[Haskell-beginners] Exception Handling with Iteratees

[Michael Craig]

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 at gmail.com> wrote:

> On Tue, Nov 22, 2011 at 4:35 AM, Michael Craig <mkscrg at 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.
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/beginners/attachments/20111122/eb47765d/attachment.htm>