CODEWITHSUNDEEP
c#asynchronousf#async-await
Natalie Perret
Natalie Perret

Reputation: 8997

F#: How to "await" a function in a handler so that exception still bubble up to the handler?

I want to offload the handler used for a subscription for RabbitMQ with EasyNetQ, in C# I get what I want cause the handler itself is tagged as async and hence inside I can then await the offloading, in other words:

var subscription = bus.PubSub.Subscribe<Message>("subscriptionId", async message =>
{
    // Await Offloading
    await Task.Run(() =>
    {
        // If it throws something here, it still bubble up to the handler
    });
});

However, in F# I am completely puzzled, does the C# code can be properly translated to F#?

I started to draft something, but this is clearly not the solution

let handle (message: Message) = async{
    printfn "%A" message
}

let subscription = bus.PubSub.Subscribe<Message>("subscriptionId", fun message ->
    handle message
    |> Async.Start
    )

This F# snippet does not properly address my question, it's two-folded:

Another improper way to tackle the problem is: 

let handle (message: Message) = async{
    printfn "%A" message
}

let subscription = bus.PubSub.Subscribe<Message>("subscriptionId", fun message ->
    handle message
    |> Async.RunSynchronously
    )

But although this is awaited (unlike the snippet above), this one runs on the same thread and does not offload the work onto a threadpool thread.

Upvotes: 3

Views: 261

Answers (1)

scrwtp
scrwtp

Reputation: 13577

A close approximation of what you're looking for would be something like this:

bus.PubSub.SubscribeAsync<Message>("subscriptionId", fun message ->
        async {
            do! handle message
        }
        |> Async.StartAsTask :> Task)

async-await constructs in C# only provide syntax sugar for composing Tasks - all the .***Async methods that are now ubiquitous in .NET apis are entirely usable from F#, though sometimes the necessary plumbing code can get unwieldy. The situation there might be improving in the future though.

Upvotes: 2

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