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>?

Is your actual method really just returning a constant? Because it doesn't make sense to do that in a way that's "actually async". If that is the case, Task.FromResult() is the right solution both in C# and F#. — svick
– svick, Commented Jun 12, 2015 at 1:59
We can assume that the value to be returned will be based off some logic. — Aaron Powell
– Aaron Powell, Commented Jun 12, 2015 at 2:16
But is that logic doing any inherently asynchronous work (e.g. accessing the file system or the network)? Or could it take relatively long time (say, more than 50 ms)? — svick
– svick, Commented Jun 12, 2015 at 2:21
Yep, file system, network, db, etc could be the async stuff that is done here. The base class also does expose a TextWriter to write to the output stream (console, network stream, etc) which is why I used it in my example there. — Aaron Powell
– Aaron Powell, Commented Jun 12, 2015 at 3:44
[slightly] Related: stackoverflow.com/questions/25605130/… — Ruben Bartelink
– Ruben Bartelink, Commented Jun 12, 2015 at 9:06

Mark Seemann · Accepted Answer · 2015-06-12 05:35:16Z

13

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>.

answered Jun 12, 2015 at 5:35

Mark Seemann

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Community · Accepted Answer · 2017-05-23 11:52:53Z

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

edited May 23, 2017 at 11:52

CommunityBot

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answered Jun 12, 2015 at 12:59

svick

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Comments

Craig Bruce · Accepted Answer · 2015-06-12 02:34:12Z

-1

I believe the FSharp way of wrapping over tasks is to use

var a = Async.AwaitIAsyncResult(somecsharpreturningtask) |> ignore

answered Jun 12, 2015 at 2:34

Craig Bruce

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