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CLIPS User's Guide

1. A familiar example of a tree organizational structure is that often used by companies which have a hierarchy consisting of president vice presidents department heads managers and so forth down to the lowest employee In this case the hierarchical structure mirrors the authority of people in the organization Trees are generally used for organizations of people because every person has exactly one boss except the top dog who has no boss Nodes in the organizational chart represent the positions such as president vice president etc Lines connecting the positions are the branches that indicate the division of responsibility Links are often called branches in a tree In Fig 1 5 every class except YUKKIE is legal or legitimate For example a SUPPIE is a UPPIE A MUPPIE is a UPPIE A YUPPIE is a UPPIE A PUPPIE is a YUPPIE no 92 middle aged YUPPIE moms allowed We would also like to say that a YUKKIE is a YUPPIE and a YUKKIE is a UPPIE by inheritance However we don t want to say that a YUKKIE is a PUPPIE which is what the is a link between YUKKIE and PUPPIE means The is a link between YUKKIE and PUPPIE is a natural mistake for a person to make since a YUKKIE is the child of a PUPPIE actually an ex PUPPIE after she gives birth Although making an is a link between YUKKIE and PUPPIE does allow the YUKKIE to inherit from YUPPIE and UPPIE as desired it also produces an illegitimate relationship by saying that a YUKKIE is a PUPP
2. Cdefclass lt class gt is a lt direct Superclasses gt The list of classes lt superclass name gt is called a direct superclass precedence list because it defines how a class is linked to its direct superclasses The direct superclasses of a class are the one or more classes named after the is a keyword In our example class DUCKLING is the direct superclass of DUCK Note that at least one direct superclass must be given in the direct superclass precedence list If the direct superclass list was as follows Cdefclass DUCK Cis a DUCKLING USER OBJECT then USER and OBJECT would also be direct superclasses of DUCK In this example it makes no difference whether USER and OBJECT are specified in addition to DUCKLING In fact since USER and OBJECT are predefined classes that are always linked such that USER is a OBJECT and OBJECT is the root you need never specify them except when defining a subclass of USER Since USER only inherits from OBJECT it is not necessary to specify OBJECT if USER is specified The indirect superclasses of a class are all the classes not named after the is a that contribute slots and message handlers by inheritance In our example the indirect superclasses are USER and OBJECT A class inherits slots and messagevhandlers from all its direct and indirect superclasses Thus DUCK inherits from DUCKLING USER and OBJECT A direct subclass is connected by a single link to the class above it An indirect sub
3. OBJECT In our example the single inheritance path of DUCK is DUCK DUCKLING USER and OBJECT You will see examples later such as Fig 1 6 in which a class has multiple inheritance paths to OBJECT Fig 1 3 illustrates a very simplified taxonomy of organisms that illustrates inheritance in Nature The term taxonomy means a classification Biological taxonomies are designed to show the kinship of organisms That is a biological taxonomy emphasizes similarities between organisms by grouping them together ORGANISM ANIMAL MAMMAL ANGIOSPERMAE DUCKWEED Fig 1 3 Simple Taxonomy of Living Organisms With is a Links In a taxonomy like Fig 1 3 the connecting lines are all is a links For example a DUCK is a BIRD A BIRD is a ANIMAL An ANIMAL is a ORGANISM and so forth Although the genetic inheritance of each individual is different the characteristics of MAN and DUCK are the same for each species In Fig 1 3 notice that the most general class ORGANISM is at the top while more specialized classes are lower in the taxonomy In CLIPS terminology we would say that each subclass inherits the slots of its parent classes For example since mammals are warm blooded and give birth to live young with the exception of the platypus the class MAN inherits the attributes of the parent MAMMAL class The direct superclass of MAMMAL is ANIMAL and the direct subclass of MAMMAL is MAN The indirect superclass of MAMMAL is ORGANISM
4. 116 CLIPS gt Cclear CLIPS gt Cdefclass DUCK is a USER Cslot sound Cdefault quack Cslot ID CslLot sex default male CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound quack CID nil Csex male CLIPS gt As you can see the default values for slot sex and slot sound were set by the default facet values Following the default keyword can be any valid CLIPS expression that does not involve a variable For example the default expression of the sound slot is the symbol quack Functions may be used in the facet expression as will be shown in the next example This default facet is a static default because the value of its facet expression is determined when the class is defined and never changed unless the class is redefined For example let s set the default value of slot ID to the gensym function which returns a new value not in the system everytime it s called CLIPS gt Cclear CLIPS gt Cdefclass DUCK is a USER Cslot sound Cdefault quack Cslot ID default Cgensym CslLot sex default male CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck 1 Dorky_Duck CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound quack CID gen1 Csex male CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck 2 Dorky_Duck CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound quack i CID gen1
5. CLIPS gt CLIPS puts different parts of the rule on different lines for the sake of readability The patterns before the arrow are still considered the LHS and the actions after the arrow are still considered the RHS of the rule The term MAIN refers to the MAIN module that this rule is in by default You can define modules to put rules in analogous to the 28 statements that may be put in different packages modules procedures or functions of other programming languages The use of modules make it easier to write expert systems having many rules since these may be grouped together with their own agendas for each module For more information see the CLIPS Reference Manual What if you want to print a rule but can t remember the name of the rule No problem Just use the rules command in response to a CLIPS prompt and CLIPS will print out the names of all the rules For example enter CLIPS gt Crules duck For a total of 1 defrule CLIPS gt Write to Me Besides asserting facts in the RHS of rules you also can print out information using the printout function CLIPS also has a carriage return linefeed keyword called erlf which is very useful in improving the appearance of output by formatting it on different lines For a change the crifis not included in parentheses As an example CLIPS gt Cdefrule duck Canimal is duck gt Cprintout t quack crlf Be sure to type in the t gt Activation duck f C
6. Other Features The test conditional element provides a very powerful way by which to compare numbers variables and strings on the LHS The test is used as a pattern on the LHS A rule will only be triggered if the test is satisfied together with other patterns Many predefined functions are provided by CLIPS as shown in the following table Predefined Functions Logical Arithmetic not Boolean not division and Boolean and F multiplication or Boolean or addition subtraction Comparison eq equal any type Compares type and magnitude neq not equal any type equal numeric type Compares magnitude gt not equal numeric type gt greater than or equal to gt greater than lt less than or equal to lt less than 79 All the comparison functions except eq and neq will give an error message if they are used to compare a number and non number If the type is not known in advance the eq and neq functions should be used The eq function checks for the same magnitude and type of its arguments while the function only checks the magnitude of its numeric arguments and doesn t care if they re integer or floating point The logical functions of CLIPS are and or and not They can be used in expressions as Boolean functions In CLIPS true and false are represented by the symbols TRUE and FALSE Note that upper case must be used for logical values in CLIPS In addition to all the predefined function
7. The other classes such as BIRD DUCK and so forth have no relationship to MAMMAL because they are not on an inheritance path from the most general class ORGANISM An inheritance path is any path from one class to another that does not involve backtracking or retracing the path A class such as PLANT is not on an inheritance path to MAMMAL because we would have to backtrack to ORGANISM before continuing down to MAMMAL Thus MAMMAL does not automatically obtain any slots from PLANT or any other classes not on the inheritance path to MAMMAL This model of inheritance mirrors the real world since otherwise we might have grass 91 growing on our heads instead of hair The Illegitimate YUKKIE Now that you ve got the basic idea of classes let s add some additional classes to the UPPIE diagram of Fig 1 1 in order to make the example more realistic This type of development by adding lower level classes is the way that OOP is done by adding classes from the most general to the most specific classes Fig 1 5 shows the inheritance diagram of the the illegitimate YUKKIE For simplicity the OBJECT and USER classes are not shown The hierarchy of Fig 1 5 is a tree because every node has exactly one parent YUPPIE PUPPIE YUKKIE Fig 1 5 The Illegitimate YUKKIE SUPPIE LEGEND MUPPIE UPPIE Urban Professional UP YUPPIE Young UP MUPPIE Middle age UP SUPPIE Senior UP PUPPIE Pregnant UP YUKKIE Yuppie kid
8. point2 send self find line point line 2 crlf CLIPS gt send Triangle1 print Line 1 point1 1 point2 0 2 CLIPS gt Using these handlers is a lot more convenient than typing in the nested messages At this point you might be tempted to define a handler called find line which returns both point values of the specified line Recall that find line point requires the specification of both the line and one of the two points which define the line So why not just send two messages in the same handler to return both point values of the specified line Shown following is the handler for find line and what it returns for line1 of Triangle CLIPS gt Cdefmessage handLler TRIANGLE find Line Line Csend send send self Csym cat get line line get point1 get position Csend send send self Csym cat get line line get point2 get position CLIPS gt send Triangle1 find Line 1 C 2 CLIPS gt As you can see the handler only returns the last message value of 0 2 Thus the first value of 1 0 is not returned by CLIPS This is like the case of deffunctions which only return the last action One way of getting around this problem and returning both point values of the line is shown following CLIPS gt Cdefmessage handLler TRIANGLE find Line line Ccreate send send send self Csym cat get Line line get point1 get position Csend send send self Csym cat get line line get point2 get pos
9. Csex male CLIPS gt As you can see the ID is always gen1 since gensym is only evaluated once and not again when the second instance is created Note that the gensym values may be different on your computer if you have already called the gensym since it increments by one each time it is called and is not reset by a clear The gensym function is reset to its starting value only if you restart CLIPS Now suppose that we want to keep track of the different Dorky_Duck instances that have been created Rather than using the static default we can use the facet called default dynamic which will evaluate its facet expression every time a new instance is created Notice the difference between the following example and the previous CLIPS gt Cclear CLIPS gt Cdefclass DUCK is a USER Cslot sound Cdefault quack Cslot ID Cdefault dynamic Cgensym CslLot sex default male CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck 1 Dorky_Duck CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound quack CID gen2 Csex male CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck 2 Dorky_Duck CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound quack CID gen3 Note ID is different from Dorky_Duck 1 Csex male CLIPS gt In this example which uses dynamic default the ID of the second instance gen3 is different from the first instance gen2 In contrast for the previous exampl
10. Only one instance name can be specified in a command and it must be surrounded by brackets if it is a user defined name For example the following will make Dorky_Duck disappear CLIPS gt Creset Create new instances again CLIPS gt Cinstances Check new instances created 108 inittal object of INITIAL OBJECT Dorky of DUCK Dorky_Duck of DUCK For a total of 3 instances CLIPS gt Csend Dorky_Duck delete TRUE CLIPS gt Cinstances initital object of INITIAL OBJECT Dorky of DUCK For a total of 2 instances CLIPS gt The in a Send will not work to delete all instances The only works with the unmake function Another alternative is to define your own handler for delete that will accept the and thus allow you to send instance name my_delete messages A send message is acted upon only by a target object which has an appropriate handler CLIPS automatically provides handlers for print init delete and so on for each user defined class It s important to realize that the message send Dorky_Duck delete works only because this instance is a user defined class If you define classes which do not inherit from USER such as a subclass of INTEGER you must also create appropriate handlers to carry out all desired tasks such as printing creating and deleting instances It s much easier to define subclasses of USER and take advantage of system supplied handlers What Did You Have For Break
