Hello Bulat, Tuesday, April 7, 2009, 6:50:14 PM, you wrote:
tid <- forkIO (executing commands from chan...) waitQSem sem killThread tid
instead of killThread we really should send pseudo-job (like my Nothing value) that will led to self-killing of job that gets this signal
this solution still may lead to a bit more or less than N threads executed at the same time. your turn!
solved! every job should go together with Bool flag `killItself`. last job should have this flag set to True. thread will execute job and kill itself if this flag is True. so we get strong guarantees that there are exactly N threads in the system: para xs = do sem <- newQSem for (init xs) $ \x -> do writeChan chan (x `finally` signalQSem sem, False) writeChan chan (last x `finally` signalQSem sem, True) -- tid <- forkIO $ do let cycle = do (x,flag) <- readChan chan x unless flag cycle cycle -- waitQSem sem btw, this problem looks a great contribution into "Haskell way" book of exercises -- Best regards, Bulat mailto:Bulat.Ziganshin@gmail.com