Constrained Matching is Type Safe

Temporally constrained matching in a persistent and declarative object-oriented system is introduced as a semantic alternative to the existing approaches to the covariance/contravariance problem. While the existing object-oriented type systems are based on subtyping, F-bounded polymorphism and matching, this language system is based entirely on inheritance, which is identified with matching. The type of matching used in this paper relies on the temporal constraint system. We prove that this constrained matching guarantees type safe substitutability even in situations where matching alone would not. This is possible only because the underlying formal system is semantically much richer than the paradigms of type systems. Its temporal constraint system can capture subtleties that go far beyond the level of expressiveness of object-oriented type systems. The temporal nature of the language and its distinctive orthogonal model of persistence make this language system successful in handling a variety of non-trivial applications.

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