Familia: unifying interfaces, type classes, and family polymorphism

Abstract

Parametric polymorphism and inheritance are both important, extensively explored language mechanisms for providing code reuse and extensibility. But harmoniously integrating these apparently distinct mechanismsÐ and powerful recent forms of them, including type classes and family polymorphismÐin a single language remains an elusive goal. In this paper, we show that a deep unification can be achieved by generalizing the semantics of interfaces and classes. The payoff is a significant increase in expressive power with little increase in programmer-visible complexity. Salient features of the new programming language include retroactive constraint modeling, underpinning both object-oriented programming and generic programming, and modulelevel inheritance with further-binding, allowing family polymorphism to be deployed at large scale. The resulting mechanism is syntactically light, and the more advanced features are transparent to the novice programmer. We describe the design of a programming language that incorporates this mechanism; using a core calculus, we show that the type system is sound. We demonstrate that this language is highly expressive by illustrating how to use it to implement highly extensible software and by showing that it can not only concisely model state-of-the-art features for code reuse, but also go beyond them.

Casting about in the dark: an empirical study of cast operations in Java programs

“…Various proposals have been made to better support family polymorphism (and the related łexpression problemž [Wadler 1998]) in object-oriented languages, including the use of design patterns [Nystrom et al 2003;Oliveira and Cook 2012;Wang and Oliveira 2016], and type systems [Ernst 2000;Kiselyov et al 2009;Nystrom et al 2006;Odersky and Zenger 2005;Zhang et al 2015;Zhang and Myers 2017] that permit some restricted form of covariant method parameters.…”