[Haskell-cafe] existential types and cast

[Paolino]

Hi
How many handlers for each type of event in the list of handlers ?
If you have only one handler for each type , it should go in the typeclass,
and you don't need typeable.
If you have more than one maybe you can avoid using type indexing at all,
because it doesn't resolve the handler selection issue.
By the way , it's not clear to me why you don't have a simple Event
datatype describing all the possible events in advance.

Regards

paolino

2012/7/3 Corentin Dupont <corentin.dupont at gmail.com>

> 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
>
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