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DefaultSignaturesを用いたMonadの理想的な定義

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{-# LANGUAGE DefaultSignatures #-}                         

import Prelude hiding (Functor(..), Monad(..))

class Functor m where                                      
  fmap :: (a -> b) -> m a -> m b                           
  default fmap :: Applicative m => (a -> b) -> m a -> m b  
  fmap f v = pure f <$> v

class Functor m => Applicative m where                     
  pure :: a -> m a                                         
  default pure :: Monad m => a -> m a                      
  pure = return                                            

  (<$>) :: m (a -> b) -> m a -> m b
  default (<$>) :: Monad m => m (a -> b) -> m a -> m b
  a <$> b = a >>= \f -> b >>= \v -> return (f v)

class Applicative m => Monad m where
  return :: a -> m a
  (>>=) :: m a -> (a -> m b) -> m b

instance Monad [] where
  return a = [a]
  m >>= f = concatMap f m
instance Applicative []
instance Functor []

main :: IO ()
main = do
  print $ [(+1), (*2)] <$> [1, 2, 3]
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