11. User s Guide Version 6 20 March 31st 2002 by Joseph C Giarratano Ph D CLIPS User s Guide Table of Contents Chapter JUSt the Facts wiciciciwicccivicsdenccescbieneeen en aune epai 1 INTRODUCTION E soi Sere ek ave eva deel eee ved E ade ae 1 The Begimning and the End orero yasen titona yy anere EEEE a uyad chacu aa Aa E RE AE ai 2 Making a LIST meran a T EA goede cites SeKS CELINA ait 2 And Checking It TWICE iienaa aaa a e Eaa a EA a E aA ao 3 Clearing Up the Facts nmo eonenna ea Aaaa ae aa aa ai anaa aat 5 Sensitive Fields and Slurping essssesseeeeeerseessstsrrtrnrersstsrnttrrerssssnntnrntrnsssnntntrereen 6 A Matter Of Sy E aana a AE ian is ae orn AG NE TA EA 13 Getting Spaced Out ssnin ion eaa inia aaa aa O a A a E EE 14 Betactihal Pact onana a aE e AEEA AN TA REAN Lae ena 16 Watch that Fat oeni serina ate bos eT ne as EEA N ANA Aa 18 With a Little Hersen a a a N E E Aa aa 19 Chapter 2 Following the Rules 0 2 eceeeeeeeeeeeeeeeeeseeeeeeeseeeeeeeseeneees 21 Making Good Rules cccceeeeeeeeee cece ee ae ete ee erst sees aaa ee eeeeee ee aaaaaeneeeeeeeeeasaaanneeseeeees 21 Let s Get QUackiNg sain ces Gee wuaeee bet eed Vinee ce Geekdad ie ee ae 24 KICK YOUN DUCK opsnorren dia asee SEESE EEA E Badd t den RNE EES 25 show Me the RUES isinai endent napaa e eaea aAa aae a aaa Eel enua ENES 27 Write tO E E A A E E E E AE T S cpa A E E NT 28 Other Feature Satanu a a a a dees a eh eed aa A Ea 29 Cha
12. 22 permutation 151 pointerp 80 pointers 138 polymorphism 84 ppdefclass 99 ppdeffacts 37 ppdeffunction 72 ppdefinstances 113 ppdefmethod 154 ppdefrule 27 ppinstance 125 predefined system class 86 predicate constraint 80 predicate function 80 predicate functions 94 prefix form 66 preview send 145 primary 127 primitive object types 102 Principle of Least Astonishment 122 print 127 printout 28 procedural 83 progn 151 propagation 115 properties 85 pseudocode 21 pseudorandom 80 pure OOP 101 put 109 Q query functions 150 query system 150 R random 80 reactive 115 read 75 readline 76 refraction 26 refresh 29 relation 13 relationship 87 remove break 36 replace 73 reset 4 24 34 106 Rete Algorithm 77 retract 17 retracted 5 return 81 return value 23 128 reusable code 85 RHS 24 right double arrow 18 role abstract 104 role concrete 103 root 86 96 root class 86 round 72 rule base 1 rule based 83 rule header 22 rules 1 28 Rules of Message handler Precedence 134 Rules of Scope 151 run 25 S salience 24 80 save 25 save facts 29 save instances 113 scope 151 select 2 semicolon 19 send 86 107 sequencep 80 set break 36 setfact duplication 4 set incremental reset 29 shadowed method 154 shadower 132 shared 119 shared facet 119 shell 1 show breaks 36
13. Cbind sum Q Cbind index 1 bind Len length send self find point 1 bind Point1 send self find point 1 bind Point2 send self find point 2 while lt index len bind dif C nth index Point1 Cnth index Point2 Cbind sum sum dif dif Cbind index index 1 bind distance sqrt sum CLIPS gt Cdefmessage handler LINE print distance printout t Distance send self find distance crlf CLIPS gt send Line1 print distance Distance 5 0 CLIPS gt The values 1 2 for point1 and 4 6 for point2 were chosen for an easy check of the handler since these coordinates define a 3 4 5 triangle As you can see the distance is 5 0 as expected Treasure Maps Now that the POINT and LINE classes have been defined by generalized inheritance why stop now Let s continue with the next simplest class that can be defined from a line the triangle Shown following are the three defclasses required CLIPS gt Cclear CLIPS gt Cdefclass POINT Cis a USER CmultisLot position propagation no inherit CLIPS gt Cdefclass LINE Cis a POINT Cslot point1 Cdefault dynamic Cmake instance Cgensym of POINT propagation no inherit Cslot point2 Cdefault dynamic Cmake instance Cgensym of POINT propagation no inherit ae CLIPS gt Cdefclass TRIANGLE is a LINE Cslot Linel Cdefault dynamic Cmake instance Cgensym of LINE Cpropagation no inherit Cslot Line2 Cdef
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15. However all of them are necessary First gen3 was created for slot line1 which required gen4 and gen5 for the slots pointi and point2 associated with it by inheritance Second gen6 was created for slot line2 which required gen7 and gen8 for its slots pointi and point2 Finally gen9 was Created for slot line3 which required gen10 and gen11 for the slots pointi and point2 associated with it The slots for Triangle1 and one of its pointer values gen3 is shown following CLIPS gt send Triangle1 print Triangle1 of TRIANGLE Cline1 gen3 Cline2 gen6 Cline3 gen9 CLIPS gt Csend gen3 print gen3 of LINE Cpoint1 gen4 Cpoint2 gen5 CLIPS gt Now lets put in the X Y coordinates of Triangle1 as follows CLIPS gt send send send Triangle1 C 1 0 CLIPS gt send send send Triangle1 C 2 CLIPS gt send send send Triangle1 C 2 CLIPS gt send send send Triangle1 1 CLIPS gt send send send Triangle1 CLIPS gt send send send Triangle1 CLIPS gt The stored values are as follows CLIPS gt send Triangle1 print Triangle1 of TRIANGLE Cline1 gen3 get Line1 get Line1 get Line2 get Line2 get Line3 get Line3 get point1 get point2 get point1 get point2 get point1 get point2 put position put position put position put position put position put position 1 2 2 1 0 1 Q 1 143 Cline2 gen6
16. crlf CLIPS gt Crun Duck Dorky_Duck says Cquack quack CLIPS gt Objects in the Database Consider the following general type of problem Given some instances how many satisfy a specified condition For example shown following are the defclasses and definstances of Joe s Home showing the various types of sensors and the appliances connected to them Notice that an abstract class DEVICE is defined since both SENSOR and APPLIANCE inherit common slots type and location CLIPS gt Cclear CLIPS gt Cdefclass DEVICE Cis a USER Crole abstract Cslot type Caccess initialize only Sensor type Cslot loc Caccess initialize only Location CLIPS gt Cdefclass SENSOR Cis a DEVICE Crole concrete Cslot reading Cslot min access initialize only Min reading Cslot max access initialize only Max reading Cslot app Caccess initialize only SEN APP CLIPS gt Cdefclass APPLIANCE Cis a DEVICE Crole concrete Cslot setting Depends on appliance Cslot status off or on CLIPS gt 149 Cdefinstances ENVIRONMENT_OBJECTS CT1 of SENSOR type temperature T2 CS1 CW1 CFR CWP CSA CLIPS gt Cloc kitchen Creading 110 Too hot Cmin 20 Cmax 100 Capp FR of SENSOR type temperature Cloc bedroom Creading 10 Too cold Cmin 20 Cmax 100 Capp FR of SENSOR type smoke Cloc bedroom reading nil Bad sensor nil reading Cmin 1000 Cmax 5000 Capp SA of SENSOR
17. or handlers for short A message handler for an object responds to messages and performs the required actions For example sending the message Csend John_Smith print would cause the appropriate message handler to print the values of the slots of the 86 instance John_Smith Instances are generally specified within brackets A message begins with the send function followed by the instance name message name and any required arguments For example in the case of the print message there are no arguments An object in CLIPS is an instance of a class The encapsulation of slots and handlers inside an object is another of the five generally accepted features of an OOP The term encapsulated means that a class is defined in terms of its slots and handlers Although an object of a class may inherit slots and handlers from its superclasses with a few exceptions discussed later the object s slot values cannot be altered or examined without sending a message to the object The root class or simply root of CLIPS is a predefined system class called OBJECT The predefined system class USER is a subclass of OBJECT How the UPPIES Get Theirs As an example suppose we wanted to define a class called UPPIE which is a colloquial term for urban professional Note that in this book we ll follow the convention of writing classes in all uppercase Fig 1 1 illustrates how the UPPIEs get their inheritance all the way up to the root class
18. the different slots of LINE can indirectly inherit the same slots of POINT It s convenient to use gensym because we don t care what the pointer names of LINE are any more than we care what the pointer addresses are in a procedural language The following examples show how the points are accessed for one dimensional points at position O and 1 CLIPS gt Cdefinstances LINE_OBJECTS CLinel of LINE CLIPS gt Creset CLIPS gt Csend send Line1 get point1 put position Q CQ CLIPS gt Csend send Line1 get point2 put position 1 1 CLIPS gt send Line1 print Line1 of LINE Cpoint1 gen1 Cpoint2 gen2 CLIPS gt Csend Csend Line1 get point1 get position Co CLIPS gt Csend send Line1 get point2 get position 1 CLIPS gt Now that you understand how the indirect access works let s define some handlers to avoid the trouble of entering the nested send messages as in the last two cases Let s define a handler called find point to print out a specified point value and a handler called print points to print out the values of both LINE points as follows The argument of find point will be either a 1 for point1 or a 2 for point2 CLIPS gt Cdefmessage handler LINE find point C point 139 Csend send self Csym cat get point point get position CLIPS gt Cdefmessage handler LINE print points Cprintout t pointl send self find point 1 crlf point2 send self find point 2 crlf
19. 24 active instance 124 active message modify instance 146 active modify instance 146 after 126 agenda 24 ampersand 65 analysis 135 and 79 and conditional element 79 angle brackets 3 any instancep 150 arc 96 arguments 66 around 128 arrow 24 artificial neural systems 83 assert 2 assert string 38 attributes 84 85 159 Index B backslash 15 backtracking 91 bar constraint 64 basic action 131 batch 29 before 126 behavior 85 121 bind function 69 binding 69 bload 29 bound 42 bound variable 69 brackets 86 branches 92 96 break 81 breakpoints 36 browse classes 96 bsave 29 build 82 C call next handler 132 cardinality 115 118 case sensitive 13 88 CE 32 child class 85 class architecture 111 class etiquette 111 class oriented 111 class restrictions 151 class role 128 class existp 151 class slotexistp 113 class slots 145 class subclasses 145 class superclasses 145 clear 4 5 CLIPS Object Oriented Language 83 close 81 cold initialization 120 colon 80 combination 151 commands 2 comment 19 composite facet 120 compound classes 102 concrete class 97 conflict resolution 26 connectionist 83 connective constraint 64 control fact 52 COOL 83 create 70 create accessor 109 crlf 28 customized software 101 D declarative implementation 131 declare salience 29 81 default 25 default and de
20. As an example of multislot suppose that you wanted to treat the name of the relation prospect as multiple fields This would provide more flexibility in processing prospects since any part of the name could be pattern matched Shown following is the deftemplate definition using multislot and the revised rule to pattern match on multiple fields Notice that a multislot pattern name is now used to match all the fields that make up the name For convenience a deffacts is also given CLIPS gt Cclear CLIPS gt Cdeftemplate prospect CmuLtislLot name Ctype SYMBOL default DERIVE Cslot assets Ctype SYMBOL Callowed symbols poor rich wealthy Loaded Cdefault rich Cslot age Ctype INTEGER Crange 80 VARIABLE The older the better 59 Cdefault 80 CLIPS gt defrule happy_relationship prospect name name assets net_worth Cage months gt printout t Prospect name crlf Note not name net_worth crlf months months old crlf CLIPS gt Cdeffacts duck bachelor prospect name Dopey Wonderful Cassets rich Cage 99 CLIPS gt Creset CLIPS gt Crun Prospect Dopey Wonderful rich 99 months old CLIPS gt In the output the parentheses around Dopey s name are put in by CLIPS to indicate that this is a multislot value If you compare the output from this multislot version to the single slot version you ll see that the double quotes around Dopey Wonderful are gone The name slot is not a str
21. CDorky_Duck of DUCK CLIPS gt Cinstances inittal object of INITIAL OBJECT For a total of 1 instance CLIPS gt Creset CLIPS gt Cinstances inittal object of INITIAL OBJECT Dorky of DUCK Dorky_Duck of DUCK For a total of 3 instances CLIPS gt 107 The Disappearing Duck Although a reset will delete all instances except initial instance it will also make new instances from definstances If you want to permanently delete an instance the function unmake instance will delete one or all instances depending on its argument To delete all instances use the The following examples illustrate the unmake instance command CLIPS gt Cunmake instance Delete all instances TRUE CLIPS gt Cinstances Check that all are gone CLIPS gt Restore the definstance Cdefinstances DORKY_OBJECTS The Dorky Cousins CDorky of DUCK Dorky_Duck of DUCK CLIPS gt Creset Create new instances again CLIPS gt Cinstances Check new instances created inittal object of INITIAL OBJECT Dorky of DUCK Dorky_Duck of DUCK For a total of 3 instances CLIPS gt Cunmake instance Dorky Delete a specific instance TRUE CLIPS gt Cinstances inittal object of INITIAL OBJECT Dorky_Duck of DUCK For a total of 2 instances CLIPS gt Another way to delete a specific instance is to send a delete message The general syntax of the send function is as follows Csend lt instance name gt lt message gt
22. CLIPS gt send Line1 find point 1 CO CLIPS gt send Line1 find point 2 1 CLIPS gt For real use it would be best to provide error detection so that only a 1 or 2 is allowed As you can see the handler works fine for one dimensional points It can be tested for two dimensional points as follows We ll assume the first number for each point is the X value and the second number is the Y value That is point has X value 1 and Y value 2 CLIPS gt send send Line1l get point1 put position 1 2 C1 2 CLIPS gt send send Line1 get point2 put position 4 6 4 6 CLIPS gt send Line1 print Line1 of LINE Cpoint1 gen1 Cpoint2 gen2 CLIPS gt send send Linel get point1 get position C1 2 CLIPS gt send send Line1 get point2 get position 4 6 CLIPS gt As expected the handler also works correctly for two dimensional points Now that we have the two point positions it s easy to find the distance between the points which is the length of the line The distance can be determined by defining a new handler called find distance which uses the Pythagorean Theorem to calculate the distance as the square root of the sum of the squares Since no assumptions were made as to the number of dimensions the nth function is used to pick out each multifield value up the the maximum number of coordinates as stored in the len variable 140 CLIPS gt Cdefmessage handler LINE find distance
