Re: [Haskell-beginners] Re: How to create a monad instance

12 Dec 2009

      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