F# async try with not catching exceptions

Question

Strange things... I just wanted to do a simple retry on exceptions in F# but the catch doesn't catch :) Any ideas?

let rec retry times next event =
 async {
 try
 return! next event
 with
 | _ when times > 0 -> return! retry (times - 1) next event
 | error -> return error.Reraise()
 } 

if the next is a function like; let handler evt = async { failwith "Oh-no" } Then the code in try executes but it is not catched. What is going on? :O

UPDATE

The reraise is an extension method as described here: https://github.com/fsharp/fslang-suggestions/issues/660 by nikonthethird.

type Exception with
 member this.Reraise () =
 (ExceptionDispatchInfo.Capture this).Throw ()
 Unchecked.defaultof<_>

Answer 1

Your code does catch the exceptions. Here's the full program I'm running to test it:

let rec retry times next event =
 async {
 try
 printfn "Retry: %A" times
 return! next event
 with
 | _ when times > 0 -> return! retry (times - 1) next event
 | error -> raise error
 }
let handler evt =
 async {
 printfn "About to fail once"
 failwith "Oh-no"
 }
[<EntryPoint>]
let main argv =
 retry 3 handler ()
 |> Async.RunSynchronously
 |> printfn "%A"
 0

Output:

Retry: 3
About to fail once
Retry: 2
About to fail once
Retry: 1
About to fail once
Retry: 0
About to fail once
Unhandled exception. System.Exception: Oh-no

You can see that the exceptions are being caught, because handler is invoked multiple times before retry gives up.

Notes:

• I replaced return error.Reraise() with raise error, since Exception.Reraise isn't a defined method. I'm not sure what you had in mind here, but it doesn't directly affect the answer to your question.
• It's important to fully invoke retry with all three arguments (I used () as the "event"), and then run the resulting async computation synchronously. Maybe you weren't doing that?
• You might want to look into using Async.Catch for handling async exceptions instead.

• JL0PD: it's recommended to use rethrow() or reraise() instead of raise exception , because latter changes stacktrace
• Brian Berns: Actually, reraise isn't allowed here, presumably because it's inside a computation expression. (The compiler error isn't clear: Calls to 'reraise' may only occur directly in a handler of a try-with.) In any case, as I mentioned, it's not directly relevant to OP's question.
• Jinx: So I think I have to apologize, it works indeed. My test case running this code had the bug :D This remind me of a question; Who gonna test your tests? Thank You @brianberns for Your time on that.
• Jinx: Thx for Async.Catch - didn't know that one! Will check it.