[Haskell-cafe] existential types and cast

Corentin Dupont corentin.dupont at gmail.com
Tue Jul 3 18:33:57 CEST 2012

Hi all,
I read somewhere (here:
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] ->
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 = ($)*

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