Dears Haskellers, As an Haskell newbie, I'm learning Haskell by trying to resolve Euler Project problems (http://projecteuler.net/ ). I'm hanging on problem 14 (Collatz problem). I've written the following program... Which does not end in a reasonable time :( My algorithm seems ok to me but I see that memory consumption is gigantic... Is this a memory problem with Data.Map ? Or an infinite loop ? (Where ?) In a more general way, how can I troubleshoot these kind of problem ? Here's the code : import qualified Data.List as List import qualified Data.Map as Map f n | even n = n `div` 2 | otherwise = 3 * n + 1 chain m n = let chain' cn cm | Map.member cn m = Map.map (+ (m Map.! cn)) cm | otherwise = chain' (f cn) $! Map.insert cn 1 (Map.map (+1) cm) in chain' n Map.empty chains n = List.foldl' (\m i -> Map.union m (chain m i)) (Map.singleton 1 1) [2..n] maxCollatz c1@(_,l1) c2@(_,l2) | l1 < l2 = c2 | otherwise = c1 maxChain = List.foldl' maxCollatz (0,0) . Map.toList . chains main = let n = 1000000 in putStrLn $ show $ maxChain n Hope someone can help me, I really don't see what is th problem... Best regards, Bruno.