Actually, using cast seems to be a perfect solution here. I can't see anything wrong with it.
Отправлено с iPad
03.07.2012, в 20:33, Corentin Dupont
Hi all, I read somewhere (here: http://stackoverflow.com/questions/2300275/how-to-unpack-a-haskell-existenti...) 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 _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe