Hi,
it works very well!
I tried to implement it. Here is my test code. Apparently I need some casts
to search the list.
The syntax looks good. Only addEvent will be on the interface, so that
should be fine. If I understand well, that's this function that enforces
the right types to be used. The events are referenced via the type of data
they use.
It just bothers me a little that I'm not able to enumerate the events, and
also that the user is able to create events with wrong types (like New ::
Event String), even if they won't be able to register them.
I also have several unrelated events that use the same type of data, so
this would be a problem. Adding more events like
*data Event d = NewPlayer | NewRule deriving (Typeable, Eq)*
is not correct because I can add wrong events like:
addEvent (NewPlayer :: Event Rule) (H(undefined::(Rule -> IO()))) []
**
Also one question: I don't understand the "where" clause in your class. If
I remove it, it works the same...
Here is my code:
*newtype Player = P Int deriving Typeable
newtype Rule = R Int deriving Typeable
data Event d = New deriving (Typeable, Eq)
class (Typeable d) => Handled d where
data Handler d = H (d -> IO ())
data EventHandler = forall d . (Handled d) => EH (Event d) (Handler d)
instance Handled Player
instance Handled Rule
addEvent :: (Handled d) => Event d -> Handler d -> [EventHandler] ->
[EventHandler]
addEvent e h ehs = (EH e h):ehs
triggerEvent :: (Handled d) => Event d -> d -> [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 h)) -> case cast h of
Just castedH -> castedH d
Nothing -> return ()
h1 :: Player -> IO ()
h1 (P a) = putStrLn $ "Welcome Player " ++ (show a) ++ "!"
h2 :: Rule -> IO ()
h2 (R a) = putStrLn $ "New Rule " ++ (show a)
eventList1 = addEvent (New :: Event Player) (H h1) []
eventList2 = addEvent (New :: Event Rule) (H h2) eventList1
trigger1 = triggerEvent (New :: Event Player) (P 1) eventList2 -- yelds
"Welcome Player 1!"
trigger2 = triggerEvent (New :: Event Rule) (R 2) eventList2 --yelds "New
Rule* 2"
Best,
Corentin
On Fri, Jun 15, 2012 at 12:40 AM, Alexander Solla
On Thu, Jun 14, 2012 at 2:04 PM, Corentin Dupont < corentin.dupont@gmail.com> wrote:
That look really nice! Unfortunately I need to have an heterogeneous list of all events with their handlers. With this test code it won't compile:
test1 = addEvent (New :: Event Player) (H (undefined::(Player -> IO ()))) [] test2 = addEvent (New :: Event Rule) (H (undefined::(Rule -> IO ()))) test1
Right, okay. Heterogenous lists are tricky, but I think we can get away with using ExistentialQuantification, since you seem to only want to dispatch over the heterogenous types. The assumption I made is a big deal! It means you can't extract the d value. You can only apply properly typed functions (your handlers) on it.
* {-# LANGUAGE ExistentialQuantification #-} *
* type Player = Int *
* type Rule = Int *
* data Event d = New d * * *
*class Handled data where -- Together with EventHandler, corresponds to your "Data" type* * *
*data EventHandler = forall d . (Handled d) => EH (Event d) (d -> IO ()) -- EventHandler takes the place of your (Event d, Handler d) pairs without referring to d.*
* *
*instance Handled Player*
*instance Handled Rule*
*addEvent :: (Handled d) => Event d -> Handler d -> [EventHandler] -> [EventHandler] -- Every [EventHandler] made using addEvent will be of "correct" types (i.e., preserve the typing invariants you want), but YOU must ensure that only [EventHandler]s made in this way are used. This can be done statically with another type and an explicit export list. We can talk about that later, if this works in principle.*
*triggerEvent :: (Handled d) => Event d -> d -> [EventHandler] -> IO ()*