Why does this usage of Async.Parallel never complete?

Question

Running this code:

open System
async {
 printfn "a"
 do!
 Async.Parallel [|
 Async.FromContinuations ignore // Sleep forever
 async { return raise (Exception("Kaboom")) }
 |]
 |> Async.Ignore<unit array>
 printfn "b"
}
|> Async.RunSynchronously

Gives this output:

a

printfn "b" is never reached and the program appears to hang.

Why is this?

Shouldn't the exception cause the program to crash?

Answer 1

The behavior you're observing is due to how Async.Parallel and Async.FromContinuations work together in F#

Async.FromContinuations:The ignore function you passed to Async.FromContinuations effectively ignores the continuations, meaning it never completes or raises an exception.

Async.Parallel: This function runs multiple asynchronous workflows in parallel and waits for all of them to complete. The key word here is "Waits for all". If any of the workflows raise an exception,Async.Parallel will propagate the exception.

Since Async.FromContinuations ignore never completes, Async.Parallel waits indefinitely for it to finish. This causes the entire asynchronous workflow to hang, and printfn "b" is never reached.

So the only way to fix this is by ensuring that all workflows in Async.Parallel can complete or handle exceptions properly. Something like below:

open System
async {
 printfn "a"
 try
 do!
 Async.Parallel [|
 async {
 // Simulate some work
 do! Async.Sleep 1000
 }
 async {
 return raise (Exception("Kaboom"))
 }
 |]
 |> Async.Ignore<unit array>
 with
 | ex -> printfn "Exception: %s" ex.Message
 printfn "b"
}
|> Async.RunSynchronously

The first workflow simulates some work with Async.Sleep, ensuring it completes. The second workflow raises an exception, which is caught by the try...with block, allowing the program to handle the exception and continue to printfn "b" . Hope it helps