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Today's Topics:
1. Exception Handling with Iteratees (Michael Craig)
2. Re: Exception Handling with Iteratees (Felipe Almeida Lessa)
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Message: 1
Date: Tue, 22 Nov 2011 01:35:46 -0500
From: Michael Craig <[email protected]>
Subject: [Haskell-beginners] Exception Handling with Iteratees
To: [email protected]
Message-ID:
<caha9zaetyazaik44qozb1nbpudri0drhscjwmu3dc0uwglk...@mail.gmail.com>
Content-Type: text/plain; charset="iso-8859-1"
I have some exception types defined ...
data POSTOnlyException = POSTOnlyException
deriving ( Show, Typeable )
instance Exception POSTOnlyException
data BadPathException = BadPathException
deriving ( Show, Typeable )
instance Exception BadPathException
... and I want to use Data.Enumerator.catchError ...
catchError :: Monad m => Iteratee a m b -> (SomeException -> Iteratee a
m b) -> Iteratee a m b
... so I define an error handler ...
handleErrors :: SomeException -> Iteratee a m String
handleErrors ex = case fromException ex of
Just POSTOnlyException -> return "POSTs only!"
Just BadPathException -> return "Bad path!"
_ -> return "Unknown exception!"
... 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?
Cheers,
Mike S Craig
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Message: 2
Date: Tue, 22 Nov 2011 04:42:14 -0200
From: Felipe Almeida Lessa <[email protected]>
Subject: Re: [Haskell-beginners] Exception Handling with Iteratees
To: Michael Craig <[email protected]>
Cc: [email protected]
Message-ID:
<CANd=ogejjdp9ejhurg15khlha53sdiy7kaxdgqu5ixscvan...@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
On Tue, Nov 22, 2011 at 4:35 AM, Michael Craig <[email protected]> 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.
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End of Beginners Digest, Vol 41, Issue 29
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