Abstract

Tony Hoare, FRS FR Eng, Hon Mem Cambridge University Computing Laboratory.

In modern agile program development, significant resources are devoted to the unit testing of modifications to a large concurrent object-oriented program in long-term and often world-wide service. We suggest that a log of the actions of a test which reveals a fault should be displayed as a diagram in the two dimensions of space and time. Such diagrams are widely used physics (eg, by multi-sensor recorders), in electronic timing diagrams, and in top-down design of software architecture, using Message Sequence Charts (or similar).

We show how many of the basic features of concurrent object oriented program execution can be simply described in an abstract (non-metric) plane geometry, combining ideas of Euclid and Descartes with those of Petri, Dahl and Nygaard, Stroustrup and many others. We suggest that the geometry is a highly intuitive denotational semantic foundation for the definition of families of programming languages, which may then be used in combination. Verification-oriented and operational semantics can then be defined and derived from the denotational.