Respectful Type Converters

In converting an object of one type to another, we expect some of the original object's behavior to remain the same, and some to change. How can we state the relationship between the original object and converted object to characterize what information is preserved and what is lost after the conversion takes place? We answer this question by introducing the new relation, respects, and say that a type converter function C : A —>■ B respects a type T. We formally define respects in terms of the Liskov and Wing behavioral notion of subtyping; types A and B are subtypes of T. We explain in detail the applicability of respectful type converters in the context of the Typed Object Model (TOM) Conversion Service, built at Carnegie Mellon and used on a daily basis throughout the world. We also briefly discuss how our respects relation addresses a similar question in two other contexts: type evolution and interoperability. This research is sponsored in part by the Defense Advanced Research Projects Agency and the Wright Laboratory, Aeronautical Systems Center, Air Force Materiel Command, USAF, F33615-93-1-1330, and Rome Laboratory, Air Force Materiel Command, USAF, under agreement number F30602-97-2-0031 and in part by the National Science Foundation under Grant No. CCR-9523972. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Defense Advanced Research Projects Agency Rome Laboratory or the U.S. Government. DISTRIBUTION STATEMENT A Approved for public release; Distribution Unlimited DTXC QUAL5W INSPECTED 8