The Essence of Lightweight Family Polymorphism

The formal core calculus .FJ has been introduced to model lightweight family polymorphism, a programming style to support reusable yet type-safe mutually recursive classes. This paper clarifies the essence of the features of .FJ, by giving a formal translation from .FJ into a variant of Featherweight GJ with a little extension of F-bounded polymorphism. The extension, which allows self types to appear in mutually recursive constraints on type variables, is significant to achieve a clear correspondence between the two languages without losing type safety. We show that the extended version of Featherweight GJ is type sound and that the formal translation preserves typing and reduction.