{-# LANGUAGE GeneralizedNewtypeDeriving #-}
import Test.QuickCheck
import Test.QuickCheck.Function

newtype ReversingList a = ReversingList [a]
    deriving (Show, Eq, Arbitrary)

-- This ReversingList is a Functor instance that reverses the elements while it
-- maps over them.
instance Functor ReversingList where
    fmap f (ReversingList xs) = ReversingList (reverse (fmap f xs))

-- The functor law is that (fmap f . fmap g) == fmap (f . g).
prop_functorLaw (Fun _ f) (Fun _ g) xs =
    (fmap f . fmap g) xs == fmap (f . g) xs

    where types = ( xs :: ReversingList Int
                  , f :: Int -> Int
                  , g :: Int -> Int )

main = quickCheck prop_functorLaw

{- Output:

*** Failed! Falsifiable (after 6 tests and 19 shrinks):    
{-5->1, _->0}
{0->-5, _->0}
ReversingList [1,0]

-}