This does not work either. It should cover all the bases, right? Fork off input, pull things from ouput as they are ready, stop when we reach end of file. If you remove the line `print partial`, the program loops forever; if you keep it, the program stops right there. -- _jsn import Data.ByteString.Lazy hiding (putStrLn) import System.Process import System.Environment import System.IO (putStrLn, hClose, hWaitForInput) import System.IO.Error import Control.Concurrent import Prelude hiding (writeFile, readFile) main = do exe:file:_ <- getArgs bytes <- readFile file foo <- simple exe bytes [] writeFile (file ++ ".foo") foo -- Manufactures a simple stream handler from a command line utility. simple :: String -> ByteString -> [String] -> IO ByteString simple exe bytes args = do (i, o, e, p) <- runInteractiveProcess exe args Nothing Nothing pushAndPull i o bytes pushAndPull i o bytes = do putStrLn "Working with:" print bytes forkIO $ hPut i bytes putStrLn "forked" readUntilDone empty where readUntilDone soFar = do (const $ return soFar) `hctac` do putStrLn "hctac" hWaitForInput o 0 -- Wait as long as it takes. putStrLn "waited" partial <- hGetContents o putStrLn "contents:" print partial readUntilDone $ append soFar partial where hctac = flip catch