[Haskell-beginners] function to read file and return list of contents not compiling

Haskellers: I want to write a function that takes a class name and reads from a file and then returns the list of String of the file lines. I have experimented with various return types, but GHC does not like any of them. Here is my function: readData :: String -> IO String -- what type here?? readData classNameP = do let fileName = classNameP ++ ".txt" contents <- readFile fileName lines contents Not to complain, but this would not be difficult in any other language I have tried, so I could use some explanation. I suspect it has to do with understanding the IO Monad. Many Thanks Geoffrey