View definitions in relational databases suffer from the shortcoming that they depend on the name rather than the type of base relations. We present the language Plop as a minimal language for studying polymorphism in a logical database programming language by means of parametrization of predicates. The main idea is to introduce generic predicates which have two lists of parameters. While the second list contains the usual arguments, the first list contains parameters which are used as predicates in the rules defining the generic predicate. Through restriction in the use of predicate parameters it is possible to give a model theoretic semantics in a two step first order deduction. We define a sound and complete proof theoretic semantics and derive an evaluation algorithm for Plop programs. Furthermore, we present an algorithm based on a necessary and sufficient condition for the evaluation algorithm to terminate for all queries. With the extensions we overcome the lack of predicate polymorphism in deductive database languages and increase their expressiveness. We also sketch some extensions of the concept.

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