1. Expert systems CLIPS Seyed Hashem Davarpanah Davarpanah@usc.ac.ir
University of Science and Culture

2. Motivation CLIPS is a decent example of an expert system shell
rule-based, forward-chaining system
it illustrates many of the concepts and methods used in other XPS shells
it allows the representation of knowledge, and its use for solving suitable problems

3. Introduction CLIPS stands for forward-chaining pattern-matching
C Language Implementation Production System
forward-chaining
starting from the facts, a solution is developed
pattern-matching
Rete matching algorithm: find ``fitting'' rules and facts
knowledge-based system shell
empty tool, to be filled with knowledge
multi-paradigm programming language
rule-based, object-oriented (Cool) and procedural

4. Rete Matching Algorithm
An expert system might check each rule against the known facts in the knowledge base, firing that rule if necessary, then moving on to the next rule.
For even moderate sized rules and facts knowledge-bases, this approach performs far too slowly.
The Rete algorithm provides the basis for a more efficient implementation.
A Rete-based expert system builds a network of nodes, where each node (except the root) corresponds to a pattern occurring in the left-hand-side (the condition part) of a rule.
The path from the root node to a leaf node defines a complete rule left-hand-side. Each node has a memory of facts which satisfy that pattern. As new facts are asserted or modified, they propagate along the network, causing nodes to be annotated when that fact matches that pattern. When a fact or combination of facts causes all of the patterns for a given rule to be satisfied, a leaf node is reached and the corresponding rule is triggered.

5. The CLIPS Programming Tool
history of CLIPS
influenced by OPS5 and ART
implemented in C for efficiency and portability
developed by NASA, distributed & supported by COSMIC
runs on PC, Mac, UNIX, VAX VMS
CLIPS provides mechanisms for expert systems
a top-level interpreter
production rule interpreter
object oriented programming language
LISP-like procedural language

6. Components of CLIPS rule-based language
can create a fact list
can create a rule set
an inference engine matches facts against rules
object-oriented language (COOL)
can define classes
can create different sets of instances
special forms allow you to interface rules and objects
[Jackson 1999]

7. Notation symbols, characters, keywords square brackets [...]
entered exactly as shown:
(example)
square brackets [...]
contents are optional:
(example [test])
pointed brackets (less than / greater than signs) < ... >
replace contents by an instance of that type
(example <char>)
star *
replace with zero or more instances of the type
<char>*
plus +
replace with one or more instances of the type
<char>+ (is equivalent to <char> <char>* )
vertical bar |
choice among a set of items:
true | false

8. Invoke / Exit CLIPS entering CLIPS exiting CLIPS
double-click on icon, or type program name (CLIPS)
system prompt appears:
CLIPS>
exiting CLIPS
at the system prompt
type (exit)
Note: enclosing parentheses are important; they indicate a command to be executed, not just a symbol

9. Fields - Examples Fields (data types) Variables
float , 2.0e+2, 2e-2
integer 4, 2, 22
symbol Alpha24*,
string “Johnny B. Good”
instance name [titanic], [PPK]
Variables
?var, ?x, ?day variables for single field value
$?names variable for multi-field value

10. CLIPS –Facts Facts (today is Thursday) a relation-name,
an ordered sequence of values (ordered facts), or
a set of (slot-name slot-value)-pairs (i.e. deftemplate-facts)
examples:
(today is Thursday)
(person (name “Johnny B. Good”) (age 25))

11. Ordered Facts Ordered facts
are facts defined without (explicit) template;
the field-values are ordered.
Examples:
(number-list )
(today is Sunday)

12. Deftemplate Facts Deftemplate-facts
are facts defined based on a template;
slots can be arranged arbitrarily, there is no specific order.
Define a template for describing a set of facts using deftemplate (record structure) .
Use deffacts to create a list of facts based on a template.

