Hi guys. I've been playing with https://www.spoj.pl/ to get better with Haskell. I'm trying to do: https://www.spoj.pl/problems/PALIN/ Basically given a number k, you have to find a number bigger than k that is palindrome. The thing is that k can be 1000000 digits long. I have learnt a bunch of things doing this exercise but I'm still getting time-limit exception. Here's the code: http://gist.github.com/401880 I believe that the bottleneck is in the addOne function (Specially in the case where you have a bunch of nines). I have rewritten it to use STArray but isn't fast enough. doing: addOne $ B.replicate 500000 'a' takes too much time! Can anybody point me out of better (faster) ways to improve the code? I don't need my code fixed, just suggestions and clues about things to try. Thanks! Diego Echeverri