"Alexandre Weffort Thenorio" writes:

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))

outputLine keyno key orgFile = do let part1 = getLeft keyno orgFile let part2 = getRight keyno orgFile let total = part1 ++ (strUpper key) ++ part2 ++ "\n" newHexFile <- openFileEx "newfile" (BinaryMode WriteMode) hPutStrLn newHexFile (orgFile!!0 ++ "\n" ++ total ++ unlines (drop 2 orgFile)) "let ... =" instead of "<-" because getLeft et al. aren't IO commands. (Why type error rather than syntax error then? Because getLeft returns a list, and list is a monad too. E.g., do { part1 <- getLeft keyno orgFile; return part1 } = [ part1 | part1 <- getLeft keyno orgFile ] so part1 is inferred to be a Char.) There will be another problem. The type of orgFile is expected to be String here, but the callsite gives it "lines(hexFile)" of type [String].