13. Examples of Facts ordered fact deftemplate fact
(person-name Franz J. Kurfess)
deftemplate fact
(deftemplate person "deftemplate example”
(slot name)
(slot age)
(slot eye-color)
(slot hair-color))

14. Defining Facts Facts can be asserted Facts can be listed
CLIPS> (assert (today is sunday))
<Fact-0>
Facts can be listed
CLIPS> (facts)
f-0 (today is sunday)
Facts can be retracted
CLIPS> (retract 0)
[Jackson 1999]

15. Instances an instance of a fact is created by
(assert (person (name "Franz J. Kurfess")
(age 46)
(eye-color brown)
(hair-color brown)) )

16. Initial Facts
(deffacts kurfesses "some members of the Kurfess family" (person (name "Franz J. Kurfess") (age 46) (eye-color brown) (hair-color brown)) (person (name "Hubert Kurfess") (age 44) (eye-color blue) (hair-color blond)) (person (name "Bernhard Kurfess") (age 41) (person (name "Heinrich Kurfess") (age 38) (eye-color brown) (hair-color blond)) (person (name "Irmgard Kurfess") (age 37) (eye-color green) (hair-color blond)) )

17. Usage of Facts adding facts deleting facts modifying facts
(assert <fact>+)
deleting facts
(retract <fact-index>+)
modifying facts
(modify <fact-index> (<slot-name> <slot-value>)+ )
retracts the original fact and asserts a new, modified fact
duplicating facts
(duplicate <fact-index> (<slot-name> <slot-value>)+ )
adds a new, possibly modified fact
inspection of facts
(facts)
prints the list of facts
(watch facts)
automatically displays changes to the fact list

18. Rules general format (defrule <rule name> ["comment"]
<patterns>* ; left-hand side (LHS)
; or antecedent of the rule
=>
<actions>*) ; right-hand side (RHS)
; or consequent of the rule

19. Rule Components rule header rule antecedent (LHS) rule arrow
defrule keyword, name of the rule, optional comment string
rule antecedent (LHS)
patterns to be matched against facts
rule arrow
separates antecedent and consequent
rule consequent (RHS)
actions to be performed when the rule fires

20. Rule - Example (defrule birthday “A person’s birthday”
comment
rule name
(defrule birthday “A person’s birthday”
(person (name ?name) (age ?age))
(has-birthday ?name ?age)
=>
(printout t “Happy Birthday, ” ?name))
template
fact
ordered fact
variables
function
terminal
text
variable

21. Examples of Rules simple rule (defrule birthday-FJK
(person (name "Franz J. Kurfess")
(age 46)
(eye-color brown)
(hair-color brown))
(date-today April-13-02)
=>
(printout t "Happy birthday, Franz!")
(modify 1 (age 47)) )

22. Wildcards question mark ? multi-field wildcard $?
matches any single field within a fact
multi-field wildcard $?
matches zero or more fields in a fact

23. Salience We can use salience measures to prioritize rules.
CLIPS provides a built-in method for prioritizing rules:
(declare (salience value))
Salience values can range from to Default is 0.
We can thus force the execution of one rule over another. We can implement sequencing of rules.

24. Rule Prioritization in Clips
for example, consider the following rules (forced order of execution)

25. Two Nifty Rules
(defrule fire-first (declare (salience 30)) (priority first) => (printout t "Print First" crlf) ) (defrule fire-second (declare (salience 20)) (priority second) (printout t "Print Second" crlf) )

26. Field Constraints not constraint ~ or constraint | and constraint &
the field can take any value except the one specified
or constraint |
specifies alternative values, one of which must match
and constraint &
the value of the field must match all specified values
mostly used to place constraints on the binding of a variable

27. Mathematical Operators
basic operators (+,-,*,/) and many functions (trigonometric, logarithmic, exponential) are supported
prefix notation
no built-in precedence, only left-to-right and parentheses
test feature
evaluates an expression in the LHS instead of matching a pattern against a fact
pattern connectives
multiple patterns in the LHS are implicitly AND-connected
patterns can also be explicitly connected via AND, OR, NOT
user-defined functions
external functions written in C or other languages can be integrated
Jess is tightly integrated with Java

