F-Bounded Polymorphism for Database Programming Languages

The relevance of F-bounded polymorphism for database programming languages (object-oriented in particular) is investigated. Two different kinds of object types are proposed as a solution for the controversy between inheritance and subtyping. It is with this duality that complex, recursive object types so typical for database application environments are constructed and the relationship with F-algebras and F-coalgebras hinted in the literature established. Surprisingly enough, the covariant (and thus unsafe) ordering for object types is shown to play an important role in interpreting formally the contravariant (type-safe) subtyping for object types. These results are established in a formal framework based on a variety of kinds (of types) of a polymorphic database type system which serves as a basis for a typed database technology. In addition to the formal theory, application-oriented and database system-oriented illustrations of the usage of F-bounded polymorphism are also presented.

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