2016 Update

Task/Ackermann-function/Haskell/ackermann-function-2.hs

@@ -1,9 +1,10 @@
+import Data.List (mapAccumL)
+
 -- everything here are [Int] or [[Int]], which would overflow
 -- * had it not overrun the stack first *
 ackermann = iterate ack [1..] where
 	ack a = s where
-		s = a!!1 : f (tail a) (zipWith (-) s (1:s))
-	f a (b:bs) = (head aa) : f aa bs where
-		aa = drop b a
+		s = snd $ mapAccumL f (tail a) (1 : zipWith (-) s (1:s))
+	f a b = (aa, head aa) where aa = drop b a
 
 main = mapM_ print $ map (\n -> take (6 - n) $ ackermann !! n) [0..5]