{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Prelude hiding (Monad(..), Functor(..))
class Functor f where
fmap :: (a -> b) -> f a -> f b
class Functor m => Monad m where
return :: a -> m a
(>>=) :: m a -> (a -> m b) -> m b
instance Functor Maybe where
fmap f m = m >>= (return . f)
instance Monad Maybe where
return = Just
Nothing >>= _ = Nothing
Just x >>= f = f x
newtype MMaybe a = MMaybe (Maybe a)
deriving (Functor, Monad)
mjust =
MMaybe . Just
mnothing =
MMaybe Nothing
-- No instance for (GHC.Base.Monad MMaybe)
f =
do x <- mjust 7
return x
On 04/01/11 23:46, Alexey Khudyakov wrote:
> On 04.01.2011 16:38, Tony Morris wrote:
>> I think you'll find a problem using do-notation with your Monad.
>>
>> Tony Morris
>>
> Do you mean that fail is absent? That's irrelevant here.
>
> I tried to demonstrate that fmap could be defined in terms of monad
> and that definition will work.
--
Tony Morris
http://tmorris.net/
_______________________________________________
Haskell-prime mailing list
Haskell-prime@haskell.org
http://www.haskell.org/mailman/listinfo/haskell-prime
