Main.hs

import System.Environment
import System.IO
import System.IO.Error
import Control.Exception

withFiles :: [FilePath] -> IOMode -> ([Handle] -> IO a) -> IO a
withFiles paths mode f = withFiles' paths mode f []

withFiles' [] mode f handles = do
  result <- f handles
  closeFiles handles result

withFiles' (path : paths) mode f handles = do
  handle <- openFile path mode
  withFiles' paths mode f (handle : handles)

closeFiles [] res = do
  return res

closeFiles (handle : handles) res = do
  hClose handle
  closeFiles handles res

process :: Handle -> [Handle] -> IO ()
process inHandle outHandles = do
  process' inHandle outHandles 0 (length outHandles)

process' inHandle outHandles cnt totalOut = do
  input <- try(hGetLine inHandle)
  case input of
    Left e ->
      if isEOFError e
        then return ()
        else ioError e
    Right inLine ->
      do hPutStrLn (outHandles !! (cnt `mod` totalOut)) inLine
         process' inHandle outHandles (cnt + 1) totalOut


main :: IO ()
main = do
  args <- getArgs
  withFiles (tail args) WriteMode (\outHandles -> (
      withFiles [head args] ReadMode (\inHandle ->
          process (head inHandle) outHandles
        ))
    )