WaitASec t
+        Assert.IsTrue (t.IsCompleted)
+        Assert.AreEqual(s, t.Result)    
+
+    [<Test>]
+    member this.StartImmediateAsTask () =
+        let s = "Hello tasks!"
+        let a = async { return s }
+#if FSCORE_PORTABLE_NEW || coreclr
+        let t = 
+#else
+        use t =
+#endif
+            Async.StartImmediateAsTask a
+        this.WaitASec t
+        Assert.IsTrue (t.IsCompleted)
+        Assert.AreEqual(s, t.Result)    
+
+
+    [<Test>]
+    member this.ExceptionPropagatesToImmediateTask () =
+        let a = async { 
+            do raise (Exception ())
+         }
+#if FSCORE_PORTABLE_NEW || coreclr
+        let t = 
+#else
+        use t =
+#endif
+            Async.StartImmediateAsTask a
+        let mutable exceptionThrown = false
+        try 
+            this.WaitASec t
+        with 
+            e -> exceptionThrown <- true
+        Assert.IsTrue (t.IsFaulted)
+        Assert.IsTrue(exceptionThrown)
+
+    [<Test>]
+    member this.CancellationPropagatesToImmediateTask () =
+        let a = async {
+                while true do ()
+            }
+#if FSCORE_PORTABLE_NEW || coreclr
+        let t = 
+#else
+        use t =
+#endif
+            Async.StartImmediateAsTask a
+        Async.CancelDefaultToken () 
+        let mutable exceptionThrown = false
+        try
+            this.WaitASec t
+        with e -> exceptionThrown <- true
+        Assert.IsTrue (exceptionThrown)   
+        Assert.IsTrue(t.IsCanceled)            
+
+    [<Test>]
+    member this.CancellationPropagatesToGroupImmediate () =
+        let ewh = new ManualResetEvent(false)
+        let cancelled = ref false
+        let a = async { 
+                use! holder = Async.OnCancel (fun _ -> cancelled := true)
+                ewh.Set() |> Assert.IsTrue
+                while true do ()
+            }
+        let cts = new CancellationTokenSource()
+        let token = cts.Token
+#if FSCORE_PORTABLE_NEW || coreclr
+        let t = 
+#else
+        use t =
+#endif
+            Async.StartImmediateAsTask(a, cancellationToken=token)
+//        printfn "%A" t.Status
+        ewh.WaitOne() |> Assert.IsTrue
+        cts.Cancel()
+//        printfn "%A" t.Status        
+        let mutable exceptionThrown = false
+        try
+            this.WaitASec t
+        with e -> exceptionThrown <- true
+        Assert.IsTrue (exceptionThrown)   
+        Assert.IsTrue(t.IsCanceled)      
+        Assert.IsTrue(!cancelled)      
+
 
     [<Test>]
     member this.TaskAsyncValue () =

src/fsharp/FSharp.Core/control.fs

@@ -1218,6 +1218,17 @@ namespace Microsoft.FSharp.Control
         static member StartWithContinuations(computation:Async<'T>, continuation, exceptionContinuation, cancellationContinuation, ?cancellationToken) : unit =
             Async.StartWithContinuationsUsingDispatchInfo(computation, continuation, (fun edi -> exceptionContinuation (edi.GetAssociatedSourceException())), cancellationContinuation, ?cancellationToken=cancellationToken)
 
+        static member StartImmediateAsTask (computation : Async<'T>, ?cancellationToken ) : Task<'T>=
+            let token = defaultArg cancellationToken defaultCancellationTokenSource.Token
+            let ts = new TaskCompletionSource<'T>()
+            let task = ts.Task
+            Async.StartWithContinuations(
+                computation, 
+                (fun (k) -> ts.SetResult(k)), 
+                (fun exn -> ts.SetException(exn)), 
+                (fun _ -> ts.SetCanceled()), 
+                token)
+            task
         static member StartImmediate(computation:Async<unit>, ?cancellationToken) : unit =
             let token = defaultArg cancellationToken defaultCancellationTokenSource.Token
             CancellationTokenOps.StartWithContinuations(token, computation, id, (fun edi -> edi.ThrowAny()), ignore)

src/fsharp/FSharp.Core/control.fsi

@@ -415,6 +415,23 @@ namespace Microsoft.FSharp.Control
             computation:Async<unit> * ?cancellationToken:CancellationToken-> unit
 
 
+        /// <summary>Runs an asynchronous computation, starting immediately on the current operating system, 
+        /// but also returns the execution as <c>System.Threading.Tasks.Task</c> 
+        /// </summary>
+        /// <remarks>If no cancellation token is provided then the default cancellation token is used.
+        /// You may prefer using this method if you want to achive a similar behviour to async await in C# as 
+        /// async computation starts on the current thread with an ability to return a result.
+        /// </remarks>
+        /// <param name="computation">The asynchronous computation to execute.</param>
+        /// <param name="cancellationToken">The <c>CancellationToken</c> to associate with the computation.
+        /// The default is used if this parameter is not provided.</param>
+        /// <returns>A <c>System.Threading.Tasks.Task</c> that will be completed
+        /// in the corresponding state once the computation terminates (produces the result, throws exception or gets canceled)</returns>
+        /// </returns> 
+        static member StartImmediateAsTask: 
+            computation:Async<'T> * ?cancellationToken:CancellationToken-> Task<'T>
+
+
 
     [<CompiledName("FSharpAsyncBuilder")>]
     [<Sealed>]