Craig Chambers. Predicate Classes. In Oscar Nierstrasz, editor, ECOOP 93 - Object-Oriented Programming, 7th European Conference, Kaiserslautern, Germany, July 26-30, 1993, Proceedings. Volume 707 of Lecture Notes in Computer Science, pages 268-296, Springer, 1993. [doi]
Predicate classes are a new linguistic construct designed to complement normal classes in object-oriented languages. Like a normal class, a predicate class has a set of superclasses, methods, and instance variables. However, unlike a normal class, an object is automatically an instance of a predicate class whenever it satisfies a predicate expression associated with the predicate class. The predicate expression can test the value or state of the object, thus supporting a form of implicit property-based classification that augments the explicit type-based classification provided by normal classes. By associating methods with predicate classes, method lookup can depend not only on the dynamic class of an argument but also on its dynamic value or state. If an object is modified, the property-based classification of an object can change over time, implementing shifts in major behavior modes of the object. A version of predicate classes has been designed and implemented in the context of the Cecil language.