F# How Async<'T> cancellation works?

Question

I was pretty comfortable with how async cancellations where done in C# with the TPL, but I am a little bit confused in F#. Apparently by calling Async.CancelDefaultTok

Answer 1

There are two things about the question:

• First, when a cancellation happens in F#, the AwaitTask does not return null, but instead, the task throws OperationCanceledException exception. So, you do not get back None value, but instead, you get an exception (and then F# also runs your finally block).

  The confusing thing is that cancellation is a special kind of exception that cannot be handled in user code inside the async block - once your computation is cancelled, it cannot be un-cancelled and it will always stop (you can do cleanup in finally). You can workaround this (see this SO answer) but it might cause unexpected things.

• Second, I would not use default cancellation token - that's shared by all async workflows and so it might do unexpected things. You can instead use Async.CancellationToken which gives you access to a current cancellation token (which F# automatically propagates for you - so you do not have to pass it around by hand as you do in C#).

EDIT: Clarified how F# async handles cancellation exceptions.