Hello! I'm writing a program that simulates multiple processes. The processes may send/receive messages, do some work, or sleep for some amount of time. I have seen that many such things can be expressed in Haskell in very elegant manner using it functional or lazy properties. For example, client/server interaction may be expressed via inifinite lists as shown in "Gentle Introduction to Haskell". Another way of process simulation is describied in http://citeseer.ist.psu.edu/harcourt95extensible.html, where simple and concise CCS interperter is constructed. I've tried to apply the idea of infinite lists or CCS-style processes, but fail. The main reason is that: 1) messages are asynchronous (it may be received and handled while process are sleeping, without waking them) 2) in some cases received message may wake up process 3) all activity in system is ordered by time 4) there are >2 process and during simulations new processes may be created I've no idea how to implement 1, 2 in CCS interpeter. The approach of infinite lists seems to have problems with 3, 4. Have somebody any ideas how this can be solved in simple and concise way? With best regards, Alexander. PS. Currently I have some code (see below), but it is VERY UGLY. The main drawback is that is very "imperative". It employs notion of "Global state". It doesn't use features of Haskell and can be rewritten in ML or C++ without changes. Another flaws is that it is very unextensible, and all processes must have the same state. -- example code latency = 0.001 type Message = String type MsgM = WriterT [(Int, Message)] IO -- process may send messages -- process states next id = (id + 1) `mod` 3 type State = (Int, Int, Double) -- proc. number, counter, interval do_step :: State -> MsgM (Double, State) --- do something and then sleep do_step (id, cnt, delay) = do tell [(next id, "ping " ++ show id ++ " cnt " ++ show cnt)] return (delay, (id, cnt + 1, delay)) handle_message :: Message -> State -> MsgM State handle_message msg (id, cnt, delay) = do -- liftIO $ putStrLn $ show id ++ " received msg " ++ msg if msg !! 0 == 'p' then tell [(next id, "reply " ++ show id ++ " to " ++ msg)] else return () return (id, cnt, delay) -- global event queue data Event = MsgRecv Message | Work deriving Show type EventQueue = [(Double, Int, Event)] compare_event (t1, n1, e1) (t2, n2, e2) = compare t1 t2 type EventM = WriterT EventQueue IO queue_event :: Int -> Event -> Double -> EventM () queue_event dest ev time = tell [(time, dest, ev)] type FullState = Map.Map Int State handle_event :: Int -> Double -> Event -> FullState -> EventM FullState handle_event procnum time ev fullstate = do let localstate = (fullstate Map.! procnum) case ev of MsgRecv msg -> do (nstate, messages) <- lift $ runWriterT (handle_message msg localstate) sequence_ $ map (\(dst, msg) -> queue_event dst (MsgRecv msg) (time + latency)) messages return $ Map.insert procnum nstate fullstate Work -> do ((pause, nstate), messages) <- lift $ runWriterT (do_step localstate) sequence_ $ map (\(dst, msg) -> queue_event dst (MsgRecv msg) (time + latency)) messages queue_event procnum Work (time + pause) return $ Map.insert procnum nstate fullstate run_queue :: FullState -> EventQueue -> IO () run_queue st eventqueue = case eventqueue of [] -> return () (time, dest, ev) : rest -> do putStrLn $ "processing event " ++ (showFFloat (Just 3) time) "" ++ " at procnum " ++ show dest ++ " " ++ show ev (nst, nev) <- runWriterT (handle_event dest time ev st) let nqueue = foldl (\res -> \e -> insertBy compare_event e res) rest nev run_queue nst nqueue init_state = Map.fromList [(0, (0, 0, 0.3)), (1, (1, 0, 0.4)), (2, (2, 0, 0.5))] main = run_queue init_state [(0, 0, Work), (0, 1, Work)]