Hi Alistair! Just a "quick" reply (I didn't had time to look at Ralf's technique). Looking at your code, it seems to me that you are missing the notion of a supertype (perhaps, that's the intended thing with BaseClass?).
I would use an existencial type to "capture" this notion: =========== data Base = forall c . Method c => Base c instance BaseClass Base instance Method Base where method1 (Base x) i = Base (method1 x i) method2 (Base x) = method2 x the modifications on the code would be: class BaseClass c => Method c where -- method1 returns a supertype Base method1 :: c -> Int -> Base method2 :: c -> Int instance Method c => Method (SubBase2 c) where -- method1 does not fail any more method1 x i = Base (SubBase2 (SubBase1 5)) method2 x = 2 makeSubType s = if s == "SubBase1" then Base (SubBase1 3) else Base (SubBase2 (SubBase1 4)) ============ Perhaps a better name for Base would be SuperMethod (since it is really trying to capture the fact that is the super type for Method). Hope it helps Cheers, Bruno _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
