hw4: exercise 3 - implement map in terms of fold

diff --git a/hw4/hw4.hs b/hw4/hw4.hs
index 780ee46864fa9404b5d4cf32f1bdeae19e8a8f30..455d8b9e6f20b7b9a78ba296ac1d7361252f896a 100644 (file)
--- a/hw4/hw4.hs
+++ b/hw4/hw4.hs
@@ -68,4 +68,16 @@ foldTree :: [a] -> Tree a
 foldTree = foldr f Leaf
     where f x acc = insert x acc
 
-
+-- exercise 3
+-- xor
+xor :: [Bool] -> Bool
+xor bs = foldr f False bs
+    where f False False = False
+          f False True  = True
+          f True False  = True
+          f True True   = False
+
+-- implement map as a fold
+map' :: (a -> b) -> [a] -> [b]
+map' f = foldr f' []
+    where f' x acc = (f x) : acc