Add quickcheck example with functions.

1 files changed, 30 insertions, 0 deletions

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]
+
+-}