(To appear in Theory and Practice of Logic Programming (TPLP)) We introduce a systematic, concurrent execution scheme for Constraint Handling Rules (CHR) based on a previously proposed sequential goal-based CHR semantics. We establish strong correspondence results to the abstract CHR semantics, thus guaranteeing that any answer in the concurrent, goal-based CHR semantics is reproducible in the abstract CHR semantics. Our work provides the foundation to obtain efficient, parallel CHR execution schemes.
It has been claimed that production systems have several advantages over other representational schemes. These include the potential for general self-augmentation (i.e., learning of new behavior) and the ability to function in complex environments. The production system language, OPS, was implemented to test these claims. In this paper we explore some of the issues that bear on the design of production system languages and try to show the adequacy of OPS for its intended purpose. Conflict resolution is a form of global control used in production systems to achieve an efficient sequential execution of a rule-based program. This type of control is not used in parallel production system models[6, 13]. Instead, only those programs are executed which make no assumptions regarding conflict resolution. Therefore, the initial sequential rule-based programs are either executed in parallel without their conflict resolution strategy, which normally results in incorrect behavior, or the programs are transformed in an ad hoc manner to execute on a particular parallel production system model. As a result, these programs do not exhibit the parallelism hoped for [10, 13]. We believe that a second reason behind the lack of parallelism is that no formal methods of verifying the correctness of rule-based programs are utilized. Correctness is especially important when conflict resolution is no longer utilized, because it necessary to transform sequential rule-based programs into equivalent programs without conflict resolution. Also, the parallel execution of a rule-based program is more complex and demands these formal methods even more than its sequential counterpart. We are concerned with designing and developing correct rule-based programs for parallel execution. In this paper, we show the difficulty in transforming a simple sequential rule-based program to a new version of the program with no conflict resolution. Also, we show that the use of a new programming paradigm and language may result in more efficient programs which are provably correct, and can be executed in parallel. 