InteractWith.hs

import System.Environment (getArgs)

interactWith function inputFile outputFile = do
  input <- readFile inputFile
  writeFile outputFile (function input)

main = mainWith myFunction
    where mainWith fn = do
            args <- getArgs
            case args of
              [input,output] -> interactWith fn input output
              _ -> putStrLn "error: exactly two arguments needed"
          myFunction = fixLines -- ここを= idとか= aaaLinesに替える

aaaLines :: String -> String
aaaLines [] = []
aaaLines (x:xs) =
    case x of
      '\r' -> '\r'
      '\n' -> '\n'
      ' '  -> ' '
      _    -> 'a'
    : aaaLines xs

splitLines :: String -> [String]
splitLines [] = []                        
splitLines cs =
    let (pre,suf) = break isLineTerminator cs
    in pre : case suf of
               ('\r':'\n':rest) -> splitLines rest
               ('\r':rest)      -> splitLines rest
               ('\n':rest)      -> splitLines rest
               _                -> []

isLineTerminator c = c == '\r' || c == '\n'

fixLines :: String -> String
fixLines input = unlines (splitLines input)