{-# 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