23. Cnumbers 3 4 lt Fact 4 gt CLIPS gt Crun Stock ID B Stock ID matches hypotenuse calculated CLIPS gt Extensive Arguments Arguments in a numeric expression can be extended beyond two for many of the math functions The same sequence of arithmetic calculations is performed for more than two arguments The following example illustrates how three arguments are used Evaluation proceeds from left to right Before entering these however you may wish to do a clear to get rid of any old facts and rules Cdefrule addition Cnumbers x y z Cassert Canswer pLus x y z 5 x y Zz 68 Enter the above program and assert numbers 2 3 4 After you run you ll see the following facts Note that the fact indices may be different if you ve done a reset instead of a clear before loading this program CLIPS gt facts f Cnumbers 2 3 4 f 1 Canswer pLus 9 For a total of 2 facts CLIPS gt The infix equivalent of a multiple argument CLIPS expression can be expressed as arg function arg where the square brackets mean that there can be multiple terms Besides the basic math functions CLIPS has Extended Math functions including trig hyperbolic and so on For a complete list see the CLIPS Reference Manual These are called Extended Math functions because they are not considered basic math functions like etc Mixed Results In dealing with expressions CLIPS tries to keep the mode the same a
24. Daemons are easily implemented using before and after handlers since these will be executed before and after their primary handler Implementing daemons like this is called declarative implementation because no explicit actions on the part of the handler is necessary for it to be executed That is CLIPS will always execute a before handler before its primary and will always execute an after handler after its primary Ina declarative daemon implementation the normal operation of CLIPS will cause the daemons to be activated when their time has come Thus the declarative implementation is implicit in the normal operation 132 The opposite of implicit execution is the imperative implementation in which the actions are explicitly programmed The around handler is very convenient to use for imperative daemons The basic idea of the around handler is as follows 1 Start before any other handlers 2 Call the next handler using either call next handler to pass the same arguments or override next handler to pass different ones 3 Continue execution when the last handler finishes 4 After any other around before primary or after handlers finish the around handler resumes execution The keyword call next handler is used to call the next handler s A handler is said to be shadowed by a shadower if it must be called from the shadower by a function such as Call next handler The callnext handler may be used multiple times to call the same h
25. OBJECT Notice that UPPIE is defined as a subclass of USER The boxes or nodes represent classes while the connecting arrows are called links Lines are often used instead of arrows for simplicity in drawing Also since CLIPS supports only is a links the is a relationship will not be explicitly written next to each link from now on Relationshi OBJECT USER is a OBJECT UPPIE is a USER Fig 1 1 The UPPIE Class 87 The convention we will follow for the relationship between classes is that the tail end of the arrow is on the subclass while the head points at the superclass The relationships in Fig 1 1 follow this convention Another possible convention is to use arrows to point at the subclasses The is a link indicates the inheritance of slots from a class to its subclass A class may have zero or more subclasses All classes except OBJECT must have a superclass Since UPPIE also inherits all slots of USER and USER inherits all slots of OBJECT it follows that UPPIE inherits all slots of OBJECT too The same principle of inheritance also applies to the message handlers of each class For example UPPIE inherits all the handlers of USER and OBJECT The inheritance of slots and handlers is particularly important in OOP since it means that you do not have to redefine the properties and behavior of each new class of objects that is defined Instead each new class inherits all the properties and behavior from its higher level cl
26. POINT class is very simple CLIPS gt Cclear CLIPS gt Cdefclass POINT Cis a USER CmultisLot position propagation no inherit CLIPS gt The no inherit facet is used to prevent a LINE from inheriting a position slot Instead a 137 LINE will be defined by two points called slot point and slot point2 These two slots will define the line and it is extraneous to have an additional position slot by inheritance The definition of the LINE class is a little more complex The reason for the added complexity is that the details of implementation are included in the defclass because Version 6 0 only supports is a relationships Cdefclass LINE Cis a POINT Cslot pointl Cdefault dynamic Cmake instance Cgensym of POINT Cpropagation no inherit Cslot point2 Cdefault dynamic Cmake instance Cgensym of POINT Cpropagation no inherit Cmessage handler find point Cmessage handler print points Cmessage handler find distance Cmessage handler print distance Note that the message handlers of LINE are forward declared for documentation purposes At this time you may be wondering why POINT and LINE are not both defined as subclasses of USER since all their slots have no inherit facets Since all the slots of POINT LINE and the TRIANGLE class to be defined later have no inherit facets all these classes could be defined as direct subclasses of USER rather than defining LINE as a subclass of POINT and TRIANGLE as a sub
27. That way if your expectations don t work out you can try again systematically such as the common algorithm for finding the Perfect Spouse by getting married over and over again One way of being organized is to keep a list Note if you really want to impress people show them a list of your lists In our case we ll keep a list of duck bachelors with the most likely prospect for matrimony at the front Once an ideal duck bachelor has been identified we ll shoot him up to the front of the list as the lucky duck The following program shows how this can be done by adding a couple of rules to the ideal duck bachelor rule Cdefrule ideal duck bachelor bill big name Cfeet wide name Cprintout t The ideal duck is name crlf Cassert Cmove to front name Cdefrule move to front move to front lt Cmove to front who old list lt list front who rear Cretract move to front old list Cassert list who front rear Cassert Cchange Llist yes Cdefrule print List change List lt Cchange List yes Clist list Cretract change List printout t List is list crlf 52 Cdeffacts duck bachelor list Clist Dorky Dinky Dicky Cdeffacts duck assets bill big Dicky bill big Dorky bill little Dinky feet wide Dicky feet narrow Dorky feet narrow Dinky The original list is given in the duck duck bachelor list deffacts When the program is run it will provide a new list of l
28. a math calculation let s first bind the answer to a variable and then print the bound variable CLIPS gt Cclear CLIPS gt Cdefrule addition Cnumbers x y gt Cassert Canswer x y 270 bind answer x y printout t answer is answer crlf CLIPS gt Cassert Cnumbers 2 2 lt Fact Q gt CLIPS gt Crun answer is 4 CLIPS gt facts f Cnumbers 2 2 f 1 Canswer 4 For a total of 2 facts CLIPS gt The bind also can be used on the RHS to bind single or multifield values to a variable The bind is used to bind zero one or more values to a variable without the operator Recall that on the LHS you can only create a multifield pattern by using the operator on a field such as x However the is unnecessary on the RHS because the arguments of bind explicitly tell CLIPS exactly how many values to bind In fact the is a useless appendage on the RHS The following rule illustrates some variable bindings on the RHS The multifield value function create is used to create a multifield value Its general syntax is as follows Ccreate lt arg1 gt lt arg2 gt lt argN gt where any number of arguments can be appended together to create a multifield value This multifield value or a single field value can then be bound to a variable as shown in the RHS actions of the following rule CLIPS gt Cclear CLIPS gt Cdefrule bind values demo Cinitial fact gt bind duck b
29. address sequencep lt arg gt multifield value stringp lt arg gt string symbolp lt arg gt symbol There are often cases in which it s convenient to have values which are globally known in an expert system For example it is inefficient to have to redefine universal constants such as x CLIPS provides the defglobal construct so that values may be universally known to all rules Another type of useful function is random numbers CLIPS has a random function which returns a random integer value The random number function of CLIPS actually returns pseudorandom numbers which means they are not truly random but are generated by a mathematical formula For most purposes the pseudorandom numbers will be fine Note that the random function of CLIPS uses the ANSI C library function rand which may not be available on all computers that do not adhere to this standard For more information on all these topics please see the CLIPS Reference Manual In addition to control facts to control the execution of programs CLIPS provides a more direct way of control by the explicit assignment of salience to rules The main 81 problem associated with explicitly using salience while you were just starting to learn CLIPS is the tendency to overuse salience and write sequential programs This overuse defeats the whole purpose of using a rule based language which is to provide a natural vehicle for those applications best represented by rules In the
30. by the pair f 0 f 1 and the other pair f 2 f 3 That is fact f 0 cannot match with f 3 because when num is bound to 0 in the first pattern the value of num in the second pattern also must be 0 Likewise when num is bound to 1 in the first pattern the value of num in the second pattern must be 1 Notice that the rule will be activated twice by these four facts one activation for the pair f 0 f 1 and the other activation for the pair f 2 f 3 As amore practical example enter the following rule Notice that the same variable name is used in both patterns Before doing a reset and run also enter a watch all command so that you can see what happens during execution CLIPS gt Cclear CLIPS gt Cdefrule ideal duck bachelLlor bill big name Cfeet wide name gt printout t The ideal duck is name crlf CLIPS gt Cdeffacts duck assets bill big Dopey bill big Dorky Cbill Little Dicky feet wide Dopey feet narrow Dorky feet narrow Dicky CLIPS gt Creset CLIPS gt Crun The ideal duck is Dopey CLIPS gt When the program is run the first pattern matches Dopey and Dorky since they both have big bills The variable name is bound to each name When CLIPS tries to match the second pattern of the rule only the variable name which is bound to Dopey also satisfies the second pattern of feet wide 5 The Lucky Duck Many situations occur in life where it s wise to do things in a systematic manner
31. command is ppdeffacts which prints the facts stored in a deffacts Other functions allow you to manipulate strings easily 7OR Function Meaning assert string Performs a string assertion by taking a string as argument and asserted as a nonstring fact str cat Constructs a single quoted string from individual items by string concatenation str index Returns a string index of first occurrence of a substring sub string Returns a substring from a string str compare Performs a string compare str length Returns the string length which is the length of a string sym cat Returns a concatenated symbol If you want to printout a multifield variable without parentheses the simplest way is by using the string implode function implode ApS Chapter 4 Variable Interests Nothing changes more than change The type of rules that you ve seen so far illustrates simple matching of patterns to facts In this chapter you ll learn very powerful ways to match and manipulate facts Let s Get Variable Just as with other programming languages CLIPS has variables to store values Unlike a fact which is static or unchanging the contents of a variable are dynamic as the values assigned to it change In contrast once a fact is asserted it s fields can only be modified by retracting and asserting a new fact with the changed fields Even the modify action described later in the chapter on deftemplate acts by retracting and asserting a modified fac
32. everything As you use more of the capabilities of CLIPS you ll find these watch commands very helpful in debugging To turn off a watch command enter an unwatch command For example to turn off watching compilations enter unwatch compilations With a Little Help CLIPS has on line help available To access the help feature just enter help and press the carriage return key In a short while you ll see a menu of topics For more information on using help read the help section on HELP_USAGE To exit from help keep pressing the carriage return key until the CLIPS prompt reappears If an error 20 message appears that says CLIPS could not find the help file clips hlp you can find out where CLIPS expected it to be by using the command help path 21 Chapter 2 Following the Rules If you want to get anywhere in life don t break the rules make the rules In the previous chapter you learned about facts Now you ll see how the rules of an expert system utilize facts in making a program execute Making Good Rules To accomplish useful work an expert system must have rules as well as facts Since you ve seen how facts are asserted and retracted it s time to see how rules work A rule is similar to an IF THEN statement in a procedural language like Ada C or Pascal An IF THEN rule can be expressed in a mixture of natural language and computer language as follows IF certain conditions are true
