2012/7/4 Corentin Dupont <corentin.dupont@gmail.com>

Hi,
for example, in my game (Nomic) if a new player arrives, I trigger a "NewPlayer" event. All handlers registered for that event should be triggered, and passed a structure "Player" containing all the infos of the incoming player.
If there is a new rule submitted, that the same: the event "NewRule" is triggered and the handlers are passed a structure "Rule". Thus I want the handlers registered on NewPlayer to have the type Player -> xxx, and on NewRule to have the type Rule -> xxx. I want to be able to associate an arbitrary data type (here Player and Rule) to an event.
The handlers are inherently of different types, but I want to store them in a unique list hence the existential...

On Wed, Jul 4, 2012 at 4:33 PM, Paolino <paolo.veronelli@gmail.com> wrote:

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 <corentin.dupont@gmail.com>

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 <paolo.veronelli@gmail.com> wrote:
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@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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