Asynchronous exeptions

[kazu-yamamoto]

Rule 1: don't catch asynchronous exceptions of others

Exceptions of others have their own semantics. If you catch them, your code does not work well. For instance, if you use the async library and catch AsyncCancelled, a thread tree created by concurrently cannot be destroyed.

If you use SomeException for catch, handle or try, check if the caught exception is asynchronous. And re-throw it via throwIO if asynchrnous. You can use the following code to check asynchronous or not.

isAsyncException :: Exception e => e -> Bool
isAsyncException e =
    case fromException (toException e) of
        Just (SomeAsyncException _) -> True
        Nothing -> False

Rule2: catch the asynchronous exception which you are using

If you don't catch, the exceptions are leaked from your threads. GHC RTS catches them and displays them to sdout.

If you are using time-manager and don't want to leak TimeoutThread, catch it.