{-# LANGUAGE FlexibleInstances #-}

import Prelude hiding (Functor, fmap)

-- A functor is a structure with a map on all elements
-- (in a container structure of type constructor tc):
class Functor tc where
  fmap :: (a -> b) -> tc a -> tc b

-- type constructor class!

-- Functor instances:
instance Functor [] where
  fmap = map

-- General increment for all Functor containers:
incAll :: Functor tc => tc Int -> tc Int
incAll = fmap (+1)

-- NOTE: functor instances can be defined for any type constructor
-- expecting one type argument, i.e., also for the Either constructor
-- applied to some type variable:

-- data Either a b = Left a | Right b

--instance Functor (Either a) where
--  fmap _ (Left x)  = Left x
--  fmap f (Right x) = Right (f x)

-- > incAll (Right 41)
-- Right 42
-- > incAll (Left "Hello")
-- Left "Hello"

-- NOTE: the following instance requires a language extension set by
-- {-# LANGUAGE FlexibleInstances #-}
instance Functor (Either String) where
  fmap _ (Left x)  = Left (x ++ x)
  fmap f (Right x) = Right (f x)

-- > incAll (Left "Hello")
-- Left "HelloHello"
-- > incAll (Right 41) :: Either String Int
-- Right 42