From: Ramakrishnan Muthukrishnan <ram@rkrishnan.org>
Date: Sat, 20 Dec 2014 07:20:36 +0000 (+0530)
Subject: hw4: exercise 3 - implement map in terms of fold
X-Git-Url: https://git.rkrishnan.org/pf/content/en/seg/priv/nxhtml.html?a=commitdiff_plain;h=5adf189982080ac960af73cf6bbfa9f973ed5910;p=yorgey.git

hw4: exercise 3 - implement map in terms of fold
---

diff --git a/hw4/hw4.hs b/hw4/hw4.hs
index 780ee46..455d8b9 100644
--- 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