33. example a defgeneric for the function can be defined with different defmethods The expression a b could add two real numbers represented by a and b or two complex numbers or two matrices or concatenate two strings and so forth depending if there is a defmethod defined for the argument classes CLIPS does this by first recognizing the type of the arguments and then calling the appropriate defmethod defined for those types A separate overloaded defmethod for would be defined for each set of argument types except for the predefined system types such as real numbers Once the defgeneric is defined it s easy to reuse in other programs Any named function that is system defined or external can be overloaded using a generic function Notice that a deffunction cannot be overloaded An appropriate use of a generic function is to overload a named function If overloading is not required you should define a deffunction or an external function The syntax of defgenerics is very simple consisting of just the legal CLIPS symbol name and an optional comment Cdefgeneric lt name gt lt comment gt As a simple example of generic functions consider the following attempt in CLIPS to 153 compare two strings using the gt function CLIPS gt Cclear CLIPS gt gt duck2 duck1i ARGACCES5 Function gt expected argument 1 to be of type integer or float CLIPS gt It s not possible to do this comparison wit
34. for the NUMBER class that will handle addition by messages The handler is defined for NUMBER rather than INTEGER because we would also like to handle FLOAT objects too Instead of defining the same handler for FLOAT and for INTEGER it s easier to just define a handler for the superclass NUMBER f a handler for a message is not defined in the class of the object CLIPS tries all the handlers in the inheritance precedence list Since is not defined for INTEGER CLIPS tries NUMBER next finds the applicable handler and returns the result of 3 CLIPS gt Cdefmessage handler NUMBER arg Argument of handler self arg Function addition of handler CLIPS gt send 1 2 3 CLIPS gt Csend 2 5 3 04 5 5 CLIPS gt send 2 5 2 6 5 1 CLIPS gt Cdescribe class NUMBER OK K K K OK K K K K K K K K OK K K K K K K K K K K K K KK OK K K K K K K K K K K K K KKK K K KKK K K KK KK KKK KK KKK Abstract direct instances of this class cannot be created Direct Superclasses PRIMITIVE Inheritance Precedence NUMBER PRIMITIVE OBJECT Direct Subclasses INTEGER FLOAT Recognized message handlers primary in class NUMBER OK K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K FK K K K K K K KKK K K K K K CLIPS gt The variable self is a special variable in which CLIPS stores the active instance The selfis a reserved word that cannot be explicitly included in a handler ar
35. functions 102 gensym 116 get 109 get defmessage handler list 145 get incremental reset 29 grouping functions 145 H handler type 126 handlers 85 121 header declaration 154 help 19 help path 20 hierarchy 91 if then else 81 imperative implementation 132 implicit method 153 implode 38 indirect access 138 indirect instances 97 120 indirect subclass 89 indirect superclasses 88 inference engine 1 inferences 1 infix form 66 inherit facet 120 inheritance 84 85 Inheritance by Generalization 135 Inheritance by Specialization 134 inheritance path 90 inheritance precedence 97 init 128 initial fact 4 initialize instance 120 initial object 34 instance 85 instance address 8 113 instance addressp 113 instance existp 113 instance list 1 instance name 8 113 instance namep 113 instance name to symbol 103 instancep 113 instances 102 105 instance set 150 instance set nember emplate 151 instance set query 151 instance set template 151 integer 8 10 72 102 integerp 80 is a 86 K knowledge base 1 L left arrow 44 left double arrow 18 legitimate 92 length 73 level 96 LEXEME 102 lexemep 80 LHS 24 link 96 links 86 list 7 list defclasses 94 list deffacts 37 list deffunctions 72 list definstances 113 list defmessage handlers 145 list defmethods 154 literal 25 literal constraint
36. instances Save instances to a file Load instances to a file Deletes the named definstance 115 Chapter 10 Fascinating Facets If you want to have class then act dress and talk like your friends In this chapter you ll learn more about slots and how to specify their characteristics by using facets The use of facets is good software engineering because there is a greater chance of CLIPS flagging an illegal value rather than risking a runtime error or crash There are many types of facets that may be used to specify slots as summarized in the following table Facet Name Description default and default dynamic Set initial values for slots cardinality Number of multifield values access type Read write read only initialize only access storage Local slot in instance or shared slot in class propagation Inherit or no inherit slots source composite or exclusive inheritance documentation Documentation of slots override message Indicate message to send for slot override create accessor Create put and get handlers visibility Public or private to defining class only reactive Changes to a slot trigger pattern matching For reasons of space we ll only describe a few facets in more detail in the rest of this chapter For more details see the CLIPS Reference Manual A Slot Named Default The default facet sets the default value of a slot when an instance is created or initialized as shown in the following example
37. is used for the LHS Let s Get Quacking CLIPS always executes the actions on the RHS of the highest priority rule on the agenda This rule is then removed from the agenda and the actions of the new highest salience rule is executed This process continues until there are no more activations or a command to stop is encountered You can check what s on the agenda with the agenda command For example CLIPS gt Cagenda duck f 0 For a total of 1 activation CLIPS gt The first number 0 is the salience of the duck activation and f 0 is the fact identifier of the fact animal is duck which matches the activation If the salience of a rule is not declared explicitly CLIPS assigns it the default value of zero where the 25 possible salience values range from 10 000 to 10 000 In this book we ll use the definition of the term default as meaning the standard way If there is only one rule on the agenda that rule will fire Since the LHS pattern of the duck sound rule is Canimal is duck this pattern will be satisfied by the fact animal is duck and so the duck sound rule should fire Each field of the pattern is said to be a literal constraint The term literal means having a constant value as opposed to a variable whose value is expected to change In this case the literals are animal is and duck To make a program run just enter the run command Type run and press the carriage return key Then do
38. keep searching through a lot of classes to find the applicable handler Getting Around Let s examine message handlers in more detail now We ll define a handler to print out a header when an object receives a message to print itself The message handler is defined using a defmessage handler construct as follows CLIPS gt Cdefmessage handler USER print before Cprintout t Starting to print crlf CLIPS gt Csend Dorky_Duck print Starting to print Dorky_Duck of DUCK Cage 2 Csound quack CLIPS gt The reason that a header is printed rather than a trailer at the end has to do with the handler type A before type handler is used before the print message To make a trailer use the after type handler as shown in the following example CLIPS gt Cdefmessage handler USER print after Cprintout t Finished printing crlf CLIPS gt Csend Dorky_Duck print Starting to print Dorky_Duck of DUCK Cage 2 Csound quack Finished printing CLIPS gt The general format of a message handler is as follows Cdefmessage handLler lt class name gt lt message name gt handler type comment SOT lt parameters gt wildcard parameter lt action gt While there may be multiple actions in a handler only the value of the last action is returned Notice that this is just like a deffunction Since Dorky_Duck is of class DUCK a subclass of USER we can take adv
39. needed after the read to put the cursor on a new line The read automatically resets the cursor to a new line CLIPS gt Cclear CLIPS gt Cdefrule read input Cinitial fact gt Cprintout t Name a primary color crlf Cassert color Cread CLIPS gt Cdefrule check input color lt color color read amp red yellow bLue gt Cretract color Cprintout t Correct crlf CLIPS gt Creset CLIPS gt Crun Name a primary color red 76 Correct CLIPS gt Creset CLIPS gt Crun Name a primary color green CLIPS gt No correct The rule is designed to use keyboard input on the RHS so it s convenient to trigger the rule with initial fact Otherwise you d have to make up some dummy fact to trigger the rule The read function is not a general purpose function that will read anything you type on the keyboard One limitation is that read will read only one field So if you try to read primary color is red only the first field primary will be read To read all the input you must enclose the input within double quotes Of course once the input is within double quotes it is a single literal field You can then access the substrings primary color is and red with the str explode or sub string functions The second limitation of read is that you can t input parentheses unless they are within double quotes Just as you can t assert a fact containing parentheses you can t read parenthe
40. quote duck 316 For a total of 2 facts CLIPS gt Retract that Fact Now that you know how to put facts into the fact list it s time to learn how to remove them Removing facts from the fact list is called retraction and is done with the retract command To retract a fact you must specify the fact index of the fact as the argument of retract For example set up your fact list as follows CLIPS gt Cclear CLIPS gt Cassert Canimal is duck lt Fact Q gt CLIPS gt assert Canimal sound quack lt Fact 1 gt CLIPS gt assert The duck says Quack lt Fact 2 gt CLIPS gt facts f Canimal is duck f 1 CanimaL sound quack f 2 The duck says Quack For a total of 3 facts CLIPS gt To remove the last fact with index f 2 enter the retract command and then check your facts as follows CLIPS gt Cretract 2 CLIPS gt facts f Canimal 1is duck f 1 CanimaL sound quack For a total of 2 facts CLIPS gt What happens if you try to retract a fact that s already retracted or a non existent fact Let s try it and see CLIPS gt Cretract 2 PRNTUTIL1 Unable to find fact f 2 a y aee CLIPS gt Notice that CLIPS issues an error message if you try to retract a non existent fact The moral of this is that you can t take back what you haven t given Now let s retract the other facts as follows CLIPS gt Cretract 1 CLIPS gt facts f Canimal is duck For a total of 1 fact CLIPS
41. same way procedural languages are best for strong control oriented applications while object oriented languages are best for representing objects CLIPS has keywords called declare salience which can be used to explicitly set the priority of rules Salience is set using a numeric value ranging from the smallest value of 10000 to the highest of 10000 If a rule has no salience explicitly assigned by the programmer CLIPS assumes a salience of zero Notice that a salience of zero is midway between the largest and smallest salience values A salience of zero does not mean that the rule has no salience but rather that it has an intermediate priority level CLIPS provides some procedural programming structures that can be used on the RHS These structures are the while and if then else that also are found in modern high level languages such as Ada C and Pascal Another useful function with while loops is the break which ends the currently executing while loop The return function immediately ends the currently executing deffunction generic function method or message handler Any function may be called from the RHS which greatly contributes to the power of CLIPS Many other CLIPS functions are available that may return with numbers symbols or strings These functions may be used for their return values or for their side effects An example of a function only used for its side effect is printout The value returned by the printout is mean
42. side effects 81 single field 7 single field constraint 45 single field variable 48 single field wildcard 45 single slot 53 slot access 110 slot boundp 113 slot delete 119 slot existp 113 145 slot facets 145 slot initablep 120 slot insert 119 slot replace 119 slots 53 85 108 slot sources 145 slot writablep 120 software engineering 8 software factory 101 source 115 specificity 96 standard output device 28 static 19 41 static default 116 step 36 storage 115 storage facet 119 strategy 32 str cat 38 str compare 38 str explode 76 str index 38 string 8 9 102 string assertion 38 string compare 38 string concatenation 38 string implode function 38 string length 38 162 stringp 80 str length 38 subclass 85 subclassp 94 subgraphs 97 subseq 73 subsetp 73 sub string 38 sub string functions 76 subtrees 97 superclasses 85 superclassp 94 symbol 8 102 symbolp 80 symbol4o instancename 103 sym cat 38 synthesis 135 system 29 T t 28 target object 108 taxonomy 90 template 84 test conditional element 78 tilde 64 tool 1 top level 2 tree 91 truth maintenance 79 tuning up 35 type 56 types 8 U unbound slot 109 undeffacts 35 undeffunction 73 undefinstances 113 undefmessage handler 145 undefmethod 154 undefrule 35 unmakeinstance 107 unordered 11 unwatch 19 unwatch facts 19 unwatch inst
43. variable such as x x 7name or name The variable can be a single field variable or a multifield variable depending on whether a or is used on the LHS Note that on the RHS only a x is used where the x can be any variable name You can think of the as a function whose argument is a single field wildcard or a single field variable and returns a multifield wildcard or a multifield variable respectively As an example of a multifield variable the following version of the rule also prints out the name field s of the matching fact because a variable is equated to the name field s that match CLIPS gt Cdefrule dating ducks Cbachelor Dopey name gt Cprintout t Date Dopey name crlf CLIPS gt Creset CLIPS gt Crun Date Dopey Dinky Mallard Date Dopey Mallard Date Dopey CLIPS gt As you can see on the LHS the multifield pattern is name but is name when used as a variable on the RHS When you enter and run you ll see the names of all eligible Dopeys The multifield wildcard takes care of any number of fields Also notice that multifield values are returned enclosed in parentheses Suppose you wanted a match of all ducks who had a Dopey somewhere in their name not necessarily as their first name The following version of the rule would match all facts with a Dopey in them and then print out the names CLIPS gt Cdefrule dating ducks bachelor first Dopey Last gt Cprintout t Date fir
