Data Constructors: On the Integration of Rules and Relations

Although the goals and means of rule-based and data-based systems are too different to be fully integrated at the present time, it seems appropriate to investigate a closer integration of language constructs and a better cooperation of execution models for both kinds of approaches. In this paper we propose a new language construct called constructor that allows the definition of new relations from existing ones by means of recursion. The constructor is semantically defined by the least fixed point of a set expression and blends well both with a strongly typed modular programming language and with a relational calculus query formalism. Moreover, it is shown to provide expressive power at least equivalent to the declarative semantics of PROLOG while avoiding some disadvantages of it, for example, poor modularity and infinite loops. Furthermore, the constructor is set-oriented thus allowing more efficient implementation techniques than those available through proof-theoretic methods typical of a rule-based approach.