On 8/23/12 1:02 PM, Patrick Browne wrote:
I am just not sure whether there is a model expansion from the super-class model to the subclass model.
If by "model expansion from..." you mean that there is a canonical/unique/special mapping from every superclass model to some subclass model, then the answer is no. Consider, for instance, applicative functors and monads. We have the (idealized) type classes: class Functor a where... class Functor a => Applicative a where... class Applicative a => Monad a where... However, there are strictly more Applicative instances than there are Monad instances. E.g., lists support an Applicative instance based on zip and an Applicative instance based on the cartesian product; however, only the latter of these can be extended to a Monad. Well, technically, that's only if we assume the appropriate laws are part of the theories defined by the type classes. Without this assumption every type class can be instantiated at every type (for every method f, define f = undefined). -- Live well, ~wren