Thesis Proposal

Nominal subtyping and structural subtyping each have their own strengths and weaknesses. Nominal subtyping allows programmers to explicitly express design intent, and, when types are associated with run-time tags, enables efficient run-time type tests (e.g., downcasts) and external/multimethod dispatch. On the other hand, structural subtyping is flexible and compositional, allowing unanticipated reuse. To date, nearly all object-oriented languages fully support only one subtyping paradigm or the other. I have designed a core language that combines the key aspects of nominal and structural subtyping in a unified framework. The goal is to combine the flexibility of structural subtyping while still allowing static typechecking of external methods. Together, these features ease software evolution: structural subtyping eases the task of maintaining interfaces, while external methods ease the task of adapting existing code to new interfaces. I also propose a language extension to Java that combines by-name and structural subtyping, and includes parametric polymorphism. The practical utility of this language will be illustrated through case studies that show that code written in the new language is more flexible and easier to modify than the original code.