Respectful Type Converters 1.1 What Information Do Type Converters Preserve?

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 deene 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 brieey discuss how our respects relation addresses a similar question in two other contexts: type evolution and interoperability. 1 Motivation The tremendous growth of the Internet and the World Wide Web gives millions of people access to vast quantities of data. While users may be able to retrieve data easily, they may not be able to interpret or display retrieved data intelligibly. For example, when retrieving a Microsoft Word document, without a Microsoft Word program, the user will be unable to read, edit, display, or print it. In general, the type of the retrieved data may be unknown to the retrieving site. Users and programs cope with this problem by converting data from one type to another, e.g., from the unknown type to one known by the local user or program. Thus, to view the Word document, we could convert it to ASCII text or HTML, and then view it through our favorite text editor or browser. A picture in an unfamiliar Windows bitmap type could be converted into a more familiar GIF image type. A mail message with incomprehensible MIME attachments could be converted from an unreadable MIME-encoded type to a text, image, or audio type that the recipient could examine directly. In general, we apply type converters on (data) objects, transforming an object of one type to an object of a diierent type. In converting objects of one type to another we expect there to be some relationship between the original object and the converted one. In what way are they similar? The reason to apply a converter in the …