F# – helper functions to convert between Async<unit> and Task

Yan Cui

I help clients go faster for less using serverless technologies.

With the official release of .Net 4.5 and Visual Studio 2012, I suspect many .Net developers will be rushing to rewrite their data access or network layers (amongst many many other things!) to take advantage of the new async-await (see the excellent 101 examples here) language feature in C#, which means you’ll likely be working with the Task and Task<T> type an awful lot.

If you have F# code that needs to interop with C# that returns or awaits some task types then you’ve probably already come across the Async.StartAsTask<T> and Async.AwaitTask<T> methods for converting between F#’s Async<T> and Task<T> types. Curiously, there are no equivalent methods on the Async class for converting between Async<unit> and Task types.

So, to fill in the gaps ourselves, here are two simple functions to do just that:

	open System.Threading.Tasks
	[<AutoOpen>]
	module Async =
	let inline awaitPlainTask (task: Task) =
	// rethrow exception from preceding task if it fauled
	let continuation (t : Task) : unit =
	match t.IsFaulted with
	| true -> raise t.Exception
	| arg -> ()
	task.ContinueWith continuation |> Async.AwaitTask
	let inline startAsPlainTask (work : Async<unit>) = Task.Factory.StartNew(fun () -> work |> Async.RunSynchronously)
	/// Usage
	> let sleepy = async {
	do! Async.Sleep(5000) // sleep for 5 seconds
	printfn "awake"
	};;
	val sleepy : Async<unit>
	sleepy |> Async.startAsPlainTask;;
	val it : Task = System.Threading.Tasks.Task {AsyncState = null;
	CreationOptions = None;
	Exception = null;
	Id = 2;
	IsCanceled = false;
	IsCompleted = false;
	IsFaulted = false;
	Status = Running;}
	// 5 seconds later
	> awake
	> let sleepy2 = async {
	do! sleepy |> Async.startAsPlainTask |> Async.awaitPlainTask
	printfn "feeling sleepy again..."
	};;
	val sleepy2 : Async<unit>
	> sleepy2 |> Async.Start;;
	val it : unit = ()
	// 5 seconds later..
	> awake
	feeling sleepy again...

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Enjoy!