Here we go, Data.Vector and the same somewhat flawed comparison as in the post: next a = Just . join (,) $ (if even a then a else 3*a+1) `div` 2 len = (+1

Here we go, Data.Vector and the same somewhat flawed comparison as in the post: next a = Just . join (,) $ (if even a then a else 3*a+1) `div` 2 len = (+1) . V.length . V.takeWhile (/=1) . V.unfoldr next result = V.maximum . V.map (\x -> (len x, x)) . V.enumFromTo 1 main = do [a0] <- getArgs let max_a0 = read a0 :: Word32 print . result $ max_a0 Results: haskell 5.0s, C 4.8s. Good enough I guess? PS: and this is how you do a proper benchmarking: import Criterion.Main main = defaultMain [ bench "data.vector/1M" $ whnf result 10000000 ] with result like http://lelf.lu/tmp/criterion.html http://lelf.lu/tmp/criterion.html