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:

On Tue, Nov 22, 2011 at 4:35 AM, Michael Craig <mkscrg@gmail.com> wrote:
> ... but of course this doesn't compile, because the types of the LHSs in 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.