44. walLk Cprintout t Don t walk crlf The previous rules are simplified and don t cover all cases such as the breakdown of the traffic light For example what does the robot do if the light is red or yellow and the walk sign says walk A way of handling this case is to use a field constraint to restrict the values that a pattern may have on the LHS The field constraint acts like constraints on patterns One type of field constraint is called a connective constraint There are three types of connective constraints The first is called a constraint Its symbol is the tilde The constraint acts on the one value that immediately follows it and will not allow that value As a simple example of the constraint suppose you wanted to write a rule that would print out Don t walk if the light was not green One approach would be to write rules for every possible light condition including all possible malfunctions yellow red blinking yellow blinking red blinking green winking yellow blinking yellow and winking red and so forth However a much easier approach is to use the constraint as shown in the following rule Cdefrule walk Clight green Cprintout t Don t walk crlf By using the constraint this one rule does the work of many other rules that required specifying each light condition Be Cautious The second connective constraint is the bar constraint I The I connective constraint is used to allow any of
45. 0 53 allowed floats 2 3 1 0 300 00056 allowed vaLues Dopey rich 99 1 e9 It doesn t make sense to specify both a numeric range and values allowed for the same deftemplate field For example if you specify allowed integers 1 4 8 this contradicts a range specification of 1 to 10 by range 1 10 If the numbers happen to be sequential such as 1 2 3 then you could specify a range which would exactly match range 1 3 However the range would be redundant to the allowed integers specification Thus range and allowed values are mutually exclusive That is if you specify a range you can t specify the allowed values and vice versa In general the range attribute cannot be used in conjunction with allowed values allowed numbers allowed integers or allowed floats Without the optional information the deftemplate and a rule which uses it follows CLIPS gt Cclear CLIPS gt CdeftempLate prospect j name of deftemplate slot name name of field default DERIVE default value of field name CslLot assets name of field Cdefault rich default value of field assets Cslot age name of field 57 Cdefault 80 defauLt value of field age CLIPS gt Cdefrule matrimonial_candidate prospect name name assets net_worth Cage months gt Cprintout t Prospect name crlf net_worth crlf months months old crlf CLIPS gt assert prospect name Dopey Wonderful Cage 99 lt Fact Q gt CLIPS gt Crun Pros
46. 25 load 29 load facts 29 load instances 113 local 119 local facet 119 logical 79 logical AND conditional element 32 logical conditional element 79 logical functions 79 logical name 81 logical support 79 logically dependent 79 make instance 104 matches 36 member 73 message dispatch 145 message handler precedence 145 message handler existp 145 message handlers 85 message handlers 121 message modify instance 146 messages 85 method body 154 method index 154 method precedence 154 modify 59 modify instance 110 146 multifield 7 102 multifield slot functions 119 multifield value function 70 multifield variable 48 multifield wildcard 47 multiparadigm 84 multiple facet 118 multiple inheritance 90 98 multislot 53 118 N name 148 name constraint 148 named function 152 next handlerp 132 nil 7 nil 109 nodes 86 no inherit facet 120 not 79 not conditional element 79 nth 73 NUMBER 102 numberp 80 numeric fields 9 161 O object 86 object oriented design 83 object oriented programming 83 object pattern match delay 146 objects 1 oddp 80 of 105 OOD 83 OOP 83 open 81 or 79 or conditional element 79 order 7 ordered 7 11 ordered list 97 overloading 152 override message 115 overridenexthandler 132 P paradigm 83 parameter 71 parent classes 85 partial matches 37 pattern
47. CLIPS gt Csymbol to instance name Dorky_Duck Dorky_Duck CLIPS gt Csymbol to instance name Csym cat Dorky _ Duck Dorky_Duck CLIPS gt Notice how standard CLIPS functions such as sym cat which concatenates items can be used with the object system of CLIPS The opposite function instance name to symbol converts an instance name to a symbol as the following examples show CLIPS gt Cinstance name to symbol Dorky_Duck Dorky_Duck CLIPS gt Cstr cat Cinstance name to symbol Dorky_Duck is a DUCK Dorky_Duck is a DUCK CLIPS gt Dorky Duck There is a difference between Nature and OOP In Nature objects are reproduced only from like objects like the birds and the bees the chicken and the egg are the exceptions to this rule However in OOP instances are only created using the class template In a pure OOP like Smalltalk an instance of a specific class is created by sending the class a message In fact the heart of OOP involves sending different types of messages from one object to another even from an object to itself In order to see how messages work let s start by entering the following commands to create a user defined DUCK class and check that it s entered Notice that the role concrete descriptor is specified in order to allow direct instances of DUCK to be created If the role is unspecified CLIPS determines the role by inheritance For 104 determining role by inheritance system class
48. Cline3 gen9 CLIPS gt send gen3 print gen3 of LINE Cpoint1 gen4 Cpoint2 gen5 CLIPS gt send gen4 print gen4 of POINT position 1 0 CLIPS gt send gen5 print gen5 of POINT position 2 CLIPS gt As you can see the line1 pointer gen1 points to pointi and point2 with their pointers gen2 and gen3 These last two pointers finally point to the actual values of 1 0 and 0 2 that define line1 of Triangle1 It s analogous to Long John Silver finding a treasure chest with a map gen1 which leads to another chest with two maps gen2 and gen3 which lead to the two buried treasures at the locations specified by gen2 and gen3 The values stored for each line of Triangle1 can be retrieved by a single command using nested messages such as the following Csend send send Triangle1 get line1 get point1 get position Although these commands work it s not much fun to type them in unless you get paid by the hour and need the typing practice As you might have guessed from the LINE handlers that we defined in the previous section it s possible to define TRIANGLE handlers as follows CLIPS gt Cdefmessage handler TRIANGLE find line point line point Csend send send self sym cat get line line Csym cat get point point get position CLIPS gt Cdefmessage handLler TRIANGLE print Line line Cprintout t pointl send self find Lline point line 1 crlf 144
49. F symbolic field if you try to read past the end of file EOF The evaluation function eval is used for evaluating any string or symbol except the def type constructs such as defrule deffacts etc as if entered at the toptevel The build function takes care of the def type constructs The build function is the complement of eval The build function evaluates a string or symbol as if it were entered at the top level and returns TRUE if the argument is a legal def type construct such as defrule deffacts and so forth Chapter 8 Matters of Inheritance The easiest way to obtain wealth is to inherit it the second best way is to make it off the labor of others marrying wealth is too much like work This chapter is an overview of object oriented programming in CLIPS Unlike rule based programming in which you can just jump right in and write a rule without caring what else is in the system object oriented programming requires some essential background material How to be Objective A key characteristic of good program design is flexibility Unfortunately the rigid methodology of structured programming techniques does not provide the needed flexibility for fast reliable and efficient changes The object oriented programming OOP paradigm provides this flexibility The term paradigm comes from the Greek word paradeigma which means a model example or pattern In computer science a paradigm is a consistent organized method
50. IE This means that a YUKKIE will inherit all the slots of a PUPPIE Assuming that one of the PUPPIE slots specifies how many months pregnant the PUPPIE is this means that every Yuppie kid will have a slot to indicate how many months pregnant he or she is too It is possible to correct the figure However we need to use a graph instead of a tree In contrast to trees in which every node except the root has exactly one parent each node in a graph may have zero or more nodes connected to them A familiar example of a graph is a roadmap in which cities are nodes and roads are the links connecting them Another difference between trees and graphs is that most types of trees have a hierarchical structure while general types of graphs do not Fig 1 6 shows the legitimate Yuppie class YUKKIE A new class CHILD has been created and is a links made between YUKKIE and its two superclasses YUPPIE and CHILD Notice there is no longer an illegitimate link between YUKKIE and PUPPIE UPPIE CHILD YUPPIE PUPPIE SUPPIE MUPPIE YUKKIE Fig 1 6 The Legitimate YUKKIE This is a graph because the YUKKIE class has two direct superclasses instead of only one as in atree This is also a hierarchical graph because the classes are arranged using is a links from the most general USER to most specific SUPPIE MUPPIE PUPPIE and YUKKIE Using Fig 1 6 we can say that a YUKKIE is a YUPPIE and also that a YUKKIE is a CHILD Shown following a
51. LIPS gt Since classes are not objects in CLIPS we can t send a message to make an object Instead the make instance function is used to make an instance object The basic syntax is as follows Cmake instance lt instance name gt of lt class gt lt slot override gt 105 Normally you specify an instance name However If you do not CLIPS will generate one using the gensym function Slot values also can be specified Now that we have a duck factory let s make some instances as follows where the name of the instance is in brackets Note the use of the of keyword to separate the instance name from the class name You must include the of or a syntax error will result Also note that the brackets in the code mean an instance name while brackets in the metasyntax such as for make instance above mean option CLIPS gt Cmake instance Dorky of DUCK Dorky CLIPS gt Cmake instance Elsie of COW PRNTUTIL1 Unable to find class COW CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck CLIPS gt Cinstances Dorky of DUCK Dorky_Duck of DUCK For a total of 2 instances CLIPS gt After the instance is successfully created CLIPS responds with the name of the instance If itis not possible to create an instance CLIPS responds with a FALSE Also like the rules and facts commands CLIPS has an instances function to print out the instances of a class For the case of make instance the brackets
52. LIPS gt assert Canimal is duck gt f Q Canimal is duck lt Fact Q gt CLIPS gt The right double arrow symbol gt means that a fact is entering memory while the left double arrow indicates a fact is leaving memory as shown following CLIPS gt Creset lt f Q Canimal is duck gt f Q Cinitial fact CLIPS gt Cassert Canimal is duck gt f 1 Canimal is duck lt Fact 1 gt CLIPS gt Cretract 1 lt f 1 Canimal is duck CLIPS gt facts f 0 Cinitial fact For a total of 1 fact CLIPS gt 22 49 The watch facts command provides a record that shows the dynamic or changing state of the fact list In contrast the facts command show the static state of the factiist since it displays the current contents of the fact list To turn off watching facts enter unwatch facts There are a number of things you can watch These include the following which are described in more detail in the CLIPS Reference Manual The comment in CLIPS begins with a semicolon Everything after the semicolon is ignored by CLIPS Cwatch facts Cwatch instances used with objects Cwatch slots used with objects Cwatch rules Cwatch activations watch messages used with objects watch message handlers used with objects watch generic functions Cwatch methods used with objects Cwatch deffunctions watch compilations on by default Cwatch statistics Cwatch globals Cwatch focus Cwatch all watch
53. LIPS gt Crun quack CLIPS gt The output is the text within the double quotes Be sure to type the letter t following the printout command This tells CLIPS to send the output to the standard output device of your computer Generally the standard output device is your terminal hence the letter t after printout However this may be redefined so that the standard output device is some other device such as a modem or disk 29 Other Features The declare salience command provides explicit control over which rules will be put on the agenda You must be careful in using this feature too freely lest your program become too controlled The set incremental reset command prohibits rules from seeing facts that are entered before the rules are entered The command to get the current value of incremental reset is getincremental reset One way to make a rule fire again is to force the rule to be re activated by the refresh rule command The load command loads in the rule that you had previously saved to disk in the file duck clp or whatever name and directory that you had saved it under You can load a file of rules made on a text editor into CLIPS using the load command A faster way to load files is to first save them in a machine readable binary format with the save binary command called bsave The load binary command bload can then be used to read these binary rules into CLIPS memory much faster since the files do not have to be re i
