[Haskell-beginners] merge two handles

[Manfred Lotz]

On Tue, 14 Jun 2011 13:10:36 -0400
David McBride <dmcbride at neondsl.com> wrote:

> It probably has to do more with parenthesis than anything:
> 
> forever $ atomically $ readTchan chan >>= print
> forever $ (atomically $ readTchan chan) >>= print
> 
> That might work.  Once you get the types to line up, that should work.
> 

Thanks for the hint. I finally got it compiled and almost working. 

The code is now like this:


{-# LANGUAGE ScopedTypeVariables #-}

import GHC.Conc.Sync
import System.IO
import System.Environment
import System.Process
import Control.Monad
import Control.Concurrent.STM.TChan
import Control.Exception.Base 
import Text.Printf
import Prelude hiding (catch)



makeThread :: Handle -> TChan String -> IO ThreadId
makeThread handle chan = forkIO $
  forever
    (do eof <- hIsEOF handle                        
        unless eof $ hGetLine handle >>= atomically . writeTChan chan)
                            `catch` (\(e :: SomeException) -> return ())

issueCmd :: String -> [String] -> IO () 
issueCmd cmd parms = do
  (_ ,Just hout ,Just herr ,_) <- createProcess (proc cmd parms) {
    std_out = CreatePipe, 
    std_err = CreatePipe
  }
  chan <- newTChanIO :: IO (TChan String)
  _ <- makeThread hout chan
  _ <- makeThread herr chan
  forever $ atomically (readTChan chan) >>= printf "%s\n"

main :: IO ()
main = do
  args <- getArgs
  let cmd = head args
  let parms = tail args
  issueCmd cmd parms
  print "Done"



If I run this with a command the command's output will be printed but
after that the program is hanging, and top shows 100% cpu usage. "Done"
will never be printed.

Any idea what I have to add to prevent it from hanging?


-- 
Thanks,
Manfred