{-# LANGUAGE InstanceSigs #-}
class Monoid' a where
  mempty' :: a
  mappend' :: a -> a -> a

class MyFunctor f where
  my_fmap :: (a -> b) -> f a -> f b


instance MyFunctor Maybe where
  my_fmap :: (a -> b) -> Maybe a -> Maybe b
  my_fmap f Nothing = Nothing
  my_fmap f (Just x) = Just (f x)
  
instance MyFunctor [] where
  my_fmap :: (a -> b) -> [a] -> [b]
  my_fmap f [] = []
  my_fmap f (x:xs) = f x : my_fmap f xs

class (MyFunctor m) => MyMonad m where
  my_return :: a -> m a
  my_join :: m (m a) -> m a

instance MyMonad [] where
  my_return :: a -> [a]
  my_return x = [x]

  my_join :: [[a]] -> [a]
  my_join = concat

my_bind :: (MyMonad m) => m a -> (a -> m b) -> m b
my_bind x f = my_join (my_fmap f x)

join' :: (Monad' m) => m (m a) -> m a
join' c = bind' c (\ x -> x)  

class (MyFunctor m) => Monad' m where
  return' :: a -> m a
  bind' :: m a -> (a -> m b) -> m b

instance Monad' Maybe where
  return' :: a -> Maybe a
  return' x = Just x

  bind' :: Maybe a -> (a -> Maybe b) -> Maybe b
  bind' Nothing f = Nothing
  bind' (Just x) f = f x


instance Monad' [] where
  return' :: a -> [a]
  return' x = [x]

  bind' :: [a] -> (a -> [b]) -> [b]
  bind' [] f = []
  bind' (x:xs) f = f x ++ bind' xs f