54. Perform major task of message Yes after Auxilliary work after primary No Summary of Message Handler Types The handler types are listed in the order that they are normally called during execution of a message Depending on the handler type CLIPS knows when to execute 128 it That is an around handler starts before any before handlers A before handler is executed before any primary handlers which are followed by the after handlers The exception to this sequential handler execution is the around type handler If an around handler is defined it will start execution before any of the others perform specified actions and then complete its actions after all the other handler types have finished You ll see a detailed example of these handlers execution soon The class role describes the intended purpose of each type The column headed Return Value describes whether the handler type is generally intended for a return value or simply to provide a useful side effect such as printing This consideration will depend on the handler For example many user defined primary handlers may be written to return a value as the result of some numeric calculation or string operation An exception to returning a useful return value is a print primary handler whose main task is the side effect of printing and does not have a return value The following table lists the predefined primary handlers of USER By inheritance these are available for all s
55. THEN execute the following actions Another term for the above statement is pseudocode which literally means false code While pseudocode cannot be directly executed by the computer it serves as a very useful guide to writing executable code Pseudocode is also helpful in documenting rules A translation of rules from natural language to CLIPS is not very difficult if you keep this IF THEN analogy in mind As your experience with CLIPS grows you ll find that writing rules in CLIPS becomes easy You can either type rules directly into CLIPS or load rules in from a file of rules created by a text editor The pseudocode for a rule about duck sounds might be IF the animal is a duck THEN the sound made is quack 22 The following is a fact and a rule named duck which is the pseudocode above expressed in CLIPS syntax The name of the rule follows immediately after the keyword defrule Although you can enter a rule on a single line it s customary to put different parts on separate lines to aid readability and editing CLIPS gt Cclear CLIPS gt Cunwatch facts CLIPS gt assert Canimal is duck lt Fact Q gt CLIPS gt Cdefrule duck Canimal is duck gt Cassert Csound is quack CLIPS gt If you type in the rule correctly as shown you should see the CLIPS prompt reappear Otherwise you ll see an error message If you get an error message it is likely that you misspelled a keyword or left out a parenthesis Remember the n
56. USER that have been discussed so far there are a number of other predefined classes You should recognize most of them as having the same name as the familiar CLIPS types that you learned about in earlier chapters The predefined types of CLIPS are also defined as classes so that they can be used with COOL The general inheritance diagram of the predefined classes from the CLIPS Reference Manual is shown in Fig 1 7 where the arrows point to the subclasses N we DN PRIMITIVE INITIAL OBJECT ae a a a INSTANCE ADDRESS MULTIFIELD we oe te ri OK of SYMBOL INSTANCE NAME INSTANCE ADDRESS FACT ADDRESS EXTERNAL ADDRESS Fig 1 7 The Predefined CLIPS Classes The OBJECT class is the root of the tree and is connected by branches to its superclass The terms branch edge link and arc are basically synonomous in that they all indicate a connection between nodes Each subclass is one level lower than its superclass The classes are numbered by level Level 0 is the root class OBJECT 96 Larger level numbers indicate that a class has higher specificity The term specificity means that a class is more restrictive For example LEXEME is a superclass of SYMBOL and STRING If you were told that an object was of the LEXEME class you would know that it could only be a SYMBOL or a STRING However if an object is a SYMBOL it cannot be a STRING and vice versa Thus the classes SYMBOL and STRING are more specific than LEXEME The browse
57. a facts to check that the fact was asserted by the rule CLIPS gt Crun CLIPS gt facts f Canimal is duck f 1 Csound 1s quack For a total of 2 facts CLIPS gt Before going on let s save the duck rule with the save command so that you don t have to type it in again if you haven t already saved it in an editor Just enter a command such as Csave duck clp to save the rule from CLIPS memory to disk and name the file duck clp where the clp is simply a convenient extension to remind us this is a CLIPS source code file Note that saving the code from CLIPS memory like this will only preserve the optional rule header comment in quotes and not any semicolon comments Kick your Duck An interesting question may occur to you at this time What if you run again There is 26 a rule and a fact which satisfies the rule so the rule should fire However if you try this and run again you ll see that the rule won t fire This may be somewhat frustrating However before you do something drastic to ease your frustration like kicking your pet duck you need to know a little more about some basic principles of expert systems A rule is activated if its patterns are matched by a 1 a brand new pattern entity that did not exist before or 2 a pattern entity that did exist before but was retracted and reasserted i e a clone of the old pattern entity and thus now a new pattern entity The rule and indices
58. a group of values to match For example suppose you wanted a rule that printed out Be cautious if the light was yellow or blinking yellow The following example shows how it s done using the I constraint CLIPS gt Cclear CLIPS gt Cdefrule cautious Clight yellow blinking yellow gt 65 Cprintout t Be cautious crlf CLIPS gt Cassert light yellow lt Fact gt CLIPS gt assert light blinking yellow lt Fact 1 gt CLIPS gt Cagenda Q cautious f 1 Q cautious f 0 For a total of 2 activations CLIPS gt And Away We Go The third type of connective constraint is the amp connective constraint The symbol of the amp connective constraint is the ampersand amp The amp constraint forces connected constraints to match in union as you ll see in the following examples The amp constraint normally is used only with the other constraints otherwise it s not of much practical use As an example suppose you want to have a rule that will be triggered by a yellow or blinking yellow fact That s easy enough just use the connective constraint as you did in a previous example But suppose that you also want to identify the light color The solution is to bind a variable to the color that is matched using the amp and then print out the variable This is where the amp is useful as shown below Cdefrule cautious Clight color amp yellow blinking yellow Cprintout t Be cautious becaus
59. a true object oriented programming language Following is an example of a defgeneric gt for STRING types and its method CLIPS gt Cdefgeneric gt Header declaraction Actually unnecessary CLIPS gt Cdefmethod gt a STRING b STRING gt Cstr compare a b CLIPS gt gt duck2 ducki The overload gt works correctly TRUE in all three cases CLIPS gt gt ducki ducki FALSE CLIPS gt gt ducki duck2 154 FALSE CLIPS gt The defgeneric acts as a header declaration to declare the type of function being overloaded It s not actually necessary to use a defgeneric in this case because CLIPS implicitly deduces the function name from the defmethod name which is the first symbol following defmethod The header is a forward declaration that is necessary if the defgeneric methods have not yet been defined but other code such as defrules defmessage handlers and so forth refers to the defgeneric name Other Features Compared to deffunctions a method has an optional method index If you don t supply this index CLIPS will provide a unique index number among the methods for that generic function that can be viewed by the list defmethods command The method body can be printed using the ppdefmethod command A method can be removed with an undefmethod function call The ranking of methods determines the method precedence of a generic function It is the method precedence which determ
60. achelors Ccreate Dopey Dorky Dinky bind happy bachelor mv Ccreate Dopey bind none create Cprintout t duck bachelors duck bachelors crlf duck bachelors no Cimplode duck bachelors crlf happy bacheLlor mv happy bachelor mv crlf none none crlf CLIPS gt Creset CLIPS gt Crun duck bachelors Dopey Dorky Dinky duck bacheLlors no Dopey Dorky Dinky happy bachelor mv Dopey none C CLIPS gt Doing Your Own Thing Just like other languages CLIPS allows you to define your own functions with deffunction The deffunction is known globally which saves you the effort of entering the same actions over and over again Deffunctions also help in readability You can call a deffunction just like any other function A deffunction may also be used as the argument of another function A printout can be used anywhere in a deffunction even if it s not the last action because printing is a side effect of calling the printout function The general syntax of a deffunction is shown following Cdeffunction lt function name gt optional comment arg1 arg2 argM argN argument list Last one may C lt action1 gt be optional multifield arg lt action2 gt action1 to ed s actionCK 1 do not lt actionCK 1 gt return a value lt actionKk gt only Last action returned The arg are dummy arguments which mean that the names of the arguments will not conflict with variable names in a rule if th
61. act identifier of f 0 because the clear command reset the fact identifiers The clear command actually does more than just remove facts Besides removing all the facts clear also removes all the rules as you ll see in the next chapter The following example shows how three facts are asserted and the facts command is used The clear command is used to get rid of all facts in memory and reset the fact indices to start with f 0 CLIPS gt Cclear CLIPS gt Cassert Ca b Cc lt Fact 2 gt CLIPS gt facts f Ca f 1 Cb f 2 Cc For a total of 3 facts CLIPS gt facts Q f Ca f 1 Cb f 2 Cc For a total of 3 facts CLIPS gt facts 1 f 1 Cb f 2 Cc For a total of 2 facts CLIPS gt facts 2 f 2 Cc For a total of 1 fact CLIPS gt Cfacts 1 f Ca f 1 Cb For a total of 2 facts CLIPS gt Cfacts 2 2 f Ca f 1 Cb For a total of 2 facts CLIPS gt Notice only one assert is used to assert the three facts a b and c The highest fact index is 2 and this is returned by CLIPS in the informational message lt Fact 2 gt The much longer alternative would be to assert one fact per command This may be done by people who like to show off their typing speed Sensitive Fields and Slurping A fact such as duck or quack is said to consist of a single field A fieldis a placeholder named or unnamed that may have a value associated with it As a simple analogy you can think
62. acters Fields are commonly delimited or bounded by one or more spaces or parentheses For example Cduck shot Brian Gary Rey has four fields to indicate all the hunters shot by the killer duck In this fact the fields are delimited by spaces and the opening and closing parentheses Facts are not allowed to be embedded within other facts For example the following is an illegal ordered fact Cduck Cshot Brian Gary Rey However this could be a legal deftemplate fact if shot is defined as the name of a field while Brian Gary Rey are the values associated with the named field CLIPS is case sensitive Also certain characters have special meaning to CLIPS C amp l lt The amp I and may not be used as stand alone symbols or as any part of a symbol Some characters act as delimiters by ending a symbol The following characters act as delimiters for symbols any non printable ASCII character including spaces carriage returns tabs and linefeeds double quotes opening and closing parentheses ampersand amp vertical bar less than lt Note that this may be the first character of a symbol tilde semicolon indicates start of a comment a carriage return ends it and may not begin a symbol but may be inside it The semicolon acts as the start of a comment in CLIPS If you try to assert a semicolon CLIPS will think you re entering a comment and wait for you to f
63. an first load assertions from a disk using a batch file An alternative way to enter facts is by using the define facts keyword deffacts For example CLIPS gt Cclear CLIPS gt Cunwatch facts CLIPS gt Cunwatch activations 34 CLIPS gt Cdeffacts walk Some facts about walking Cstatus walking fact to be asserted Cwalk sign walk 3 fact to be asserted CLIPS gt Creset causes facts from defacts to be asserted CLIPS gt facts f Cinitial fact f 1 Cstatus walking f 2 Cwalk sign walk For a total of 3 facts CLIPS gt The required name of this deffacts statement walk follows the deffacts keyword Following the name is an optional comment in double quotes Like the optional comment of a rule the deffacts comment will be retained with the deffacts after it s been loaded by CLIPS After the name or comment are the facts that will be asserted in the fact list The facts in a deffacts statement are asserted using the CLIPS reset com mand The initial fact is put in automatically by a reset The fact identifier of the initial fact is always f 0 Even without any deffacts statements a reset always will assert an initial fact The utility of initial fact lies in starting the execution of a program A CLIPS program will not start running unless there are rules whose LHS s are satisfied by facts Rather than having to type in some fact to start things off the reset command asserts it for you as wel
64. ances 113 urban professional 86 USER 86 user defined functions 79 user defined object types 102 V V amp V 87 value 8 variable identifier 41 variables 41 verification and validation 87 visibility 115 volatile patterns 77 W warmnitialization 120 watch activations 35 watch all 35 watch compilations 35 watch facts 18 watch instances 113 watch rules 35 watch statistics 35 while 81 white space 11 15 wildcard 45
