As usual a beginner in Haskell. Trying to write a simple program in haskel shown below main :: IO() main =catch (do hexFile <- readFile "file" putStr "What is the key number (0 or 1)?\n" keyno <- getLine putStr "Input key.\n" key <- getLine newLine <- outputLine keyno key (lines(hexFile)) bla bla bla........ getLeft :: String -> String -> String getLeft _ [] = "" getLeft key line |key == "0" = take 9 line |otherwise = take 25 line getRight :: String ->String -> String getRight _ [] = "" getRight key line |key == "0" = drop 25 line |otherwise = drop 41 line outputLine keyno key orgFile = do part1 <- getLeft keyno orgFile part2 <- getRight keyno orgFile total <- part1 ++ (strUpper key) ++ part2 ++ "\n" newHexFile <- openFileEx "newfile" (BinaryMode WriteMode) hPutStrLn newHexFile (orgFile!!0 ++ "\n" ++ total ++ unlines (drop 2 orgFile)) strUpper :: String -> String strUpper [] = "" --strUpper x:xs = (toUpper x):(strUpper xs) And I keep getting the error changecode.hs:42: Couldn't match `[a]' against `Char' Expected type: [a] Inferred type: Char In the first argument of `(++)', namely `part1' In a 'do' expression: total <- part1 ++ ((strUpper key) ++ (part2 ++ "\n")) I have tried thousands and thousand of modifications (Originally getLeft and getRight didn't exist, the code was part of outputLine) but it keeps thinking that orgFile is a String when clearlly it is a [String]. Also I can get my function strUpper (Which simply puts strings to Upper case) to work. Can anybody see what is wrong here? Best Regards Alex