[Haskell-cafe] event handler
alex.solla at gmail.com
Thu Jun 14 22:05:33 CEST 2012
On Thu, Jun 14, 2012 at 12:15 PM, Corentin Dupont <corentin.dupont at gmail.com
> Hi folks,
> I'm trying to make a simple event driven engine. It simply consists of two
> - "addEvent", where you pass the event name with a callback,
> - "triggerEvent" where you pass the event name with the data.
> the data shall be passed to the callback of the corresponding event.
> I have trouble making it correctly typed.
> Here is my try:
> type Player = Int --dummy types for the example
> type Rule = Int
> data EventEnum = NewPlayer | NewRule deriving Eq
> data Data = P Player | R Rule
> data Handler = H (Data -> IO ())
> addEvent :: EventEnum -> Handler -> [(EventEnum, Handler)] -> [(EventEnum,
> addEvent e h es = (e,h):es
> triggerEvent :: EventEnum -> Data -> [(EventEnum, Handler)] -> IO ()
> triggerEvent e d es = do
> let r = lookup e es
> case r of
> Nothing -> return ()
> Just (H h) -> h d*
> The trouble is that I want the user to be only able to add an event that
> is compatible with its handler:
> For example the event NewPlayer should have a handler of type Player -> IO
> (). The data passed when triggering this event should be only of type
> How can I do that? It sound like dependant typing...
Haven't tried it, and I don't know if it actually does what you want in the
big picture. But you can do "dynamic" dependent typing with dummy (free)
*type Player = Int --dummy types for the example
type Rule = Int
data Event d = New deriving Eq -- not necessary for this example, but you
might want to enumerate other events.
*class Handled data where -- Corresponds to your "Data" type
*data Handler d = H (d -> IO ())*
*instance Handled Player*
*instance Handled Rule*
*addEvent :: (Handled d) => Event d -> Handler d -> [(Event d, Handler d)]
-> [(Event d, Handler)]*
*triggerEvent :: (Handled d) => Event d -> d -> [(Event d, Handler d)] ->
Basically, this means that Events are "keyed" into separate spaces by the
Handled types. (New :: Event Player) has a different type as (New :: Event
You might want to look into ScopedTypeVariables.
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Haskell-Cafe