Report Number: CS-TR-81-865
Institution: Stanford University, Department of Computer Science
Title: Toward a unified logical basis for programming languages
Author: Tang, Chih-sung
Date: June 1981
Abstract: In recent years, more and more computer scientists have been paying attention to temporal logic, since there are many properties of programs that can be described only by bringing the time parameter into consideration. But existing temporal logic languages, such as Lucid, in spite of their mathematical elegance, are still far from practical. I believe that a practical temporal-logic language, once it came into being, would have a wide spectrum of applications. XYZ /E is a temporal-logic language. Like other logic languages, it is a logic system as well as a programming language. But unlike them, it can express all conventional data structures and control structures, nondeterminate or concurrent programs, even programs with branching-time order. We find that the difficulties met in other logic languages often stem from the fact that they try to deal with these structures in a higher level. XYZ /E adopts another approach. We divide the language into two forms: the internal form and the external form. The former is lower level, while the latter is higher. Just as any logic system contains rules of abbreviation, so also in XYZ /E there are rules of abbreviation to transform the internal form into the external form, and vice versa. These two forms can be considered to be different representations of the same thing. We find that this approach can ameliorate many problems of formalization.