add third exercise of lecture 4

1 files changed, 11 insertions, 0 deletions

diff --git a/lec04/hw.hs b/lec04/hw.hs
index bb5a026..923ec01 100644
--- a/lec04/hw.hs
+++ b/lec04/hw.hs
@@ -37,3 +37,14 @@ treeInsertBalanced t@(Node h tl@(Node hl _ _ _) e tr@(Node hr _ _ _)) a
 
 foldTree :: [a] -> Tree a
 foldTree = foldr (flip treeInsertBalanced) Leaf
+
+-- Exercise 3
+
+xor :: [Bool] -> Bool
+xor = odd . foldl (\s x -> if x then s+1 else s) 0
+
+map' :: (a -> b) -> [a] -> [b]
+map' f = foldr (\x s -> f x : s) []
+
+myFoldl :: (a -> b -> a) -> a -> [b] -> a
+myFoldl f base xs = foldr (flip f) base (reverse xs)