An Informal Definition Of Alphard Preliminary Edited By W A Wulf

An Informal Definition of Alphard (preliminary). Edited by W.A. Wulf

Alphard: Form and Content

Alphard is a design for a programming system that supports the abstraction and verification techniques required by modern program'ming methodology. During the language design process, we were concerned simultaneously with problems of methodology, correctness, and efficiency. Methodological concerns are addressed through facilities for defining new, task·specific abstractions that capture complex notions in terms of their intended properties, without explicating them in terms of specific low· level implementations. Techniques for verifying certain properties of these programs address the correctness concerns. Finally, the language has been designed to permit compilation to efficient object code. Although a compiler was not implemented, the research shed light on specification issues and on programming methodology. an abstraction, specifying its behavior Alphard language constructs allow a programmer to isolate publicly while localizing knowledge about its implementation. The verification of such an abstraction consists of showing that its implementation behaves in accordance with the public specification. Given such a verification, the abstraction may be used with confidence to construct higher·level, more abstract, programs. The most common kind of abstraction in Alphard corresponds to what is now called an abstract data type. An abstract data type comprises a set of values for elements of the type and a set of operations on those values. A new language construct, the form, provides a way to encapsulate the definitions of data structures and operations in such a way that only public information could be accessed by the rest of the program.

Studies in Ada Style

The major problems of modern software involve finding effective techniques and tools for organizing and maintaining large, complex programs. The key concept in modern programming for controlling complexity is abstraction; that is, selective emphasis on detail. This monograph discusses how the Ada programming language provides ways to support and exploit such abstraction techniques. The monograph is organized into two parts. The first part traces the important ideas of modern programming languages to their roots in the languages of the past decade and shows how modern languages, such as Ada, respond to contemporary problems in software development. The second part examines five problems to be programmed using Ada. For each problem, a complete Ada program is given, followed by a discussion of how the Ada language affected various design decisions. These problems were selected to be as practical as possible rather than to illustrate any particular set of language features. Much of this material has appeared previously in print. An earlier version of the first section, by Mary Shaw, was published as "The Impact of Abstraction Concerns on Modern Programming Languages" in the Proceedings of the IEEE special issue on Software Engineering, September 1980, Vol. 68, No. 9, pages 1119·1130. It is reprinted with the IEEE's permission. The article has been updated to reflect the revised Ada syntax and semantics.