OK, so here's my last attempt. What do you think?
The Event class is optional (it works without because of EventData is enforcing the use of the right types) however, I find it more clear because it clearly specifies which types are events.

data NewPlayer = NewPlayer deriving (Typeable, Eq)
data NewRule = NewRule deriving (Typeable, Eq)

class (Eq e, Typeable e) => Event e
instance Event NewPlayer
instance Event NewRule

data family EventData e
data instance EventData NewPlayer = P Int
data instance EventData NewRule = R Int
instance Typeable1 EventData where
    typeOf1 _ = mkTyConApp (mkTyCon "EventData") []

data EventHandler = forall e . (Event e) => EH e (EventData e -> IO ())

addEvent :: (Event e) => e -> (EventData e -> IO ()) -> [EventHandler] -> [EventHandler]
addEvent e h ehs = (EH e h):ehs

triggerEvent :: (Event e) => e -> (EventData e) -> [EventHandler] -> IO ()
triggerEvent e d ehs = do
    let r = find (\(EH myEvent _) -> cast e == Just myEvent) ehs
    case r of
       Nothing -> return ()
       Just (EH _ h) -> case cast h of
        Just castedH -> castedH d
        Nothing -> return ()

-- TESTS
h1 :: EventData NewPlayer -> IO ()
h1 (P a) = putStrLn $ "Welcome Player " ++ (show a) ++ "!"
h2 :: EventData NewRule -> IO ()
h2 (R a) = putStrLn $ "New Rule " ++ (show a)
eventList1 = addEvent NewPlayer h1 []
eventList2 = addEvent NewRule h2 eventList1

trigger1 = triggerEvent NewPlayer (P 1) eventList2 --Yelds "Welcome Player 1!"
trigger2 = triggerEvent NewRule (R 2) eventList2 --Yelds "New Rule 2"

Thanks again!!
Corentin

On Sun, Jun 17, 2012 at 12:46 AM, Alexander Solla <alex.solla@gmail.com> wrote:

On Sat, Jun 16, 2012 at 3:31 PM, Corentin Dupont <corentin.dupont@gmail.com> wrote:

Hi Alexander,
sorry my initial example was maybe misleading. What I really what to do is to associate each event with an arbitrary data type. For example, consider the following events:
NewPlayer
NewRule
Message
User

I want to associate the following data types with each, to pass to there respective handlers:
NewPlayer ---> Player
NewRule ---> Rule
Message ---> String
User ---> String

Message and User have the same data type associated, that's why we can't use this type as a key to index the event...

In that case, you definitely want FunctionalDependencies or TypeFamilies, and will probably want to drop the constraint (Handler e d) on Event e (if it doesn't work), and maybe enforce it with explicit exports.