Subtyping constraints for incomplete objects

Abstract

We extend the type system for the Lambda Calculus of Objects to account for a notion of width subtyping. The main novelties over previous work are the use of bounded quantification to achieve a new and more direct rendering of MyType polymorphism, and a uniform treatment for other features that were accounted for via different systems in subsequent extensions of. In particular, the new system provides for (i) appropriate type specialization of inherited methods, (ii) static detection of errors, (iii) width subtyping compatible with object extension, and (iv) complete freedom in the order of method addition.