It is interesting, that the naive implementation import Data.List (tails) neighbours :: Int -> [a] -> [[a]] neighbours w = rotL . take w . map (take 3) . tails . cycle rotL :: [a] -> [a] rotL xs = last xs : init xs type Rule a = [a] -> a step :: Int -> Rule a -> [a] -> [a] step w f = map f . neighbours w rule110 :: Rule Char rule110 " " = ' ' rule110 "X " = ' ' rule110 "XXX" = ' ' rule110 _ = 'X' main = let f = step 149 rule110 init = replicate 148 ' ' ++ "X" in mapM_ putStrLn $ take 10000 $ iterate f init is only 3 times slower than your quite complex, hard to follow and hard to debug implementation. As always, I prefer to write most code in Haskell, quick, easy, nice, reasonable fast, ... If speed matters, I switch to some lower level language, as you did staying inside Haskell. /BR, Mirko Rahn