Hi folks, I'm trying to make a simple event driven engine. It simply consists of two functions: - "addEvent", where you pass the event name with a callback, - "triggerEvent" where you pass the event name with the data. the data shall be passed to the callback of the corresponding event. I have trouble making it correctly typed. Here is my try: * type Player = Int --dummy types for the example type Rule = Int data EventEnum = NewPlayer | NewRule deriving Eq data Data = P Player | R Rule data Handler = H (Data -> IO ()) addEvent :: EventEnum -> Handler -> [(EventEnum, Handler)] -> [(EventEnum, Handler)] addEvent e h es = (e,h):es triggerEvent :: EventEnum -> Data -> [(EventEnum, Handler)] -> IO () triggerEvent e d es = do let r = lookup e es case r of Nothing -> return () Just (H h) -> h d* The trouble is that I want the user to be only able to add an event that is compatible with its handler: For example the event NewPlayer should have a handler of type Player -> IO (). The data passed when triggering this event should be only of type Player. How can I do that? It sound like dependant typing... Thanks! Corentin