Re: [Haskell-cafe] existential types and cast

Hi Corentin, If you could explain *why* there should be a type associated to each event value, it would help, maybe. If it's a design choice , maybe it's wrong design. One reason to use dynamic typing would be to plug in new type of events. But if you already have the events semantics , this is not useful. If the language of events is complex , possibly recursive, you can use GADTs to enforce their validity by construction and you don't need to typefy the event values, but some of their characteristics. Remember type machinery is good to give correctness at the compilation time which Typeable defeats moving checks at runtime. So lifting values to types and eliminating this information with existentials and casting seems wrong. paolino 2012/7/4 Corentin Dupont

Hi Paolino, the user can add as many handlers he wants for each event. When a event is triggered along with a data, all handlers associated to that event should be triggered and passed the data. The trick is, there is one type of data associated with each event. That's why I cannot use a Event datatype: how to associate a data type to each event value? This would be some sort of dependant typing if I'm not mistaken. That's why my events exists both on type level and value level: *data NewPlayer = NewPlayer *wich allows me to associate it a typf data with type indexing.*.. * Regards Corentin

On Wed, Jul 4, 2012 at 12:58 PM, Paolino wrote:

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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

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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

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