If I understand, the SomeEvent event acts as a proxy to hide the diversity
of the events? That's interesting.
This way I don't have to use an heterogeneous list and a lot of casting...
On Wed, Sep 12, 2012 at 7:44 AM,
Let me see if I understand. You have events of different sorts: events about players, events about timeouts, events about various messages. Associated with each sort of event is a (potentially open) set of data types: messages can carry payload of various types. A handler specifies behavior of a system upon the reception of an event. A game entity (player, monster, etc) is a collection of behaviors. The typing problem is building the heterogeneous collection of behaviors and routing an event to the appropriate handler. Is this right?
There seem to be two main implementations, with explicit types and latent (dynamic) types. The explicit-type representation is essentially HList (a Type-indexed Record, TIR, to be precise). Let's start with the latent-type representation. Now I understand your problem better, I think your original approach was the right one. GADT was a distraction, sorry. Hopefully you find the code below better reflects your intentions.
{-# LANGUAGE ExistentialQuantification, DeriveDataTypeable #-} {-# LANGUAGE StandaloneDeriving #-}
import Data.Typeable
-- Events sorts
data Player = Player PlayerN PlayerStatus deriving (Eq, Show, Typeable)
type PlayerN = Int data PlayerStatus = Enetering | Leaving deriving (Eq, Show)
newtype Message m = Message m deriving (Eq, Show)
deriving instance Typeable1 Message
newtype Time = Time Int deriving (Eq, Show, Typeable)
data SomeEvent = forall e. Typeable e => SomeEvent e deriving (Typeable)
-- They are all events
class Typeable e => Event e where -- the Event predicate what_event :: SomeEvent -> Maybe e what_event (SomeEvent e) = cast e
instance Event Player instance Event Time instance Typeable m => Event (Message m)
instance Event SomeEvent where what_event = Just
-- A handler is a reaction on an event -- Given an event, a handler may decline to handle it type Handler e = e -> Maybe (IO ())
inj_handler :: Event e => Handler e -> Handler SomeEvent inj_handler h se | Just e <- what_event se = h e inj_handler _ _ = Nothing
type Handlers = [Handler SomeEvent]
trigger :: Event e => e -> Handlers -> IO () trigger e [] = fail "Not handled" trigger e (h:rest) | Just rh <- h (SomeEvent e) = rh | otherwise = trigger e rest
-- Sample behaviors
-- viewing behavior (although viewing is better with Show since all -- particular events implement it anyway)
view_player :: Handler Player view_player (Player x s) = Just . putStrLn . unwords $ ["Player", show x, show s]
-- View a particular message view_msg_str :: Handler (Message String) view_msg_str (Message s) = Just . putStrLn . unwords $ ["Message", s]
-- View any message view_msg_any :: Handler SomeEvent view_msg_any (SomeEvent e) | (tc1,[tr]) <- splitTyConApp (typeOf e), (tc2,_) <- splitTyConApp (typeOf (undefined::Message ())), tc1 == tc2 = Just . putStrLn . unwords $ ["Some message of the type", show tr] view_msg_any _ = Nothing
viewers = [inj_handler view_player, inj_handler view_msg_str, view_msg_any]
test1 = trigger (Player 1 Leaving) viewers -- Player 1 Leaving
test2 = trigger (Message "str1") viewers -- Message str1
test3 = trigger (Message (2::Int)) viewers -- Some message of the type Int