Once a knowledge base (in the form of rules) is built and the factlist and instancelist is prepared, CLIPS is ready to execute rules. In a conventional language, the starting point, the stopping point, and the sequence of operations are defined explicitly by the programmer. With CLIPS, the program flow does not need to be defined quite so explicitly. The knowledge (rules) and the data (facts and instances) are separated, and the inference engine provided by CLIPS is used to apply the knowledge to the data. The basic execution cycle is as follows:
a) If the rule firing limit has been reached or there is no current focus, then execution is halted. Otherwise, the top rule on the agenda of the module which is the current focus is selected for execution. If there are no rules on that agenda, then the current focus is removed from the focus stack and the current focus becomes the next module on the focus stack. If the focus stack is empty, then execution is halted, otherwise step a is executed again. See sections 5.4.10.2, 10.6, 12.2, and 13.7 for information on the focus stack and the current focus.
b) The righthand side (RHS) actions of the selected rule are executed. The use of the return function on the RHS of a rule may remove the current focus from the focus stack (see sections 10.6 and 12.6.7). The number of rules fired is incremented for use with the rule firing limit.
c) As a result of step b, rules may be activated or deactivated. Activated rules (those rules whose conditions are currently satisfied) are placed on the agenda of the module in which they are defined. The placement on the agenda is determined by the salience of the rule and the current conflict resolution strategy (see sections 5.3, 5.4.10, 13.7.5, and 13.7.6). Deactivated rules are removed from the agenda. If the item is being watched (see section 13.2), then an informational message will be displayed each time a rule is activated or deactivated.
d) If dynamic salience is
being used, the salience values for all rules on the agenda are
reevaluated (see sections 5.4.10, 13.7.9, and 13.7.10). Repeat
the cycle beginning with step a.