A Deductive and Typed Object-Oriented Language

1 I n t r o d u c t i o n a n d r e s u l t s In the last decade, the merge of object-oriented programming with object-oriented data structuring principles has lead to a rapid increasement of new developments in the field of databases and logical languages. Object-oriented databases have the advantages of a clean conceptual design as well as the possibility of enforcing better software engineering. Systems equipped with subtyping facilities, such as the Cardelli object-oriented type system (cf. [Card88]), offer a concise and clear way to deal with (multiple) inheritance. Inheritance is a very powerful modelling tool and forms the backbone of many object-oriented data models. Also th'g availability of complex objects, such as records, lists, variants, and sets offer a wide range of expressiveness. Examples of data models with facilities as mentioned above are 02 (ILeal89]), Iris ([LyVi87]) and Machiavelli ([OhBB89]). Object-orientation has also not left the field of logical languages untouched. Languages like LIFE (lASt-K91]), F-Logic ([KiLW90]), and [BrLM90,IbCu90, McCa92,MoPog0] are examples of such languages that make extensive use of object-oriehted principles to enhance the field of logic programming with the expressiveness and concise modelling possibilities, typical for the object-oriented paradigm. Especially the use of subtyping makes logic programs more structured and easier to understand. In short, the combination of logic programming and object orientation is very promising. Relational databases and logic programming have been combined resulting in so-called deductive databases. Deductive databases highlight the ability to use a * Our E-mai l addresses are, resp.: r e n e ~ c s . u t w e n t e , n l , b a l s t e r s 9 u t w e n t e . n l