An introduction to the formal definition of ALGOL 68

Abstract

The aim of this paper is to provide an introduction or guide to the Revised ALGOL 68 Report [15]. The earlier revised ALGOL 60 Report[12] constituted one of the first attempts at formally defining a programming language. This was a major advance and opened up new areas of interest and research in computer science. But there were some deficiencies in both the language and in its formal definition. In the course of time other attempts were made at improving and correcting the earlier oversights. Our intention here is to begin by considering the formal definition of ALCOL 60 [12] and to show how these early ideas gradually evolved to produced the method of formally defining ALGOL 68. The discussion will, for the most part, be restricted to the ALGOL movement. But over the years other advances have been made in formally defining languages. The most notable of these is probably the development from McCarthy's LIPS [7–11] through to the Vienna Definition Language [5, 6, 16, 17] and beyond. But see also [4]. In discussing formal definition of programming languages one must distinguish between the programming language itseld and its method of formal definition. These are quite separate. Yet they tend to be confused since new methods of formal definition often accompany new programming languages. And a formlal definition tends to provide a vocabulary of terms by which various constructions become known. It will be assumed that the reader has a certain knowledge of the programming languages ALGOL 60, ALGOL W and ALGOL 68. Moreover it would be convenient if he had available a copy of the formal definition of each language. Although it should be possible to proceed without these documents frequent references are made to them.