On Dec 12, 2005, at 11:31 AM, Bulat Ziganshin wrote:
Hello Joel,
this code really looks strange: you asks to create global veriable, but don't say its type :) polymorhism is for functions definitions, any concrete data in Haskell have concrete type
It's a long story but I'll try to explain. I would also emphasize that the code works and the type checker complains if I try to use, say, Event String once Event Int has been used. So this is how I do it... This chunk lives in my internal libraries that I deliver to the client. I have some pre-defined events and let the user of the library come up with custom ones. data Event a = Go | Quit | ForcedQuit | NetworkError Exception | Timeout String | Cmd Command | Custom a deriving Show Yes, this looks bad but keep reading! The 'a' is the same throughout. type Child a = (MVar (), TMVar (ClockTime, (Event a)), MVar ThreadId) {-# NOINLINE children #-} children :: MVar [Child a] children = unsafePerformIO $ newMVar [] forkChild :: Show a => (TMVar (ClockTime, (Event a)) -> IO ()) -> IO ThreadId forkChild io = do mvar <- newEmptyMVar mbx <- atomically $ newEmptyTMVar childs <- takeMVar children thread <- newEmptyMVar putMVar children ((mvar, mbx, thread):childs) tid <- forkIO (io mbx `finally` putMVar mvar ()) putMVar thread tid return tid This is the poker bot state. I use the 'b' for the user data type. data World a b = World { ... dispatchers :: ![(String, Dispatcher a b)], trace_filter:: Event a -> Bool, ... user_data :: !(Maybe b) } The monad... type ScriptState a b = ErrorT String (StateT (World a b) IO) type ScriptResult a b = IO (Either String (), World a b) This is the type signature for the bot fun... type Dispatcher a b = Event a -> ((ScriptState a b) (Status a)) data Status a = Start | Eat (Maybe (Event a)) | Skip deriving Show What each bot should return. Eat means do not process any further dispatchers in the list of dispatchers kept in the bot state (World above). Skip will continue processing by calling dispatchers upstream with the same event. Dispatchers can fail thus they are in the ScriptState monad. This bit is actually exported getdata :: Show b => (ScriptState a b) b getdata = do w <- get return $ fromJust $ user_data w setdata :: b -> (ScriptState a b) () setdata b = do w <- get put_ $ w { user_data = Just $ b } launch :: (Show a, Show b) => HostInfo -> Dispatcher a b -> IO () launch hi script = do forkChild $ run hi script liftIO $ sleep_ 10 return () This is what a user "script" looks like. We are almost there, I promise! data CustomEvent = Tables [TableInfo] | LoggedIn | JoinedTable Word32 | SeatTaken Word8 | SeatNotTaken Word8 | DealerChip | Quorum deriving Show main = do initSnippets launch host script sleep_ 2000 -- 2 seconds waitToFinish Notice the call to setdata (). The type of 'b' will not be known without it and ghc will not compile the program. script Go = do startScript setdata () trace 10 "Kicking off" push "goToLobby" $ goToLobby [28] return $ Eat $ Just Go script (Custom (JoinedTable 0)) = do trace 10 "We are in the lobby" return $ Eat $ Just Quit script (Timeout _) = fail $ "Script: Timeout received" script event = do fail $ "script: event: " ++ show event return Skip Now, as soom as I use my custom event in the script, 'a' in the dispatcher signature and everywhere else will be "bound" to CustomEvent and thus the chunk of code below will be resolved. {-# NOINLINE children #-} children :: MVar [Child a] children = unsafePerformIO $ newMVar [] As soon as you try to post a different event somewhere in the script ghc will complain of a type mismatch and suggest that you use CustomEvent instead. Problem solved, everything works. Did I explain this to everyone's satisfaction? Have I supplied enough context? Is my code beautiful and efficient? Finally, does anyone have _constructive_ criticism to offer? :D Thanks, Joel -- http://wagerlabs.com/