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 was probably the most widely used expert system tool as of 2005. 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.

Facts and rules
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:
 (deftemplate car_problem
     (slot name)
     (slot status))

 (deffacts trouble_shooting
     (car_problem (name ignition_key) (status on))
     (car_problem (name engine) (status wont_start))
     (car_problem (name headlights) (status work)))

 (defrule rule1
     (car_problem (name ignition_key) (status on))
     (car_problem (name engine) (status wont_start))
      =>
     (assert (car_problem (name starter) (status faulty))))
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.
CLIPS> (reset)
CLIPS> (agenda)
0      rule1: f-1,f-2
For a total of 1 activation.
CLIPS> (facts)
f-0     (initial-fact)
f-1     (car_problem (name ignition_key) (status on))
f-2     (car_problem (name engine) (status wont_start))
f-3     (car_problem (name headlights) (status work))
For a total of 4 facts.
CLIPS> (run)
CLIPS> (facts)
f-0     (initial-fact)
f-1     (car_problem (name ignition_key) (status on))
f-2     (car_problem (name engine) (status wont_start))
f-3     (car_problem (name headlights) (status work))
f-4     (car_problem (name starter) (status faulty))
For a total of 5 facts.
CLIPS>
(assert <fact>+)
 (Assert (object (name "chair")
(Material wood)
(height 3feet)
(Color brown))
CLIPS(retract <fact-index+)
(retract 0)
for a color

In CLIPS, salience allows a user to assign priority (or weight) to a rule.

Descendants
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).

See also 
 Constraint Handling Rules
 Drools
 OpenL Tablets

References

External links
 NASA CLIPS: downloads, information, news, and history
 CLIPS expert system tool: a candidate for the Diagnostic System engine
 CLIPS tutorial on Code Project
 Code editor for CLIPS

Expert systems
Public-domain software with source code
Domain-specific programming languages
Knowledge representation languages