Safe, Flexible Recursive Types for Featherweight Java

This paper presents a type assignment system with recursive types for Featherweight Java, inspired by the work of Nakano. Nakano’s innovation consists in adding a modal type constructor which acts to control the folding of recursive types, resulting in a head-normalisation guarantee. We build on this approach by introducing a second modal type constructor which prevents the unfolding of types in contexts where doing so results in non-termination. Moreover our system inherits the flexibility of Nakano’s approach, allowing object-oriented features (such as binary methods) to be typed in a safe and intuitive way. The work described in this paper is preliminary, and no formal results are claimed. However, we conjecture that our type system enjoys strong normalisation and we motivate this by working through some apposite examples.