That works well, but is there an extension free way of doing this cleanly? I tried handleErrors :: (Monad m, Exception e) => Maybe e -> Iteratee a m Response handleErrors (Just POSTOnlyException) = return "POSTs only!" handleErrors (Just BadPathException) = return "Bad path!" handleErrors _ = return "Unknown exception!" app = catchError myApp (handleErrors . fromException) But this won't compile because GHC "Couldn't match type `POSTOnlyException' with `BadPathException'". I think I'm settling towards something like this: data MyAppException = POSTOnlyException | BadPathException deriving ( Show, Typeable ) instance Exception MyAppException handleErrors :: (Monad m) => SomeException -> Iteratee a m Response handleErrors = hErr . fromException where hErr (Just POSTOnlyException) = return "POSTs only!" hErr (Just BadPathException) = return "Bad path!" hErr Nothing = return "Unknown exception!" Mike S Craig On Tue, Nov 22, 2011 at 1:42 AM, Felipe Almeida Lessa < felipe.lessa@gmail.com> wrote:

...

... but of course this doesn't compile, because the types of the LHSs in

On Tue, Nov 22, 2011 at 4:35 AM, Michael Craig wrote: the

...

case statement are different. I can get around it with some ugliness ... handleErrors :: SomeException -> Iteratee a m String handleErrors ex = case fromException ex of Just POSTOnlyException -> return "POSTs only!" _ -> case fromException ex of Just BadPathException -> return "Bad path!" _ -> return "Unknown exception!" ... but there must be a better way. Enlighten me?

If you enable the ViewPatterns extension

{-# LANGUAGE ViewPatterns #-}

then you can write handleErrors as

handleErrors :: SomeException -> Iteratee a m String handleErrors (fromException -> Just POSTOnlyException) = return "POSTs only!" handleErrors (fromException -> Just BadPathException) = return "Bad path!" handleErrors _ = return "Unknown exception!"

Cheers,

-- Felipe.