28. Examples of Rules more complex rule (defrule find-blue-eyes
(person (name ?name)
(eye-color blue))
=>
(printout t ?name " has blue eyes."
crlf))

29. Example Rule with Field Constraints
(defrule silly-eye-hair-match
(person (name ?name1)
(eye-color ?eyes1&blue|green)
(hair-color ?hair1&~black))
(person (name ?name2&~?name1)
(eye-color ?eyes2&~?eyes1)
(hair-color ?hair2&red|?hair1))
=>
(printout t ?name1 " has "?eyes1 " eyes and " ?hair1 " hair." crlf)
(printout t ?name2 " has "?eyes2 " eyes and " ?hair2 " hair." crlf))

30. Using Templates (slot name (type STRING))
(deftemplate student “a student record”
(slot name (type STRING))
(slot age (type NUMBER) (default 18)))
CLIPS> (assert (student (name fred)))
(defrule print-a-student
(student (name ?name) (age ?age))
=>
(printout t ?name “ is “ ?age) )
[Jackson 1999]

31. An Example CLIPS Rule
(defrule sunday “Things to do on Sunday”
(salience 0) ; salience in the interval [-10000, 10000]
(today is Sunday)
(weather is sunny)
=>
(assert (chore wash car))
(assert (chore chop wood)) )
[Jackson 1999]

32. Variables & Pattern Matching
Variables make rules more applicable
(defrule pick-a-chore
(today is ?day)
(chore is ?job)
=>
(assert (do ?job on ?day)) )
if conditions are matched, then bindings are used
[Jackson 1999]

33. Retracting Facts from a Rule
(defrule do-a-chore
(today is ?day) ; ?day must have a consistent binding
?chore <- (do ?job on ?day)
=>
(printout t ?job “ done”)
(retract ?chore) )
a variable must be assigned to the item for retraction
[Jackson 1999]

34. Procedural Control in Actions
Procedural Control Elements can appear on the RHS of a rule or in message-handlers of classes.
(if <predicate-expression>
then <expression>+
[else <expression>+ ]) ;;else-part optional
(while <predicate-expression>
[do] <expression>* ]) ;;‘do’ not mandatory

35. Example – if-then-else
(defrule special-age “18, 21, 100”
(or (person (name ?name) (age ?age&18))
(person (name ?name) (age ?age&21))
(person (name ?name) (age ?age&100)))
=>
(if (= ?age 18) then (printout t ?name “ can buy beer in Canada.”)
else
(if (= ?age 21) then (printout t ?name “ can buy beer in the USA.”)
(if (= ?age 100) then (printout t “The major will visit ” ?name ))...)

36. Condition Patterns with Logical Connectives
Complex Conditions with logical connectives:
(or (pattern1) (pattern2))
Rule becomes active if one of the patterns matches.
example: (or (birthday) (anniversary))
matches fact base with facts (birthday) or (anniversary)
Equivalent for:
and (is default)
not
exists to be fulfilled for one matching fact
forall to be fulfilled for all facts which match based on first fact and variable binding

37. Complex Condition Elements - or
(defrule report-emergency
(or (emergency (emergency-type fire) (location ?building))
(emergency (emergency-type bomb) (location ?building)) )
=>
(printout t “evacuate “ ?building)
reports a building if there is a fire or bomb emergency in this building

38. Complex Condition Elements – exists
(defrule emergency-report
(exists
(or (emergency (emergency-type fire))
(emergency (emergency-type bomb))) )
=>
(printout t “There is an emergency.“ crlf )
prints one emergency-message if there is a fire or bomb emergency. (no further matching, firing, or printout)

39. Complex Condition Elements – forall
(defrule evacuated-all-buildings
(forall (emergency (emergency-type fire | bomb)
(location ?building) )
(evacuated (building ?building)))
=>
(printout t “All buildings with emergency are evacuated “ crlf))
prints evacuated-message if for all buildings, which have a fire or bomb emergency, the building is evacuated.

