Sometimes the qualification of an error changes while it is propagated.

There was a discussion about this recently (on the ghc list I think)( The proposed (partial) solution is to use mapException: mapExceptionIO :: (Exception -> Exception) -> IO a -> IO a mapExceptionIO f io = Exception.catch io (\e -> throw $ f e) which would be used like: main = mapExceptionIO (\x -> ErrorCall $ showString "in main: " $ show x) $ do Exceptions can be thrown from anywhere, but can only be caught in the IO monad hence mapException must be in the IO monad. Map exception allows a kind of 'backtrace' allowing all functions in the call stack of a function which throws an exception to annotate the error message. Exceptions should only really be used for unpredictcable events, I find that the defintion of functions like head is lacking rigor... I would prefer to see: head :: [a] -> Maybe a head (a0:_) = Just a0 head _ = Nothing I think functions should only throw an exception if they are subject to the halting problem (the only way to tell if an algorith will fail is to run that algorith) - head and lots of other functions in the prelude are not subject to this, and in my opinion should really be returning Maybe... nobody said anything about me previous post about using the type system to have 'validated' types - does this mean everyone thinks its a good idea, or was it too off topic - too obvious? Keean.