2012/1/23, Edward Z. Yang
Excerpts from Heka Treep's message of Mon Jan 23 13:56:47 -0500 2012:
adding the message queue (with Chan, MVar or STM) for each process will not help in this kind of imitation.
Why not? Instead of returning a thread ID, send the write end of a Chan which the thread is waiting on. You can send messages (normal or errors) using it.
Edward
Yes, one can write this: -------------------------------------------------------------------------------- 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 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. * Caller and callee need to perform synchronizations (for the @Chan@) or atomically transactions (for the @TChan@). With exception-like messages one can write: -------------------------------------------------------------------------------- actor = receive $ \message -> case message of -- ... PM over message constructors ... -------------------------------------------------------------------------------- and then: -------------------------------------------------------------------------------- child <- spawn actor child ! MessageCon1 child ! MessageCon2 -- ... -------------------------------------------------------------------------------- where @receive@ is similar to @catch@ and (!) is similar to @throwTo@. Scheduler will be the one who will wake the actor and give him the message (well, like the @catch@ function passes an exception to a thread). No need for busy waiting on the channel or it synchronization. I can say that Erlang's concurrency works like this. As well as GHC exceptions, but they stop threads. I'm just interested in whether it is their fundamental limitation?