Haskell program to demux input

demux.hs

#!/usr/bin/env runhaskell

import System.Environment
import System.Exit
import System.IO

usage = "usage: cat somefile | demux outfile1 outfile2 ..."
help = "This program alternates sending its input to outfile1 and outfile2."

main = do
    args <- getArgs
    case args of
        ["-h"] -> doHelp
        ["--help"] -> doHelp
        [] -> doUsage
        [_] -> doUsage
        _ -> do
            outfiles <- mapM (\path -> openFile path WriteMode) args
            contents <- getContents
            demux outfiles . lines $ contents
            mapM_ hClose outfiles

doUsage = do
    putStrLn usage
    exitFailure

doHelp = do
    putStrLn usage
    putStrLn help
    exitSuccess

demux :: [Handle] -> [String] -> IO ()
demux outfiles lines =
    zipWithM_ hPutStrLn (cycle outfiles) lines