Implementing C# method returning Task<T> in F#

Question

I'm creating a type in F# that inherits from a C# class that exposes a method that returns Task<T> in C#. I'm trying to work out what'd be the best way to do that in F#

Say my C# looks like this:

public class Foo {
    public TextWriter Out { get { return Console.Out; } }

    public virtual Task<SomeEnum> DoStuff() {
        return Task.FromResult(SomeEnum.Foo);
    } 
}

public enum SomeEnum {
    Foo,
    Bar
}

My first pass of inheriting that type in F# looks like so:

type Bar() =
    inherits Foo()

    override this.DoStuff() =
        this.Out.WriteLineAsync("hey from F#") |> ignore

        System.Threading.Task.FromResult(SomeEnum.Bar)

But a) it doesn't feel like it's actually async and b) it just feels not-F#.

So how would I go about inheriting the Foo class and implement the DoStuff method that expects to return a Task<T>?

Answer 1

You can use Async.StartAsTask:

type Bar() =
    inherit Foo()
    override this.DoStuff() =
        async { return SomeEnum.Bar } |> Async.StartAsTask

Async.StartAsTask takes Async<T> as input, and returns a Task<T>.

Answer 2

The F# way of doing async is using asynchronous workflows. Unfortunately, they don't support awaiting non-generic Tasks. But using one of the workarounds from the above question, your code could look like this:

override this.DoStuff() =
    async {
        do! this.Out.WriteLineAsync("hey from F#") |> Async.AwaitVoidTask
        return SomeEnum.Bar
    } |> Async.StartAsTask