Tim@CIS.UPENN.EDU.UUCP (12/04/86)
DBIG Meeting
Computer and Information Science
University of Pennsylvania
10:30am; 12-5-86; 555 Moore
ON NON-STRICT CLASS HIERARCHIES IN CONCEPTUAL MODELING LAGUAGES.
Alexander Borgida
Rutgers University
One of the cornerstones of the conceptual modeling languages devised for the
specification and implementation of Information Systems is the idea of objects
grouped into classes. I begin by reviewing the various roles played by this
concept: specification of type constraints, repository of logical constraints
to be verified, and maintenance of an associated set of objects (the "extent").
I then consider a second feature of these languages -- the notion of class
hierarchies -- and after outlining its benefits, present arguments against a
strict interpretation of class specialization and the notion of inheritance.
Additional consideration of the concept of "default inheritance" leads to a
list of desirable features for a language mechanism supporting non-strict
taxonomies of classes: ones in which some class definitions may contradict
portions of their superclass definitions, albeit in a controlled way.
I conclude by presenting some preliminary thoughts on a type system and type
verification mechanism which would allow one to check that programs written in
the presence of exceptional types will not go wrong.