Introduction to Part VII

Summary

Object-oriented programming is widely practised. It is based on an analogy between concrete physical models, built from real objects, and abstract software models, whose components are intangible computational entities - an example being a physical model of an aircraft wing, for testing in a wind tunnel, and a software model of the wing, for testing by computer simulation. Following the analogy, the intangible entities that comprise the software models are themselves called objects. In general, as an object-oriented system evolves, objects are created and the interconnection structure among objects changes. Although some implementations of object-oriented programming languages are sequential, activity within a system of objects is naturally thought of as concurrent.

This final Part of the book is about how π-calculus can be useful in object-oriented design and programming. The most important point is an indirect one. It is that a good theory offers a conceptual framework, a way of thinking about design and analysis of systems that helps one see the wood as well as the trees. The π-calculus captures in a tractable mathematical form a particular conception of mobile system. That conception is closely related to ideas that are important in object-orientation, especially its behavioural aspects. It may also be that particular concepts from π-calculus can usefully be transferred to objects, for instance ways of understanding behaviour, analytical techniques, type constructs, and process operators.

The π-calculus can be used to express systems of objects mathematically. This modelling can be carried out at all points on the spectrum from design-sketch to implementation in a programming language. And π-calculus techniques can be used to reason about object systems, both their static aspects, for instance types, and their dynamic properties.

Part VII illustrates some of these points by giving a semantic definition for a simple object-oriented language by translation to the π-calculus, and using techniques from earlier in the book to show some properties of the language and programs written in it.

The requirements placed on π-calculus descriptions may vary from one region of the design-implementation spectrum to another. At the design-sketch end, an informal understanding of the notation or language used may be sufficient to grasp the accuracy of π-calculus descriptions. Indeed, the act of modelling may help to sharpen understanding.