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COMP 4200: Expert Systems

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Title: COMP 4200: Expert Systems

1
Expert systemsCLIPS
Seyed Hashem Davarpanah Davarpanah_at_usc.ac.ir Unive
rsity 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
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
entered exactly as shown
(example)
square brackets ...
contents are optional
(example test)
pointed brackets (less than / greater than signs)
lt ... gt
replace contents by an instance of that type
(example ltchargt)
star
replace with zero or more instances of the type
ltchargt
plus
replace with one or more instances of the type
ltchargt (is equivalent to ltchargt ltchargt )
vertical bar
choice among a set of items
true false

8
Invoke / Exit CLIPS

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

9
Fields - Examples

Fields (data types)
float 4.00, 2.0e2, 2e-2
integer 4, 2, 22
symbol Alpha24, !?_at_
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
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 1 2 55 6 7 42)
(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
(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
CLIPSgt (assert (today is sunday))
ltFact-0gt
Facts can be listed
CLIPSgt (facts)
f-0 (today is sunday)
Facts can be retracted
CLIPSgt (retract 0)
CLIPSgt (facts)

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)
(eye-color blue) (hair-color blond))
(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
(assert ltfactgt)
deleting facts
(retract ltfact-indexgt)
modifying facts
(modify ltfact-indexgt (ltslot-namegt ltslot-valuegt)
)
retracts the original fact and asserts a new,
modified fact
duplicating facts
(duplicate ltfact-indexgt (ltslot-namegt
ltslot-valuegt) )
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 ltrule namegt "comment"
ltpatternsgt left-hand side (LHS)
or antecedent of the rule
gt
ltactionsgt) right-hand side (RHS)
or consequent of the rule

19
Rule Components

rule header
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
comment
rule name
(defrule birthday A persons birthday (person
(name ?name) (age ?age)) (has-birthday ?name
?age) gt (printout t Happy Birthday,
?name))
template fact
ordered fact
variables
terminal
variable
text
function
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)
gt
(printout t "Happy birthday, Franz!")
(modify 1 (age 47))
)

22
Wildcards

question mark ?
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 -10000 to 10000.
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)
gt
(printout t "Print First" crlf) )
(defrule fire-second
(declare (salience 20))
(priority second)
gt
(printout t "Print Second" crlf) )

26
Field Constraints

not 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))
gt
(printout t ?name " has blue eyes."
crlf))

29
Example Rule with Field Constraints

(defrule silly-eye-hair-match
(person (name ?name1)
(eye-color ?eyes1bluegreen)
(hair-color ?hair1black))
(person (name ?name2?name1)
(eye-color ?eyes2?eyes1)
(hair-color ?hair2red?hair1))
gt
(printout t ?name1 " has "?eyes1 " eyes and "
?hair1 " hair." crlf)
(printout t ?name2 " has "?eyes2 " eyes and "
?hair2 " hair." crlf))

30
Using Templates

(deftemplate student a student record
(slot name (type STRING))
(slot age (type NUMBER) (default 18)))
CLIPSgt (assert (student (name fred)))
(defrule print-a-student
(student (name ?name) (age ?age))
gt
(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)
gt
(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)
gt
(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 lt- (do ?job on ?day)
gt
(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 ltpredicate-expressiongt then
ltexpressiongt else ltexpressiongt
) else-part optional (while
ltpredicate-expressiongt do ltexpressiongt
) do not mandatory
35
Example if-then-else
(defrule special-age 18, 21, 100 (or (person
(name ?name) (age ?age18)) (person (name
?name) (age ?age21)) (person (name ?name)
(age ?age100))) gt (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.)
else (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))
)
gt
(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)))
)
gt
(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)))
gt
(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 -10000 to 10000.
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)
gt
(printout t "Print First" crlf) )
(defrule fire-second
(declare (salience 20))
(priority second)
gt
(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 ltdefrule-namegt), (ppdeftemplate
ltdeftemplate-namegt), (ppdeffacts ltdeffacts-namegt)
displays the text of the construct (pretty
print'')
deleting constructs
(undefrule ltdefrule-namegt), (undeftemplate
ltdeftemplate-namegt), (undeffacts ltdeffacts-namegt)
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
(open ltfile-namegt ltlogical-namegt r)
ltfile-namegt is physical file-name (path)
ltlogical-namegt is name used in program
r indicates read-access (w, r)
example (open example.dat my-file r)
(read my-file)
Close file
(close ltlogical-namegt)

46
Input read, readline

read input of single field
readline input of complete (line as string)
general
(read ltlogical namegt)
ltlogical namegt 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

(read / readline ltlogical namegt)
default is keyboard/terminal
file has to be opened using
(open ltfile-namegt ltlogical-namegt r)
ltfile-namegt is physical file-name (can include
path)
ltlogical-namegt 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 ltlogical-namegt ... )
t terminal is standard
otherwise ltlogical-namegt refers to a file-name
file has to be opened using
(open ltfile-namegt ltlogical-namegt w )
ltfile-namegt is physical file-name (can include
path)
ltlogical-namegt 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
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 ltrule-namegt)
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 ltlimitgt) starts the execution
breakpoints
(set-break ltrule-namegt)
stops the execution before the rule fires,
continue with (run)
(remove-break ltrule-namegt)
(show-breaks)

51
Watching

watching the execution
(watch ltwatch-itemgt) prints messages about
activities concerning a ltwatch-itemgt
(facts, rules, activations, statistics,
compilation, focus, all)
(unwatch ltwatch-itemgt)
turns the messages off

52
Watching Facts, Rules and Activations

facts
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
information about the program execution
(number of rules fired, run time, ... )
compilation (default)
shows information for constructs loaded by (load)
Defining deftemplate ...
Defining defrule ... jj
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
CLIPSgt (instances)
corvette of car
CLIPSgt (send corvette print)
corvette of car
(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
CLIPSgt (send corvette delete)
deleting all instances
CLIPSgt (unmake-instance )
resetting creates an initial object
CLIPSgt (reset)
CLIPSgt (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