Re: [Haskell-beginners] Exception Handling with Iteratees

22 Nov 2011

      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.