Gradual Typing for Objects

Abstract

Static and dynamic type systems have well-known strengths and weaknesses. In previous work we developed a gradual type system for a functional calculus named \(\lambda^?_\to\). Gradual typing provides the benefits of both static and dynamic checking in a single language by allowing the programmer to control whether a portion of the program is type checked at compile-time or run-time by adding or removing type annotations on variables. Several object-oriented scripting languages are preparing to add static checking. To support that work this paper develops \(\mathbf{Ob}^{?}_{<:}\), a gradual type system for object-based languages, extending the Ob < : calculus of Abadi and Cardelli. Our primary contribution is to show that gradual typing and subtyping are orthogonal and can be combined in a principled fashion. We also develop a small-step semantics, provide a machine-checked proof of type safety, and improve the space efficiency of higher-order casts.KeywordsOperational SemanticDynamic TypingType AnnotationMethod InvocationTyping DerivationThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.