[Haskell-cafe] existential types and cast
paolo.veronelli at gmail.com
Wed Jul 4 16:33:33 CEST 2012
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.
2012/7/4 Corentin Dupont <corentin.dupont at 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
> 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.*..
> On Wed, Jul 4, 2012 at 12:58 PM, Paolino <paolo.veronelli at gmail.com>wrote:
>> 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.
>> 2012/7/3 Corentin Dupont <corentin.dupont at gmail.com>
>>> 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]
>>> -> [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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>>> Haskell-Cafe at haskell.org
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