The Impact of Mesa on System Design

The Mesa programming language supports program modularity in ways that permit subsystems to be developed separately but to be bound together with complete type safety. Separate and explicit interface definitions provide an effective means of communication, both between programs and between programmers. A configuration language describes the organization of a system and controls the scopes of interfaces. These facilities have had a profound impact on the way we design systems and organize development projects. This paper reports our recent experience with Mesa, particularly its use in the development of an operating system. It illustrates techniques for designing interfaces, for using the interface language as a specification language, and for organizing a system to achieve the practical benefits of progrmn modularity wjthout sacrificing strict type-checking. Mesa is a programming language designed for system implementation. It is used within the Xerox Corporation both by research laboratories as a vehicle for experiments and by development organizations for 'production' programming. Some of our initial experience with Mesa was reported previously [Geschke et ai, 1977]. Since that time, the language has evolved in several directions and has acquired a larger and more diverse community of users. That community has accumulated a substantial amount of experience in using Mesa to design and implement large systems, a number of which are now operational. It has beCOme increasingly clear that the value of Mesa extends far beyond its enforcement of type-safety within individual programs. It has profounuly affected the ways we think about system design, organize development projects, and communicate our ideas about the systems we build. This paper reports some of our recent experience with Mesa. It is based primarily upon the development of one particular system-what we refer to as the Pilot operating system-tbr a small, personal computer. We also draw upon the lessons learned from other systems. These represent a non-trivial amount of programming; a survey of just the authors' immediate colleagues at the end of 1978 uncovered several hundred thousand lines of stable, operational Mesa code. Pilot itself is a 'second generation' client of Mesa. It is the first major system to take advantage of explicit interface and configuration descriptions (discussed below) in its original design. rn addition, its designers were ai>le to make careful assessments of earlier systems to discover both the benefits i.md pitfalls of using Mesa. As a result, we were able to benefit from, as well as add to, the accumulated 'institutional learning' about the practical problems of developing large systems in Mesa. The purpose of this paper is to communicate those lessons, which deserve more emphasis and discussion than they have received to date in the literature. We concentrate upon the impact and adequacy of the Mesa programming language and its illfluenceupon system design; a companion paper [Lynch and Horsley, 1979J focuses upon organizational and marwgement issues. This paper contains three main sections. First, the fm:ilities provided by Mesa for supporting the development 2 H. C. LAUER AND E. H. SATIERTHWAITE and organization of modular programs are discussed. In the second section, we describe the role played by the Mesa interface 'and configuration languages in system design, particularly from the perspective of Pilot. The final section is a qualitative assessment of the adequacy of Mesa as a system implementation language.