Intelligence TCSS435A
Artificial & Knowledge Acquisition
Winter 2009
Jess: an Introduction Jess is a first-order logic (FOL) inference engine for developing expert systems in Java. Jess derives from a FOL inference engine written in LISP, named CLIPS. CLIPS is defined as an expert system tool developed by the Software Technology Branch (STB) at the NASA/ Lyndon B. Johnson Space Center. The Java port of CLIPS has consisted mainly in writing a LISP interpreter in Java, under which the native CLIPS runs. Consequently, Jess syntax is that of LISP, and LISP programs can also be interpreted under Jess.
How to Program in Jess
In the labs, Jess is installed under the G: drive in a folder corresponding to its version the latest one being Jess71p2. It is recommended to work in the directory where your Jess code will be developed. For example, to run the Monkey and Bananas example fullmab.clp located in the folder C:\TCSS435\labs: 1. Start Jess from development folder by invoking at the DOS-prompt G:\Jess71p2\bin\jess.bat:
C:\TCSS435\labs>G:\Jess71p2\bin\jess.bat
2. Load the CLIPS file fullmab.clp
Jess, the Rule Engine for the Java Platform Copyright (C) 2008 Sandia Corporation Jess Version 7.1p2 11/5/2008 Jess> (batch fullmab.clp) Jess> (reset) Jess> (run) Monkey jumps off the green-couch onto the floor. Monkey walks to t2-2. Jess>
3. Tips o To reinitialize the set of facts, command is (reset). o To run the inference engine, command is (run). o To remove all rules and facts from memory, command is (clear). Whenever you modify the set of rules or the set of facts, it is important to clear the previous knowledge base and working memory entirely by using this (clear) command.
Structure of a Jess Program
A Jess program has three main parts, which can be separated into several files, like in the Wumpus example, where the facts are separated from the templates and rules:
TCSS435A
Artificial Intelligence & Knowledge Acquisition
Winter 2009
1. Templates declarations, corresponding to classes and attributes declarations, for example:
(deftemplate hunter A hunter (slot agent (default Xena)) (slot x (type INTEGER)) (slot y (type INTEGER)) (slot gold (default 0) (type INTEGER)) (slot alive (default TRUE)) )
2. Rules declarations, corresponding to processes declarations, for example:
(defrule desire-to-leave-caves (task think) (hunter (agent ?a)(x ?x)(y ?y)(gold ~0)) (cave (x ?x)(y ?y)(has-exit TRUE)) => (printout t "Having found the gold and killed the wumpus, " ?a " want to leave the caves." crlf) (assert (desire (agent ?a)(strength ?*veryhigh*) (action leavecaves))))
3. Facts definition, corresponding to problem situation description, populating the working memory, for example:
(deffacts caves (worldsize 4 4) (wumpus (x 1) (y 3)) (pit (x 3)(y 1)) (pit (x 3)(y 3)) (pit (x 4)(y 4)) (gold (x 2)(y 3)(amount 10)) (exit (x 1)(y 1)) (nocave (x 2)(y 4)) (hunter (agent Xena)))
Jess Syntax
Jess syntax is that of Lisp, with the addition of rules, templates, and facts concepts. 1. Lisp syntax: Symbols: can be letters, digits and punctuation symbols. Symbols cannot start with a digit. They are case sensitive. Example: hunter, _abc. Numbers: can be integer, long, and floating point numbers. Example: 1, 4.2. Strings: are enclosed in double quotes (). Example: Hello. Lists: are composed of a set of parentheses enclosing symbols. Example: (+ 1 2) represents the 1 + 2 and will return the value 3. Comments: are preceded by a semi-colon, or enclosed within /* and */. Example: ; comment. Function calls: there is no operator, only function calls. All function calls are lists where the function is the leftmost symbol. Example: (+ (+ 2 3) (* 3 3)).
TCSS435A
Artificial Intelligence & Knowledge Acquisition
Winter 2009
Printout: is used to display a message. Example: (printout t Hello world ! crlf). Variables: start with the question mark ? character. Variables do not have to be declared before being used. Assignment of a value to a variable can be obtained with (bind ?x 2). Example: ?x. Global variables: defined are with a defglobal function. All other variables are local to the rules in which they will be used. Example: (defglobal ?*highest* = 6 ? *veryhigh* = 5 ). Branching: is defined by if then else pattern. Example: (if (> ?x 100) then (printout t X is large crlf) else (printout t X is small crlf) ) 2. Template syntax: Templates are composed of a name and slots corresponding to variables. Inheritance can be defined between templates. A slot can have a type within INTEGER, FLOAT, NUMBER, SYMBOL, STRING, LONG, LEXEME, and OBJECT. 3. Fact syntax: Facts can be added or removed from the working memory. Add a fact: assert the fact. Example: (assert (goal (action ?act) (x ?x) (y ?y) )). Remove a fact: retract the fact. Example: (retract ?desire). Modify a fact: modify the fact. Example: (modify ?f (has-wumpus FALSE)). 4. Rule syntax: A rule is composed of a left-hand side (LHS) containing conditions or a pattern to be matched against the facts in working memory, and a right-hand side (RHS) containing actions or consequents. A rule of the form IF LHS THEN RHS is expressed in Jess as: (defrule rule-name (LHS) => (RHS) ) Both LHS and RHS are fully parenthesized expressions. LHS conditions are prefixed by an AND which is implicit (not represented). Boolean expressions use operators such as <= (less equal), == (equals), <> or != (not equals), && (and), || (or). It is recommended to look at several rules to try to understand the many different ways rules can be expressed. Some examples are represented below. Example 1:
(defrule safe-cave (task think) ?f <- (cave (x ?x)(y ?y) (has-wumpus FALSE)(has-pit FALSE)(safe UNKNOWN)) =>
TCSS435A
Artificial Intelligence & Knowledge Acquisition
Winter 2009
(printout t "With neither wumpus nor pit, (" ?x "," ?y ") is safe." crlf) (modify ?f (safe TRUE)))
This rule will be executed when the working memory contains both a fact with a variable name task of value think and a cave template instance with slots has-wumpus with value FALSE, has-pit with value FALSE, slot safe with value UNKNOWN, and any value for slots x and y. In addition, the LHS of the rule stores the cave fact in a variable ?f (remember that variables do not have to be declared in advance, and are local to the rule). This saving of the fact is necessary because the rule will modify the cave slot values. Therefore the LHS of the rule will make the rule e...