Jess (programming language). Jess is a rule engine for the Java computing platform, written in the Java programming language. It was developed by Ernest Friedman-Hill of Sandia National Laboratories. It is a superset of the CLIPS language. It was first written in late 1995. The language provides rule-based programming for the automation of an expert system, and is often termed as an expert system shell. In recent years, intelligent agent systems have also developed, which depend on a similar ability.

Rather than a procedural paradigm, where one program has a loop that is activated only one time, the declarative paradigm used by Jess applies a set of rules to a set of facts continuously by a process named pattern matching. Rules can modify the set of facts, or can execute any Java code. It uses the Rete algorithm to execute rules.

License
The licensing for Jess is freeware for education and government use, and is proprietary software, needing a license, for commercial use. In contrast, CLIPS, which is the basis and starting code for Jess, is free and open-source software.

Code examples
Code examples:
; is a comment

(bind ?x 100)

; x = 100

(deffunction max (?a ?b)
             (if (> ?a ?b) then ?a else ?b))

(deffacts myroom
          (furniture chair)
          (furniture table)
          (furniture bed)
          )

(deftemplate car
             (slot color)
             (slot mileage)
             (slot value)
             )

(assert (car (color red) (mileage 10000) (value 400)))
Sample code:
(clear)
(deftemplate blood-donor (slot name) (slot type))
(deffacts blood-bank ; put names & their types into [[working memory]]
          (blood-donor (name "Alice")(type "A"))
          (blood-donor (name "Agatha")(type "A"))
          (blood-donor (name "Bob")(type "B"))
          (blood-donor (name "Barbara")(type "B"))
          (blood-donor (name "Jess")(type "AB"))
          (blood-donor (name "Karen")(type "AB"))
          (blood-donor (name "Onan")(type "O"))
          (blood-donor (name "Osbert")(type "O"))
          )
(defrule can-give-to-same-type-but-not-self ; handles A > A, B > B, O > O, AB > AB, but not N1 > N1
         (blood-donor (name ?name)(type ?type))
         (blood-donor (name ?name2)(type ?type2 &:(eq ?type ?type2) &: (neq ?name ?name2) ))
         =>
         (printout t ?name " can give blood to " ?name2 crlf)
         )
(defrule O-gives-to-others-but-not-itself ; O to O cover in above rule
         (blood-donor (name ?name)(type ?type &:(eq ?type "O")))
         (blood-donor (name ?name2)(type ?type2 &: (neq ?type ?type2) &: (neq ?name ?name2) ))
         =>
         (printout t ?name " can give blood to " ?name2 crlf)
         )
(defrule A-or-B-gives-to-AB ; case O gives to AB and AB gives to AB already dealt with
         (blood-donor (name ?name)(type ?type &:(or (eq ?type "A") (eq ?type "B" ))))
         (blood-donor (name ?name2)(type ?type2 &: (eq ?type2 "AB") &: (neq ?name ?name2) ))
         =>
         (printout t ?name " can give blood to " ?name2 crlf)
         )
;(watch all)
(reset)
(run)

See also
 Semantic reasoner
 Decision Model and Notation

Related systems
 CLIPS: public-domain software tool to build expert systems
 ILOG rules: business rule management system
 JBoss Drools: business rule management system (BRMS)
 Prolog: general purpose logic programming language
 OpenL Tablets: business centric rules and BRMS
 DTRules: decision table based, open-source rule engine for Java

References

Further sources

External links
 

Expert systems
Rule engines
Knowledge representation languages
Java APIs
Sandia National Laboratories

Articles with example Lisp (programming language) code