Eric wrote:
(1) Have Handlers implement a method handle(m: Msg). To add new types of message, we declare message types which extend Msg. The Handler then uses runtime type testing to decide how to deal with each message. The advantage of this design is that we can add new Handler and new Msg types without recompiling the Dispatcher.
Note however that the type of the handle method is a lie (your handlers don't actually accept arbitrary messages). I guess, you actually want a polymorphic "Dispatcher", which will actually turn out to be a function. Of course, you give insufficient information to actually solve your problem.
Type classes allow us to adopt approach (0) in Haskell, but don't seem to allow approach (1)....
I consider that a good thing. -Udo