2012/1/23, Edward Z. Yang
Excerpts from Heka Treep's message of Mon Jan 23 15:11:51 -0500 2012:
-------------------------------------------------------------------------------- import Control.Monad.STM import Control.Concurrent import Control.Concurrent.STM.TChan
spawn f = do mbox <- newTChanIO forkIO $ f mbox return mbox
(!) = writeTChan
actor mbox = do empty <- atomically $ isEmptyTChan mbox if empty then actor mbox else do val <- atomically $ readTChan mbox
Uh, don't you want to combine isEmptyChan and readTChan into one single atomic action?
putStrLn val actor mbox
test = do mbox <- spawn actor atomically $ mbox ! "1" atomically $ mbox ! "2" atomically $ mbox ! "3"
-- > test -- 1 -- 2 -- 3 --------------------------------------------------------------------------------
But there are several problems:
* The @actor@ function is busy checking the channel all the time.
GHC's runtime system is clever. It will block appropriately.
* Caller and callee need to perform synchronizations (for the @Chan@) or atomically transactions (for the @TChan@).
The synchronization for Chan is very cheap, and you would have needed to synchronize anyway in Erlang (Erlang message queues are not lock free!)
Cheers, Edward
Ok, I have tried to write the test: -------------------------------------------------------------------------------- import Data.Maybe import Control.Monad import Control.Monad.STM import Control.Concurrent import Control.Concurrent.STM.TChan actor :: TChan String -> IO () actor mbox = forever $ do putStrLn "call to actor..." msg <- atomically $ do isEmpty <- isEmptyTChan mbox if isEmpty then return Nothing else readTChan mbox >>= return . Just when (isJust msg) $ putStrLn $ fromJust msg main :: IO () main = do -- spawn mbox <- newTChanIO tid <- forkIO $ actor mbox -- communicate atomically $ mbox `writeTChan` "1" threadDelay (10 ^ 6) atomically $ mbox `writeTChan` "2" threadDelay (10 ^ 6) atomically $ mbox `writeTChan` "3" threadDelay (10 ^ 6) killThread tid -------------------------------------------------------------------------------- -------------------------------------------------------------------------------- $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.0.20111219 $ ghc -O2 -rtsopts -threaded --make test.hs [1 of 1] Compiling Main ( test.hs, test.o ) Linking test ... $ ./test +RTS -N8 -RTS > test.log $ wc -l test.log 220804 test.log -------------------------------------------------------------------------------- looks like it performs thousands of calls to the `actor' function. When I think about an asynchronous messaging approach, then forward just a call, or at least three. Maybe I'm missing something, or just doing it wrong?