Hi Folks, I am redesigning a system previously implemented in C++ using Haskell. I am keen to adopt a native programming style, but certain things about the original implementation still make perfect sense to me, including the C++-style subtype polymorphism. I'd be grateful if someone could take a look at the following sample and tell me if I'm missing some neater possible implementation in Haskell. {-# OPTIONS -fglasgow-exts #-} -- for existential types -- all subclasses take the same input format and can be created factory-pattern style data Input = Input Int Int -- this is the class we have to implement when creating a 'subtype' class InternalAPI i where doitImpl :: i -> Input -> Int -- factory function, everything is initialised by giving an "i" and the Input create :: i -> Input -> PublicAPI create a b = PublicAPI a b -- this is the object clients see, regardless of the underlying implementation data PublicAPI = forall p. InternalAPI p => PublicAPI p Input doit (PublicAPI p i) = doitImpl p i -- Here's an example implementation of something trivial data Adder = Adder instance InternalAPI Adder where doitImpl _ (Input a b) = a + b -- first argument is superfluous -- There's the factory function in use: test = create Adder (Input 1 2) result = doit test I'm actually pretty happy with this. I'm not trying for a complete OOP implementation but it lets me: - have the 'base class' (InternalAPI) implement default versions of methods with full access to the 'Input' structure containing the member data - do further inheritance, since I can select the implementation of a method using that first argument (e.g. on doitImpl) - base and derived classes see the same member data - Implementations of 'InternalAPI' can reference each other through the PublicAPI interface. Admittedly it's a rather vague question. I wonder if there are any articles out there showing how to reformulate problems solved using C++-style object models into Haskell programs? After all, it's hardly as if the C++ model is particularly elegant... thanks, David