gistfile1.hs

{-# LANGUAGE FlexibleInstances, UndecidableInstances, ContextConstraints #-}

{- in response to http://www.reddit.com/r/haskell/comments/bxdi2/ask_what_would_be_your_dream_xs/c0p1r55 -}

import Control.Monad

class Func m where
  func :: m Int -> m Int

instance (Monad m) => Func m where
  func = liftM (+1)

instance (Functor m) => Func m where
  func = fmap (+2)

-- Overrides both instances above, since its context is a proper superset of the others'
instance (Monad m, Functor m) => Func m where
  func = fmap (+3)


data MyMonad a = MyMonad a
  deriving (Show)
data MyFunctor a = MyFunctor a
  deriving (Show)
data MyBoth a = MyBoth a
  deriving (Show)

instance Monad MyMonad where
  return = MyMonad
  (MyMonad x) >>= f = f x

instance Functor MyFunctor where
  fmap f (MyFunctor x) = MyFunctor $ f x

instance Functor MyBoth where
  fmap f (MyBoth x) = MyBoth $ f x

instance Monad MyBoth where
  return = MyBoth
  (MyBoth x) >>= f = f x


monad :: MyMonad Int
monad = return 1

functor :: MyFunctor Int
functor = MyFunctor 1

both :: MyBoth Int
both = MyBoth 1

main = do print $ func monad
          print $ func functor
          print $ func both

{- OUTPUT:
MyMonad 2
MyFunctor 3
MyBoth 4
-}