Hi Emmanuel, On Fri, Feb 08, 2013 at 10:50:30AM +0100, Emmanuel Touzery wrote:
The "obvious" way to do in haskell what I would do in OO would be through type classes. However I realize type classes are not quite interfaces. I'm wondering what would be the "haskell way" to solve this problem?
For sure type classes do the job. But is it the idiomatic way of solving this problem?
The problem is, that without the use of extensions it's not possible to have something like: class EventProvider a where events :: a -> IO [Event] instance EventProvider Prov1 where ... instance EventProvider Prov2 where ... -- won't compile, because Prov1 and Prov2 have different types providers :: EventProvider a => [a] providers = [Prov1, Prov2] To express something like this you need existential quantification: {-# LANGUAGE ExistentialQuantification #-} data AnyProvider = forall a. (EventProvider a) => AnyProvider a providers :: [AnyProvider] providers = [AnyProvider Prov1, AnyProvider Prov2] But after trying ExistentialQuantification I got the impression, that it just doesn't fit nicely into the language, that you can get quite fast to a point where your head explodes by looking at the type errors ;). So, like others already said (thanks Oleg ;), a record of functions can get you quite far. In your case youd could have something like: data EventProvider = EventProvider {events = IO [Event]} mkProv1 prov1 = EventProvider {events = do -- read from prov1 } mkProv2 prov2 = EventProvider {events = do -- read from prov2 } Greetings, Daniel