Hi all, I have a simple piece of code which is giving me stack overflow. I guess I need to make it stricter sowhere but I can't figure out extactly where. So I thought I'd ask the experts. import Data.List (foldl') import Control.Monad.State.Strict hammingDistance [] _ = 0 hammingDistance _ [] = 0 hammingDistance (x:xs) (y:ys) | x==y = hammingDistance xs ys | otherwise = 1 + hammingDistance xs ys meanHammingDistanceM xss yss = evalState (mhd xss yss) (0,0) mhd xss yss = do for xss $ \xs -> for yss $ \ys -> do modify (\ (sum,count) -> ((,) $! hammingDistance xs ys + sum) $! count + 1) (sum,count) <- get return $ fromIntegral sum/fromIntegral count where for = flip mapM_ -- Grzegorz