From 475872efcb9bc8496cedcbf4f978fab44402c046 Mon Sep 17 00:00:00 2001
From: Kjetil Orbekk <kjetil.orbekk@gmail.com>
Date: Fri, 11 Mar 2016 03:43:39 +0100
Subject: Add quickcheck example with functions.

---
 qcheck.hs | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 qcheck.hs

diff --git a/qcheck.hs b/qcheck.hs
new file mode 100644
index 0000000..382179b
--- /dev/null
+++ b/qcheck.hs
@@ -0,0 +1,30 @@
+{-# 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]
+
+-}
-- 
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