Gradual Typing for Objects

13 years 10 months ago

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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 λ? →. 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 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.

Jeremy G. Siek, Walid Taha

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