gistfile1.hs

import Control.Monad
import Control.Monad.Trans.Class
import Control.Monad.IO.Class
import Data.Conduit
import Data.Conduit.List
import System.Environment
import System.IO

{- | Accept file paths on input, output opened file handle, and ensure that the
 - handle is always closed after its downstream pipe finishes whatever work on it. -}
fileConduit :: MonadResource m => IOMode -> Conduit FilePath m Handle
fileConduit mode = awaitForever process
  where
    process file = bracketP (openFile file mode) closeWithMsg yield
    closeWithMsg h = do
        putStrLn "Closing file"
        hClose h

{- | Print the first line from each handle on input. -}
firstLine :: MonadIO m => Sink Handle m ()
firstLine = awaitForever (liftIO . (hGetLine >=> putStrLn))

main = do
    args <- getArgs
    runResourceT $ sourceList args =$= fileConduit ReadMode $$ firstLine