Report Number: CS-TR-79-716
Institution: Stanford University, Department of Computer Science
Title: A framework for control in production systems
Author: Georgeff, Michael P.
Date: January 1979
Abstract: A formal model for representing control in production systems is defined. The formalism allows control to be directly specified independently of the conflict resolution scheme, and thus allows the issues of control and nondeterminism to be treated separately. Unlike previous approaches, it allows control to be examined within a uniform and consistent framework. It is shown that the formalism provides a basis for implementing control constructs which, unlike existing schemes, retain all the properties desired of a knowledge based system --- modularity, flexibility, extensibility and explanatory capacity. Most importantly, it is shown that these properties are not a function of the lack of control constraints, but of the type of information allowed to establish these constraints. Within the formalism it is also possible to provide a meaningful notion of the power of control constructs. This enables the types of control required in production systems to be examined and the capacity of various schemes to meet these requirements to be determined. Schemes for improving system efficiency and resolving nondeterminism are examined, and devices for representing such meta-level knowledge are described. In particular, the objectification of control information is shown to provide a better paradigm for problem solving and for talking about problem solving. It is also shown that the notion of control provides a basis for a theory of transformation of production systems, and that this provides a uniform and consistent approach to problems involving subgoal protection.