I have a list (more precisely a string) which I'm trying to recurse through, editing sections as I come to it. It's obvious how to do it with a for loop, but I'm doing it all wrong in haskell, as this simple task is running mind-numbingly slowly. addOneNum cycles through each line, and when it gets to character 23, it calls changeNo, which edits the string. The problem is clearly the adding on to the end of the string, but the only other way I can see of doing it is writing the 'new string' backwards and then reverse it at the end. This looks like it would make the function even less legible than it already is though. addOneNum :: String -> String -> Integer -> String addOneNum x [] _ = x addOneNum x (y:ys) n = case n of 23 -> changeNo x (y:ys) n _ -> case y of '\n' -> addOneNum (x ++ [y]) ys 1 _ -> addOneNum (x ++ [y]) ys (n+1) changeNo :: String -> String -> Integer -> String changeNo v (w:x:y:z:xs) n = case length(show((read([w,x,y,z])::Integer) + 1)) of 0 -> addOneNum (v ++ " ") xs (n+4) 1 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ " ") xs (n+4) 2 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ " ") xs (n+4) 3 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1) ++ " ") xs (n+4) 4 -> addOneNum (v ++ show((read([w,x,y,z])::Integer) + 1)) xs (n+4) _ -> error "asdf" changeNo w _ n = error "changeNo error" Also, while I'm at it, it looks like I want to be using an as-pattern (or some C-style #define thing): changeNo v (w:x:y:z:xs) n = case length(s@(show((read([w,x,y,z])::Integer) + 1))) of 0 -> addOneNum (v ++ " ") xs (n+4) 1 -> addOneNum (v ++ s ++ " ") xs (n+4) 2 -> addOneNum (v ++ s ++ " ") xs (n+4) 3 -> addOneNum (v ++ s ++ " ") xs (n+4) 4 -> addOneNum (v ++ s) xs (n+4) _ -> error "asdf" but that gives me a 'not in scope' error. Any suggestions? Cheers, PhiJ