First order programming logic

• Authors: Robert Cartwright;John McCarthy
• Venue: POPL '79 Proceedings of the 6th ACM SIGACT-SIGPLAN symposium on Principles of programming languages
• Year: 1979

First Order Programming Logic is a simple, yet powerful formal system for reasoning about recursive programs. In its simplest form, it has one major limitation: it cannot establish any property of the least fixed point of a recursive program which is false for some other fixed point. To rectify this weakness, we present two intuitively distinct approaches to strengthening First Order Programming Logic and prove that either extension makes the logic relatively complete. In the process, we prove that the two approaches are formally equivalent. The relative completeness of the extended logic is significant because it suggests it can establish all "ordinary" properties (obviously we cannot escape the Godelian incompleteness inherent in any programming logic) of recursive programs including those which compute partial functions.The second contribution of this paper is to establish that First Order Programming Logic is applicable to iterative programs as well. In particular, we show that the intermittent assertions method--an informal proof method for iterative programs which has not been formalized--is conveniently formalized simply as sugared First Order Programming Logic applied to the recursive translations of iterative programs.