dlm@allegra.att.COM (09/21/87)
Time: Thursday, September 10, 1987 1:00pm. Place: AT&T Bell Laboratories Murray Hill 3D-473 Speaker: Alex Borgida Affiliation: Rutgers University Title: Of Quakers and Republicans: A Syntax, Semantics, and Type Theory for Class Hierarchies with Contradictions Abstract: Disparate fields such as Artificial Intelligence, Databases and Programming Languages have discovered the joys of object-oriented programming. One of the principal features of this paradigm is the presence of classes of objects organized in subclass hierarchies, which provide a form of polymorphism and the notion of inheritance. The arguments in favour of these mechanisms are concerned with the ease of developing and modifying programs, but we show that in several circumstances the same kinds of arguments can be used to undermine the usual strict interpretation of specialization: namely that a subclass must be in every way a subtype of its superclass(es). We therefore propose a syntax that allows the definition of subclasses appearing to contradict their superclasses, albeit in an explicit and controlled way. After demonstrating the proper semantics for this construct, we examine the difficulties of writing correct programs when statements made about the objects in some class may be contradicted for elements belonging to a subclass. To solve these difficulties, we propose a type theory which admits "exceptional subclasses", and consider the problem of reasoning with these types. Sponsor: Ron Brachman