Assertions

Abstract

An assertion is a Boolean formula written in the text of a program, which the programmer asserts will always be true when that part of the program is executed. It species an internal interface between that part of the program that comes before it and all that follows it. In the software industry today, assertions are conditionally compiled in test runs of a program, and help in the detection and diagnosis of errors. Alan Turing first proposed assertions as a means of checking a large routine. They were rediscovered independently by Naur as generalised snapshots, and by Floyd, who used them to assign meanings to programs. Floyd suggested that if the internal assertions were strong enough, they would constitute a formal proof of the correctness of a complete program. In this lecture, I will summarise the subsequent development of the idea, and describe some of its practical impact.