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 <corentin.dup...@gmail.com> написал(а): > 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 > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe@haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe
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