View operations on objects with roles for a statically typed database language

To deal with the evolution of data and applications and with the existence of multiple views for the same data, the object data model needs to be extended with two different sets of operations: object extension operations, to allow an object to dynamically change its type; and object viewing operations, to allow an object to be seen as if it had a different structure. Object extension and object viewing operations are related in that they are both identity-preserving operations, but different in that object extension may modify the behavior of the original object while object viewing creates a new view for the original object without modifying its behavior. A set of object viewing operations is defined in the context of a statically and strongly typed database programming language which supports objects with roles, and the relationships with object extension and role mechanisms are discussed. We then show how the object viewing operations can be used to give the semantics of a higher level mechanism to define views for object databases. Examples of the use of these operations are given with reference to the prototype implementation of the language Galileo 97.