An Introduction to the Construction and Verification of Alphard Programs

The programming language Alphard is designed to provide support for both the methodologies of "well-structured" programming and the techniques of formal program verification. Language constructs allow a programmer to isolate an abstraction, specifying its behavior publicly while localizing.knowledge about its implementation. The verification of such an abstraction consists of showing that its implementation behaves in accordance with its public specifications; the abstraction can then be used with confidence in constructing other programs, and the verification of that use employs only the public specifications. This paper introduces Alphard by developing and verifying a data structure definition and a program that uses it. It shows how each language construct contributes to the development of the abstraction and discusses the way the language design and the verification methodology wete tailored to each other. It serves not only as an introduction to Alphard, but also as an example of the symbiosis between verification and methodology in language design. The strategy of program structuring, illustrated for Alphard, is also applicable to most of the "data abstraction" mechanisms now appearing. Index Terns-Abstract data types, abstraction and representation, assertions, correctness, information hiding, levels of abstraction, modular decomposition, program specifications, program verification, programming languages, programming methodology, structured programming.