2009/12/12 Maciej Piechotka :

On Sat, 2009-12-12 at 01:17 +0100, Marco De Oliveira wrote:

...

Hi,

Is it possible to create an instance of Monad for IOException? Each time a try I stay blocked by IO monad.

with the definition:

data IOException a = IOExceptionCons (IO (Exception a))

data Exception a = SuccessCons (Maybe Warning) a                           | ErrorCons Error

data Warning = WarningCons1                     | WarningCons2

data Error = ErrorCons1                 | ErrorCons2

Regards

Yes - to begin with:

newtype IOException a = IOException {runIOException :: IO (Exception a)}

data Exception a = Success (Maybe Warning) a                 | Exception Error

data Warning = Warning1             | Warning2 data Error = Error1           | Error2

instance Monad Exception where    return = Success Nothing    (Success w v) >>= f = f v --    (Success w v) >>= f = case f v of --                            Success _ v' -> Success w v' --                            Exception e  -> Exception e    (Exception e) >>= _ = Exception e instance Monad IOException where    return = IOException . return . return    m >>= f = IOException $ runIOException m >>= runIOException . f

However: 1. What should be the combination of warnings I'd rather threat them as some 'proper' monoid (for ex. List instead of Maybe) 2. It is much easier to use mtl: type IOException = ErrorT (Either Error) (WriterT [Warning] IO)

Regards

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Thanks for your help. I have try your code ghci and I think there is a mistake in the definition of (>>=) operator. test1 m f = IOException $ runIOException m >>= runIOException . f :t test1 test1 :: IOException a -> (Exception a -> IOException a) -> IOException a I think the defintion is: test :: IOException a -> ( a -> IOException a) -> IOException a 1. why I am not using monoid? I try to bind C code and the function return only one warning. But if I change i little the structure of IOException, it may have sense to use a List. newtype IOException a = IOException {runIOException :: ([Warning] -> IO (Exception a))} 2. Sorry but I have a lot of difficulties to understand the concept of Monad Transformers