Grzegorz wrote:
hammingDistance [] _ = 0 hammingDistance _ [] = 0 hammingDistance (x:xs) (y:ys) | x==y = hammingDistance xs ys | otherwise = 1 + hammingDistance xs ys
hammingDistance xs ys = h xs ys 0 where h [] _ n = n h _ [] n = n h (x:xs) (y:ys) n | x==y = h xs ys n | otherwise = h xs ys $! (n+1) It is also possible to use a bang pattern on the parameter n. I'm too lazy to look up how to do it. But that is not the end of the problem.
modify (\ (sum,count) -> ((,) $! hammingDistance xs ys + sum) $! count + 1)
modify still delays state update, i.e., its code says s <- get put (f s) therefore s nor f s is evaluated now. Loop over it, and you will accumulate a thunk equivalent to f (f (f (f (... (f s)... and that costs stack. Try your own version of modify, e.g., modifies f = do s <- get put $! f s and that does it.