Example of checking a law for applicative functors using QuickCheck

applicative_quickcheck.hs

#!/usr/bin/env runhaskell

import Control.Applicative
import Test.QuickCheck

newtype ZipList2 a = ZipList2 { getZipList :: [a] }
instance Applicative ZipList2 where
    pure x = ZipList2 $ repeat x
    (ZipList2 gs) <*> (ZipList2 xs) = ZipList2 (zipWith ($) gs xs)

instance Functor ZipList2 where
        fmap f (ZipList2 xs) = ZipList2 (map f xs)

main = quickCheck prop_law

-- See pages 25 and 26 of the Typeclassopedia.
-- http://www.haskell.org/wikiupload/8/85/TMR-Issue13.pdf
prop_law :: [Int] -> Bool
prop_law x = (fmap g x) == ((pure g) <*> x)
    where g = id