Excerpts from Heka Treep's message of Mon Jan 23 16:20:51 -0500 2012:
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
There are several things wrong with this: - You're only synchronizing over one variable: use an Chan, not a TChan (if you make that change, btw, it will automatically start working.) - You don't want the transaction to succeed in all cases; you only want the transaction to succeed if you manage to get something from the TChan. What you've done is convert the interface from blocking to non-blocking, but in a busy-loop. - You don't even need to run isEmptyTChan. Just do a readTChan. It will block if there is nothing in the chan (technically, it will ask the STM transaction to retry, but STM is clever enough not to try running again unless any of the touched STM variables changes.) Edward