CLIPS is a public domain software tool for building expert systems. The name is an acronym for "C Language Integrated Production System." The syntax and name were inspired by Charles Forgy's OPS5. The first versions of CLIPS were developed starting in 1985 at NASA-Johnson Space Center (as an alternative for existing system ART*Inference) until the mid-1990s when the development group's responsibilities ceased to focus on expert system technology. The original name of the project was NASA's AI Language (NAIL).
CLIPS is probably the most widely used expert system tool. CLIPS itself is written in C, extensions can be written in C, and CLIPS can be called from C. Its syntax resembles that of the programming language Lisp.
CLIPS incorporates a complete object-oriented language for writing expert systems. COOL combines the programming paradigms of procedural, object oriented and logical (theorem proving) languages.
CLIPS uses forward chaining. Like other expert system languages, CLIPS deals with rules and facts. Various facts can make a rule applicable. An applicable rule is then fired. Facts and rules are created by first defining them, as shown below:
Having set this up, the (reset) command causes CLIPS to read the facts and rules. In this case, that would lead to the three "trouble_shooting" facts being asserted. Then the (run) command is used. Given that the two facts listed in rule1 have both been asserted, the conditions for doing its action have been met so the additional fact is asserted as a result of the run.
In CLIPS, salience allows a user to assign priority (or weight) to a rule.
Descendants of the CLIPS language include Jess (rule-based portion of CLIPS rewritten in Java, it later grew up in different direction), and FuzzyCLIPS (which adds concept of relevancy into the language).