On Thu, Jun 14, 2012 at 12:15 PM, Corentin Dupont wrote: Hi folks,
I'm trying to make a simple event driven engine. It simply consists of two
functions:
- "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,
Handler)]
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
Player.
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 variables.
*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)] ->
IO ()*
*
*
Basically, this means that Events are "keyed" into separate spaces by the
Handled types. (New :: Event Player) has a different type as (New :: Event
Rule).
You might want to look into ScopedTypeVariables.