Small haskell program to trim whitespace in files

trim-whitespace.hs

import System.Environment (getArgs)
import System.Directory (doesFileExist)
import Data.Char (isSpace)

main :: IO ()
main = do
  args <- getArgs
  case args of
    [] -> putStrLn msg
    [_] -> putStrLn msg
    [file, outFile] -> processFile file outFile
    otherwise -> putStrLn "Too many arguments"
    where
      msg = "Not enough arguments"

type Filename = String

processFile :: Filename -> Filename -> IO ()
processFile file outFile = do
  exists <- doesFileExist file
  case exists of
    False -> putStrLn $ "File does not exist: " ++ file
    True -> do
      buf <- readFile file
      let str = foldr cleanLine [] (lines buf)
      writeFile outFile $ unlines str where
        cleanLine line xs = if success then (empty:xs) else (line:xs) where
          success = all isSpace line
          empty = ""