It's possible for code to throw an exception that itself throws an imprecise exception. Such an exception is a bit tricky to catch. For example: import Control.Exception strange = throwIO (undefined :: SomeException) `catch` \ex -> case () of _ | Just _ <- (fromException ex :: Maybe IOError) -> print "IOError" | otherwise -> print "Something else" You might think that this would catch the exception and print "Something else", but in fact it does not. If others think this is as surprising as I do, then I think we should make throwIO and throw strict, so an exception will never itself be bottom. Using throwIO' !e = throwIO e in the code above instead of throwIO allows the exception to be caught. A more conservative approach might be to just force result of toException before calling raise#, but this only works when users use an explicit type signature to fix the expression type, rather than an exception constructor. David