slow copy: using zipped conduit sinks

slowcp.hs

import Control.Concurrent 
import Control.Monad
import Control.Monad.Trans
import qualified Data.ByteString as B
import Data.Conduit 
import Data.Conduit.Binary as CB
import qualified Data.Conduit.List as CL
import qualified Data.Conduit.Util as CU
import System.Environment
import System.Exit
import System.IO

waitSink :: Int -> Int -> Sink B.ByteString IO Int
waitSink nbytes niter = do 
  CB.drop nbytes 
  mp <- CL.peek
  liftIO $ putStrLn $ "iteration number = " ++ show niter 
  liftIO $ threadDelay 50000 
  case mp of 
    Nothing -> return niter
    Just _ -> waitSink nbytes (niter+1) 

main = do
  args <- getArgs 
  when (length args /= 2) $ do 
    putStrLn "num of args must be 2"
    exitWith (ExitFailure 1) 
  let srcfile = args !! 0 
      tgtfile = args !! 1 

  putStrLn $ "slow copying " ++ srcfile ++ " to " ++ tgtfile  

  ih <- openFile srcfile ReadMode
  oh <- openFile tgtfile WriteMode 
  sourceHandle ih $$ CU.zipSinks (waitSink 1024 0) (sinkHandle oh) 
  -- r <- sourceHandle ih $$ waitSink 1024 0 
  -- print r 
  hClose oh 
  hClose ih