Hi all,
I read somewhere (here:
http://stackoverflow.com/questions/2300275/how-to-unpack-a-haskell-existential-type) that it's bad to try to unbox an existential type using a cast. OK, but without I really can't figure out how to do what I want:
data NewPlayer = NewPlayer deriving (Typeable, Eq)
data NewRule = NewRule deriving (Typeable, Eq)
class (Eq e, Typeable e) => Event e where
data EventData e
instance Event NewPlayer where
data EventData NewPlayer = P Int
instance Event NewRule where
data 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 ()How to remove the casts from triggerEvent? All that I want is to apply the handler found on the data passed in parameter.
I tried to add a function apply in the class, without success:
apply :: (EventData e -> IO ()) -> (EventData e) -> IO ()
apply = ($)Thanks!
Corentin