65. and age initially contain no data i e ni values CLIPS gt Cclear CLIPS gt Cdefclass DUCK is a USER Crole concrete CsLot sound Ccreate accessor read write Cslot age Ccreate accessor read write CLIPS gt Cdefinstances DORK_OBJECTS Dorky_Duck of DUCK CLIPS gt Creset CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound nil Cage nil CLIPS gt Csend Dorky_Duck put sound quack quack CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound quack Cage nil CLIPS gt Notice that the slots are printed in the order defined in the class However if the instance inherits slots from more than one class the slots from the more general classes will be printed first Just as slots describe instances facets describe slots A special facet is used in this example create accessor which tells CLIPS to create put and get handlers for the class By default all slots have a read write create accessor so you don t actually have to specify this facet as was done in the prior example If you define your own handlers then you need to use NONE with the create accessor facet The other facet types for create accessor are read write read and write Facets are described in more detail in the next chapter 110 Whether you use the handlers created by CLIPS or else define your own you can also set the slot access to read write read only and initialize only The read write access is the d
66. andler s A predicate function called next handlerp is used to test for the existence of a handler before the call is made If no handler exists then next handlerp will return FALSE The following example illustrates the around handler through a truthful daemon that tells on Dorky_Duck whenever he lies about his age CLIPS gt Cdefmessage handler DUCK 1lie about age around change bind old age self age Cif Cnext handlerp then Ccall next handler bind new age self age Cif lt gt old age new age then Cprintout t Dorky_Duck is lying crlf Dorky_Duck is Lying crlf He s really old age crlf CLIPS gt Cmake instance Dorky_Duck of DUCK Cage 3 Dorky_Duck CLIPS gt send Dorky_Duck lie about age 1 Starting to print I am only 2 Finished printing Dorky_Duck is Lying Dorky_Duck is Lying He s really 3 CLIPS gt 433 Although Dorky_Duck may still lie about his age the daemon tells the truth Notice the change argument Although the around handler does not use change the lie about age primary that is called by the call next handler does need it to change the age Thus change must be passed to the primary by the around handler An error message will occur if you leave out the change The call next handler always passes the arguments of the shadower to the shadowed handler It s possible to pass different arguments to a shadowed handler by use of the overridenext handler function a
67. andler uses the variable new age to store the changed age which is then put into the age slot of the instance The Truthful Daemon A daemon is a handler which executes whenever some basic action like initialization deletion get or put is performed on an instance A rule cannot be considered a daemon because it s not certain that it will be executed just because its LHS patterns are satisfied The only thing that is certain is that a rule will become activated when its LHS is satisfied not that it will execute There is no explicit keyword for daemon since it s just a concept The before and after handlers that printed strings can be considered print daemons These handlers waited for a send Dorky_Duck print age message to trigger their action First the before handler printed its string then the primary handler printed and finally the after handler printed One daemon is the before handler which waits for a print message The second daemon is the after handler that waits for the print primary to finish printing Printing is not considered a basic action because there is no return value associated with a Send lt instance gt print The print message is only sent for the side effect of printing In contrast a send lt instance gt get lt slot gt message will return a value that may be used by other code Likewise the initialization deletion and put all have an effect on an instance and so are considered basic actions like get
68. antage of the print handler that is predefined by CLIPS for the USER class All subclasses of USER can take advantage of the handlers of USER which saves you the trouble of writing handlers for every class that you define Notice how the concept of inheritance from USER to its subclass DUCK simplifies program development by allowing reuse of existing code i e the print handler of USER The empty parentheses that follow the before handler type mean that there are neither parameters nor a wildcard parameter In other words the header handler takes no arguments and so the parentheses are empty but required Note that while multiple parameters may be used there can be only one wildcard Primary Considerations As you can see the trailer handler is the same as the header handler except that an after handler type is used and the action text is different Thus a before handler type does its task before the primary type handler and an after handler does its task after the primary handler A primary is intended to do the major task An around handler type is intended to set up the environment for the rest of the handlers The before and after types are intended for minor tasks such as initializing variables or printing while the primary does the major task The following table summarizes the handler types Handler Type Class Role Return Value around Set up environment for other handlers Yes before Auxilliary work before primary No primary
69. around the instance name is optional for a USER defined class As an example let s create Dorky s cousin Dorky_Duck without brackets as follows CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck Instance Dorky_Duck is made CLIPS gt There are two important rules about instances to keep in mind Only one instance of the same name may be used in a module A class cannot be redefined if instances of the class exist For example let s make a clone of Dorky_Duck as follows It s this evil clone that always gets Dorky_Duck in trouble because no one can tell them apart Kids often have 106 clones like this too and some adults CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck CLIPS gt Cinstances Dorky of DUCK Dorky_Duck of DUCK Still only one Dorky_Duck For a total of 2 instances CLIPS gt Much Ado About Instances If a reset command is issued all the instances in memory are deleted and an instance initial instance is created analogous to the fact initial fact CLIPS gt Creset CLIPS gt Cinstances inittal object of INITIAL OBJECT For a total of 1 instance CLIPS gt Just as deffacts defines facts there is also a definstances to define instances when a reset is issued The following definstances also illustrates the optional comment in double quotes after the instance name DORKY_OBJECTS CLIPS gt Cdefinstances DORKY_OBJECTS The Dorky Cousins CDorky of DUCK
70. as the following example shows CLIPS gt assert prospect Cage 99 name Dopey lt Fact 1 gt CLIPS gt facts f prospect name Cassets rich Cage 80 f 1 prospect name Dopey Cassets rich Cage 99 For a total of 2 facts CLIPS gt Note that the order that the fields are typed in does not matter since these are named fields In the deftemplate it s important to realize that NUMBER is not a primitive field type like symbol string integer and float The NUMBER is really a compound type that can be integer or float It is used for cases in which the user doesn t care what type of numbers are stored An alternative to NUMBER would be specifying the types as follows Cslot age type INTEGER FLOAT Cdefault 80 Bye Bye In general a deftemplate with N slots has the following general structure CdeftempLate lt name gt CsLot 1 CsLot 2 slot N In a deftemplate the attribute values may be specified more precisely than a simple value such as 80 or rich For example in this deftemplate a type of value is specified The field values can be specified by either explicitly listing them or giving a range of values The allowed values can be any primitive type such as SYMBOL STRING INTEGER FLOAT and so on For example Deftemplate Enumerated Values Example allowed symbols rich filthy rich Loaded allowed strings Dopey Dorky Dicky allowed numbers 123 4 5 2 001 1 3e 4 allowed integers 10
71. asses Since the new behavior is inherited it may substantially reduce the verification and validation V amp V of the handlers V amp V essentially means that the product was built properly and that it meets the requirements The task of verifying and validating software may take more time and money than the software development itself especially if the software affects human life and property Inheritance of handlers allows for efficient reuse of existing code and speeds up development Classes are defined in CLIPS using the defclass construct The UPPIE class is defined in one statement as follows Cdefclass UPPIE Cis a USER Notice the similarity between the UPPIE USER relationship in Fig 1 1 and the defclass construct You do not have to enter the USER or the OBJECT classes since these are predefined classes and so CLIPS already knows their relationship In fact if you try to define USER or OBJECT an error message will result since you cannot change the predefined classes unless you change the source code of CLIPS Since CLIPS is case sensitive commands and functions must be entered in lowercase Predefined system classes such as USER and OBJECT must be entered in uppercase Although you can enter user defined classes in lowercase or uppercase we will follow the convention of using all uppercase for classes for the sake of readability The basic format of the defclass command to define classes only and not slots is 88
72. at the duck must have more than one name So a plain bachelor Richard isn t adequate because there is only one name in the fact This type of situation in which only part of the fact is specified is very common and very important To solve this problem a wildcard can be used to fire the Richards The simplest form of wildcard is called a single field wildcard and is shown by a question mark The is also called a single field constraint A singlefield wildcard stands for exactly one field as shown following 46 CLIPS gt Cclear CLIPS gt Cdefrule dating ducks bachelor Dopey gt printout t Date Dopey crlf CLIPS gt Cdeffacts duck bachelor Dicky bachelor Dopey bachelor Dopey Mallard bachelor Dinky Dopey bachelor Dopey Dinky Mallard CLIPS gt Creset CLIPS gt Crun Date Dopey CLIPS gt The pattern includes a wildcard to indicate that Dopey s last name is not important So long as the first name is Dopey the rule will be satisfied and fire Because the pattern has three fields of which one is a single field wildcard only facts of exactly three fields can satisfy it In other words only Dopeys with exactly two names can satisfy this duckette Suppose you want to specify Dopeys with exactly three names All that you d have to do is write a pattern like bachelor Dopey or if only persons with three names whose middle name was Dopey bachelor Dopey or if only the last name wa
73. ault dynamic Cmake instance Cgensym of LINE Cpropagation no inherit Cslot Line3 Cdefault dynamic Cmake instance Cgensym of LINE propagation no inherit CLIPS gt Notice that the no inherit facets in TRIANGLE are techically not necessary since there is no subclass of TRIANGLE defined The reason for including the no inherit facets is because of defensive programming which is analogous to defensive driving If another subclass is added either by you or someone else the defclass of TRIANGLE must be modified to include the no inherit facets This is bad style because it means that existing debugged code must be modified If you re going to enhance existing debugged code you shouldn t have to modify it It s better to plan ahead for enhancements Next we ll define a triangle instance and check the instances generated Note that your gen values may be different from those shown unless you ve just started or restarted CLIPS or have not used gensym or gensym since you started CLIPS gt Cdefinstances TRIANGLE_OBJECTS CTrianglel of TRIANGLE CLIPS gt Creset CLIPS gt Cinstances initital object of INITIAL OBJECT Triangle1 of TRIANGLE gen3 of LINE gen4 of POINT gen5 of POINT gen6 of LINE gen7 of POINT gen8 of POINT gen9 of LINE gen10 of POINT geni1 of POINT For a total of 11 instances 4 CLIPS gt At first you may be surprised at all the gensym values created
74. bjects These objects can be used or modified as desired in a new program In addition to public domain object libraries a number of companies market commercial object libraries Although the concept of reusable software components has been around since the early days of FORTRAN subroutine libraries in the 1960s the concept has never before been so successfully used for general software development In order to define a class you must specify one or more parent classes or superclasses of the class to be defined As an analogy to superclasses every person has parents people do not spontaneously come into existence although sometimes you may wonder if certain people really had parents The opposite of a superclass is a child class or subclass This determines the inheritance of the new class A subclass inherits attributes from one or more superclasses The term attribute in COOL refers to the properties of an object which are named slots that describe it For example an object to represent a person might have slots for name age address and so forth An instance is an object that has values for the slots such as John Smith 28 1000 Main St Clear Lake City TX Lower level classes automatically inherit their slots from higher level classes unless the slots are explicitly blocked New slots are defined in addition to the inherited slots to set all the attributes that describe the class An object s behavior is defined by its message handlers
75. ce lists all the superclasses of a class back to the root class OBJECT You can also see that the direct superclasses information indicates the superclass that is one link above a class while the inheritance precedence list shows all superclasses Even if aclass has no direct instances it will have indirect instances if it has subclasses which have instances The indirect instances of a class are all the instances of its subclasses A concrete class is allowed to have direct instances For example given a concrete class COW the direct instance Elsie would be the famous TV salescow Normally classes inheriting from abstract classes are also abstract classes However classes inheriting from system classes such as USER are considered to be concrete unless otherwise specified So UPPIE and YUPPIE are also concrete classes It is strongly recommended that all classes you define in CLIPS be subclasses of USER CLIPS will automatically provide handlers for print init and delete if your classes are subclasses of USER The YUKKIE class has multiple inheritance since it has two direct superclasses 98 CHILD and YUPPIE If you think back to the analogy of inheritance to bosses in an organization the issue of multiple inheritance brings up an interesting question Who s the boss In the case of a tree structure every class has only one direct superclass boss and so its easy to figure out who to take orders from However in the case of mu
