The motivation behind the work in very-high-level languages is to ease the programming task by providing the programmer with a language containing primitives or abstractions suitable to his problem area. The programmer is then able to spend his effort in the right place; he concentrates on solving his problem, and the resulting program will be more reliable as a result.Clearly, this is a worthwhile goal.Unfortunately, it is very difficult for a designer to select in advance all the abstractions which the users of his language might need.If a language is to be used at all, it is likely to be used to solve problems which its designer did not envision, and for which the abstractions embedded in the language are not sufficient. This paper presents an approach which allows the set of built-in abstractions to be augmented when the need for a new data abstraction is discovered.This approach to the handling of abstraction is an outgrowth of work on designing a language for structured programming.Relevant aspects of this language are described, and examples of the use and definitions of abstractions are given.

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The main purposes in writing this paper are to discuss the importance of formal specifications and to survey a number of promising specification techniques. The role of formal specifications both in proofs of program correctness, and in programming methodologies leading to programs which are correct by construction, is explained. Some criteria are established for evaluating the practical potential of specification techniques. The importance of providing specifications at the right level of abstraction is discussed, and a particularly interesting class of specification techniques, those used to construct specifications of data abstractions, is identified. A number of specification techniques for describing data abstractions are surveyed and evaluated with respect to the criteria. Finally, directions for future research are indicated.

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

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