An Implementation-oriented Semantics for Module Composition 1.1 Introduction

This paper describes an approach to module composition by executing \module expressions" to build systems out of component modules; the paper also gives a novel semantics intended to aid implementers. The semantics is based on set the-oretic notions of tuple set, partial signature, and institution, thus avoiding more diicult mathematics theory. Language features include information hiding, both vertical and horizontal composition, and views for binding modules to interfaces. Vertical composition refers to the hierarchical structuring of a system into layers, while horizontal composition refers to the structure of a given layer. Modules may involve information hiding, and views may involve behavioral satisfaction of a theory by a module. Several \Laws of Software Composition" are given, which show how the various module composition operations relate. Taken together, this gives foundations for an algebraic approach to software engineering. The approach to module composition described in this paper can be used in many diierent ways. For example, it can be added to a programming language, by providing a simple module connection language (MCL) with \module expressions" that say how to manipulate and connect modules in the programming language. The general approach, called \parameterized programming" Gog89], involves a module speciication capability; however, programmers can specify as little of a module as they like, as long as they declare the syntax. The approach has been validated with experiments using lileanna (for Library Interconnect Language Extended with Anna, where Anna itself is an acronym for Annotated Ada) Tra93a] on real exam-1