40. Salience We can use salience measures to prioritize rules.
CLIPS provides a built-in method for prioritizing rules:
(declare (salience value))
Salience values can range from to Default is 0.
We can thus force the execution of one rule over another. We can implement sequencing of rules.

41. Rule Prioritization in Clips
for example, consider the following rules (forced order of execution)

42. Two Nifty Rules
(defrule fire-first (declare (salience 30)) (priority first) => (printout t "Print First" crlf) ) (defrule fire-second (declare (salience 20)) (priority second) (printout t "Print Second" crlf) )

43. Manipulation of Constructs
show list of constructs
(list-defrules), (list-deftemplates), (list-deffacts)
prints a list of the respective constructs
show text of constructs
(ppdefrule <defrule-name>), (ppdeftemplate <deftemplate-name>), (ppdeffacts <deffacts-name>)
displays the text of the construct (``pretty print'')
deleting constructs
(undefrule <defrule-name>), (undeftemplate <deftemplate-name>), (undeffacts <deffacts-name>)
deletes the construct (if it is not in use)
clearing the CLIPS environment
(clear)
removes all constructs and adds the initial facts to the CLIPS environment

44. bind-function
bind-function – explicitly binds value to variable (bind ?age (read)) stores value of single field which is read into single-field variable ?age (bind ?name (readline)) stores line which is read as STRING into single-field STRING-variable ?address (bind ?address (explode$ (readline))) explode$ splits line which is read as STRING into multifield-value which is stored in multislot-variable ?address

45. Open, Close File Open file for read/write: Close file:
(open “<file-name>” <logical-name> “r”)
<file-name> is physical file-name (path)
<logical-name> is name used in program
“r” indicates read-access (“w”, “r+”)
example: (open “example.dat” my-file “r”)
(read my-file)
Close file:
(close <logical-name>)

46. Input – read, readline
read – input of single field readline – input of complete (line as string) general: (read <logical name>) <logical name> refers to file-name in program (read) keyboard is default read with bind-function to bind input to variable: (bind ?input (read)) (bind $?input (readline))

47. Input – read, readline default is keyboard/terminal
(read / readline <logical name>)
default is keyboard/terminal
file has to be opened using
(open “<file-name>” <logical-name> “r”)
<file-name> is physical file-name (can include path)
<logical-name> is name used in read command
“r” indicates read-access
example: (open “example.dat” example “r”)
(read example)
use with bind-function to bind input to variable

48. Output - printout
(printout <logical-name> ... )
t terminal is standard
otherwise <logical-name> refers to a file-name
file has to be opened using
(open “<file-name>” <logical-name> “w” )
<file-name> is physical file-name (can include path)
<logical-name> is name used in printout command
“w” indicates write-access
example: (open “example.dat” my-output “w” )
(printout my-output ?name crlf)

49. Program Execution agenda salience refraction
if all patterns of a rule match with facts, it is put on the agenda
(agenda) displays all activated rules
salience
indicates priority of rules
refraction
rules fire only once for a specific set of facts
prevents infinite loops
(refresh <rule-name>)
reactivates rules

50. Execution of a Program (reset) prepares (re)start of a program:
all previous facts are deleted
initial facts are asserted
rules matching these facts are put on the agenda
(run [<limit>]) starts the execution
breakpoints
(set-break [<rule-name>])
stops the execution before the rule fires,
continue with (run)
(remove-break [<rule-name>])
(show-breaks)

51. Watching watching the execution
(watch <watch-item>) prints messages about activities concerning a <watch-item>
(facts, rules, activations, statistics, compilation, focus, all)
(unwatch <watch-item>)
turns the messages off

