Privacy via Subsumption

We describe an object calculus that allows both extension of objects and full width subtyping (hiding arbitrary components). In contrast to other proposals, the types of our calculus do not mention “missing” methods. To avoid type unsoundness, the calculus mediates all interaction with objects via “dictionaries” that resemble the method dispatch tables in conventional implementations. Private fields and methods can be modeled and enforced by scoping restrictions: forgetting a field or method through subsumption makes it private. We prove that the type system is sound, give a variant which allows covariant self types, and give some examples of the expressiveness of the calculus.