This has to be one of the most irritating ways a program can fall over. Can't the Haskell RTS try just a /little/ harder to help the poor programmer? For example by saying what sort of exception it is, and (if it's a dynamic exception) what type it has?
An "unknown exception" is a dynamic exception. GHC just uses the ordinary instance of Show on Exception to print out the value of an exception, and it so happens that all it prints out for a dynamic exception is "unknown exception". Arguably the show instance should print out the type too. It could; I just looked into it and unfortunately there's an annoying module loop in the libraries which means doing this would entail some restructuring. Why can't GHCi just print out the type itself? Because it has to be done in the context of the interpreted program, which means GHCi would have to compile a little snippet of code to show the exception. Annoying, but doable. Cheers, Simon