52. Watching Facts, Rules and Activations
assertions (add) and retractions (delete)
of facts
rules
message for each rule that is fired
activations
activated rules: matching antecedents
these rules are on the agenda

53. More Watching ... statistics compilation (default) focus
information about the program execution
(number of rules fired, run time, ... )
compilation (default)
shows information for constructs loaded by (load)
Defining deftemplate: ...
Defining defrule: ... +j=j
+j, =j indicates the internal structure of the compiled rules
+j join added
=j join shared
important for the efficiency of the Rete pattern matching network
focus
used with modules
indicates which module is currently active

54. Defining Functions in CLIPS
Uses a LISP or Scheme-like syntax
(deffunction function-name (arg ... arg)
action ... action)
(deffunction hypotenuse (?a ?b)
(sqrt (+ (* ?a ?a) (* ?b ?b))))
(deffunction initialize ()
(clear)
(assert (today is sunday)))
[Jackson 1999]

55. Defining Classes & Instances
defining the class CAR
(defclass car
(is-a user)
(name)
(made-by))
defining an instance of CAR
(make-instance corvette of car
(made-by chevrolet))
[Jackson 1999]

56. Managing Instances Commands to display instances
CLIPS> (instances)
[corvette] of car
CLIPS> (send [corvette] print)
(made-by chevrolet)
Command to group instances (in a file)
(definstances
(corvette of car (made-by chevrolet))
(thunderbird of car (made-by ford)))
[Jackson 1999]

57. Clearing & Resetting Instances
deleting an instance
CLIPS> (send [corvette] delete)
deleting all instances
CLIPS> (unmake-instance *)
resetting creates an initial object
CLIPS> (reset)
CLIPS> (instances)
[initial-object] of INITIAL-OBJECT
[Jackson 1999]

58. Limitations of CLIPS single level rule sets
in LOOPS, you could arrange rule sets in a hierarchy, embedding one rule set inside another, etc
loose coupling of rules and objects
rules can communicate with objects via message passing
rules cannot easily be embedded in objects, as in Centaur
CLIPS has no explicit agenda mechanism
the basic control flow is forward chaining
to implement other kinds of reasoning you have to manipulate tokens in working memory
[Jackson 1999]

59. Alternatives to CLIPS JESS Eclipse NEXPERT OBJECT see below
enhanced, commercial variant of CLIPS
has same syntax as CLIPS (both are based on ART)
supports goal-driven (i.e., backwards) reasoning
has a truth maintenance facility for checking consistency
can be integrated with C++ and dBase
new extension RETE++ can generate C++ header files
not related to the (newer) IBM Eclipse environment
NEXPERT OBJECT
another rule- and object-based system
has facilities for designing graphical interfaces
has a ‘script language’ for designing user front-end
written in C, runs on many platforms, highly portable
[Jackson 1999]

60. JESS JESS stands for Java Expert System Shell
it uses the same syntax and a large majority of the features of CLIPS
tight integration with Java
can be invoked easily from Java programs
can utilize object-oriented aspects of Java
some incompatibilities with CLIPS
COOL replaced by Java classes
a few missing constructs
more and more added as new versions of JESS are released

61. CLIPS Summary notation facts rules variables, operators, functions
similar to Lisp, regular expressions
facts
(deftemplate), (deffacts), assert / retract
rules
(defrule ...), agenda
variables, operators, functions
advanced pattern matching
input/output
(printout ...), (read ...), (load ...)
program execution
(reset), (run), breakpoints
user interface
command line or GUI

62. Important Concepts and Terms
agenda
antecedent
assert
backward chaining
consequent
CLIPS
expert system shell
fact
field
forward chaining
function
inference
inference mechanism
instance
If-Then rules
JESS
knowledge base
knowledge representation
pattern matching
refraction
retract
rule
rule header
salience
template
variable
wild card