76. class has more than one link Fig 1 2 summarizes the class terminology 89 OBJECT lt Indirect Superclass gt Superlkesses lt Direct Superclass gt lt Direct Subclass gt lt Indirect Subclass gt Fig 1 2 Class Relationships 3 Subclasses L The root class OBJECT is the only class that does not have a superclass Using this fancy new terminology allows us to state the A class may inherit from all its superclasses This is a simple yet powerful concept fully exploited in OOP This principle means that slots and message handlers may be inherited to save us the trouble of redefining them for new subclasses In addition slots may be easily customized for new subclasses as modifications and as composites of superclass slots By allowing easy and flexible reuse of existing code program development time and cost are decreased In addition the reuse of working existing code minimizes the amount of verification and validation needed All these advantages facilitate the program maintenance tasks of debugging modification and enhancement once the code is released The reason for using may in the principle is to emphasize that inheritance of slots from a class may be blocked by including a no inherit facet in the class slot definition The direct and indirect classes of a class are all those that lie on an inheritance path to OBJECT An inheritance pathis a set of connected nodes between the class and 90
77. class of LINE However the whole point of this example is to illustrate Inheritance by Generalization which is a logical concept that is not directly supported by Version 6 0 Thus defining LINE as a subclass of POINT and TRIANGLE as a subclass of LINE is done for reasons of documenting the logical concept of Inheritance by Generalization Admittedly a comment could be added by the defclass LINE is a USER and defclass TRIANGLE is a USER stating that we are trying to implement Inheritance by Generalization but seeing the code in place is better documentation If Inheritance by Generalization is ever directly supported by CLIPS these defclass statements will make it easy to convert The reason for including the make instance gensym in the LINE slots is to provide the inheritance from the POINT class With the standard Inheritance by Specialization only one position slot of LINE is possible because POINT has only one position slot It is not possible for both slot point1 and slot point2 of LINE to inherit the position slot of 138 POINT The actual slot value of each LINE will be a gensym value Each gensym value will be the instance name of a point instance The point position can then be accessed through the gensym value Thus the gensym values act as pointers to different instances This indirect access technique of gensym values is analogous to using a pointer to access a value in a procedural language Thus
78. classes and subclasses because these check slots dynamically In order for a superclass to dynamically reference a slot however the visibility facet of the slot must be public The following example would not work if DUCK is changed to USER CLIPS gt Cdefmessage handler DUCK print age primary Cprintout t Starting to print crlf Age self age crlf Finished printing crlf CLIPS gt Csend Dorky_Duck print age Starting to print Age 2 Finished printing CLIPS gt As an example of using a dynamic put function in a handler suppose we want to help Dorky_Duck regain some of his youth The following example shows how his age can be changed using a handler This example also illustrates how a value can be passed to a handler through the change variable CLIPS gt Cclear CLIPS gt Cdefclass DUCK is a USER Cslot sound default quack Cslot age CLIPS gt Cdefmessage handler DUCK 1lie about age change bind new age self age change Cdynamic put age new age Cprintout t Starting to print crlf I am only new age crlf Finished printing crlf CLIPS gt Cmake instance Dorky_Duck of DUCK Cage 3 Dorky_Duck 131 CLIPS gt send Dorky_Duck lie about age 2 Starting to print I am only 1 Finished printing CLIPS gt As you can see Dorky_Duck s belief is so strong that the changed age is put in his age slot Notice how the h
79. classes command shows the class hierarchy through indentation CLIPS gt browse classes OBJECT PRIMITIVE NUMBER INTEGER FLOAT LEXEME SYMBOL STRING MULTIFIELD ADDRESS EXTERNAL ADDRESS FACT ADDRESS INSTANCE ADDRESS INSTANCE INSTANCE ADDRESS INSTANCE NAME USER INITIAL OBJECT UPPIE SUPPIE MUPPIE YUPPIE PUPPIE YUKKIE CHILD YUKKIE CLIPS gt The asterisk after a class name indicates that it has multiple superclasses The browse classes command has an optional argument which specifies the starting class for the subclasses you want to see This is convenient if you are not interested in listing all the classes The following examples illustrate how to show 97 portions of the YUPPIE graph of Fig 1 6 called subtrees or subgraphs depending on whether the nodes and links form a tree or graph CLIPS gt Cbrowse classes UPPIE UPPIE SUPPIE MUPPIE YUPPIE PUPPIE YUKKIE CLIPS gt Cbrowse classes YUPPIE YUPPIE PUPPIE YUKKIE CLIPS gt Cbrowse classes YUKKIE YUKKIE CLIPS gt An abstract class is designed for inheritance only The abstract class USER cannot have direct instances defined for it which are instances defined directly for a class In addition to the class information the inheritance precedence of the classes is described This is an ordered list in which the order from left to right indicates the highest to lowest precedence that classes contribute by inheritance The inheritance preceden
80. computer games and so forth are not intended to embody human expertise or knowledge One definition of an expert is someone more than 50 miles from home and carrying a briefcase CLIPS is called an expert system tool because it is a complete environment for developing expert systems which includes features such as an integrated editor anda debugging tool The word shell is reserved for that portion of CLIPS which performs inferences or reasoning The CLIPS shell provides the basic elements of an expert system 1 fact list and instance list Global memory for data 2 knowledge base Contains all the rules the rule base 3 inference engine Controls overall execution of rules A program written in CLIPS may consist of rules facts and objects The inference engine decides which rules should be executed and when A rule based expert system written in CLIPS is a data driven program where the facts and objects if desired are the data that stimulate execution via the inference engine 2 This is one example of how CLIPS differs from procedural languages such as Pascal Ada BASIC FORTRAN and C In procedural languages execution can proceed without data That is the statements are sufficient in those languages to cause execution For example a statement such as PRINT 2 2 could be immediately executed in BASIC This is a complete statement that does not require any additional data to cause its execution However in CLIPS data are
81. cutables CLIPS executables are available at http www ghg net clips download executables The source code can be downloaded from http www ghg net clips download source Documentation The CLIPS Reference Manuals and User s Guide are available in Portable Document Format PDF at http www ghg net clips download documentation Adobe Acrobat Reader is needed to view PDF files It can be downloaded at http Awww adobe com prodindex acrobat readstep html Expert Systems Principles and Programming 3rd Edition by Giarratano and Riley ISBN 0 534 95053 1 available for 75 95 comes with a CD ROM containing CLIPS 6 05 executables DOS Windows 3 1 95 and MacOS documentation and source code The first half of the book is theory oriented and the second half covers rule based programming using CLIPS For more information contact International Thompson Publishing 7625 Empire Dr Florence KY 41042 Phone 800 354 9706 Phone 606 525 2230 or 157 PWS Publishing Company 20 Park Plaza Boston MA 02116 4324 Phone 617 542 3377 Fax 617 338 6134 WWW http www pws com pws html SYMBOL 47 amp 65 amp connective constraint 65 amp 80 107 bile 67 80 45 self 124 86 15 64 64 constraint 64 lt 44 67 gt 18 gt 24 A abstract class 97 abstraction 84 access facet 119 access type 115 action 151 actions 23 activations
82. d to more carefully show every fact and activation The arrow going to the right means an entering fact or activation while an arrow to the left would mean an exiting fact or activation CLIPS gt Cclear CLIPS gt Cdefrule duck Canimal is duck gt Cassert Csound is quack CLIPS gt Cwatch facts CLIPS gt Cwatch activations CLIPS gt Cassert Canimal is duck gt f Q Canimal is duck gt Activation Q duck f Activation salience is by lt Fact Q gt default then rule name pattern entity index CLIPS gt Cassert Canimal is duck Notice that duplicate fact FALSE cannot be entered CLIPS gt Cagenda duck f 0 For a total of 1 activation CLIPS gt Crun gt f 1 Csound 1s quack CLIPS gt Cagenda Nothing on agenda after rule fires CLIPS gt Cfacts Even though fact matches rule f Canimal is duck refraction will not allow this f 1 Csound 1is quack activation because the rule already For a total of 2 facts fired on this fact CLIPS gt Crun CLIPS gt You can make the rule fire again if you retract the fact and then assert it as a new fact Show Me the Rules Sometimes you may want to see a rule while you re in CLIPS There s a command called ppdefrule the pretty print rule that prints a rule To see a rule specify the rule name as argument to pprule For example CLIPS gt Cppdefrule duck Cdefrule MAIN duck Canimal is duck gt Cassert Csound is quack
83. default value of field name slot assets name of field Ctype SYMBOL type of field Cdefault rich default value of field assets Cslot age name of field Ctype NUMBER type NUMBER can be INTEGER or FLOAT Cdefault 8Q defauLt value of field age In this example the components of deftemplate are structured as A deftemplate relation name Attributes called fields The field type which can be any one of the allowed types SYMBOL STRING NUMBER and others The default for the field value This particular deftemplate has three single slot slots called name assets and age The deftemplate default values are inserted by CLIPS when a reset is done if no explicit values are defined For example enter the deftemplate for prospect and assert it as shown CLIPS gt assert Cprospect lt Fact gt CLIPS gt facts f prospect name Cassets rich Cage 80 For a total of 1 fact CLIPS gt 55 As you can see CLIPS has inserted the default value of the null string for the name field since that is the default for a STRING Likewise the assets and age defaults were also inserted by CLIPS Different types have different default symbols such as the null string for STRING the integer 0 for INTEGER the float 0 0 for FLOAT and so on The DERIVE keyword selects the appropriate type of constraint for that slot e g the null string for a slot of type STRING You can explicitly set the field values
84. dence shadowed primaries if they are to execute Since only around and primary handlers can return values and the around shadows primaries it follows that the return value of a send will be the around return value If there is no around then the return value will be that of the highest precedence primary As mentioned before there must always have at least one applicable primary handler The following table summarizes the return value of the handler types Handler Type Return Value around Ignore or capture return value of next most specific around or primary before Ignore Side effect only primary Ignore or capture return value of more general primary after Ignore Side effect only Message handler Return Values Get the Point Up to now we ve discussed inheritance using only is a links As you ve seen this type of inheritance relationship is good for defining classes that are more and more specialized That is you start off by defining the most general classes as a subclass of USER and then define more specialized classes with more slots in the lower levels of the class hierarchy Normally you design new classes as specializations of existing ones This paradigm is Inheritance by Specialization As you recall from the quadrilateral example of Fig 1 8 of chapter 1 the highest level is quadrilateral and the lower levels are trapezoid parallelogram rectangle and then square along one inheritance path Trapezoid is a sp
85. duckshoot duck Missing third field lt Fact 3 gt CLIPS gt Crun CLIPS gt Rule doesn t fire no output Notice what a big difference the order of fields makes in determining who shot who You can also see that the rule did not fire when the fact duckshoot duck was asserted The rule was not activated because no field of the fact matched the second pattern constraint who The Happy Bachelor Retraction is very useful in expert systems and usually done on the RHS rather than at the top level Before a fact can be retracted it must be specified to CLIPS To retract a fact from a rule the fact address first must be bound to a variable on the LHS There is a big difference between binding a variable to the contents of a fact and binding a variable to the fact address In the examples that you ve seen such as duck sound sound a variable was bound to the value of a field That is sound was bound to quack However if you want to remove the fact whose contents are duck sound quack you must first tell CLIPS the address of the fact to be retracted The fact address is specified using the left arrow lt To create this just type a lt symbol followed by a As an example of fact retraction from a rule CLIPS gt Cclear CLIPS gt assert bachelor Dopey lt Fact Q gt CLIPS gt facts f 0 bachelor Dopey For a total of 1 fact CLIPS gt Cdefrule get married duck lt bachelor Dopey gt printout t Do
86. e Rete Algorithm devised by Charles Forgy of Carnegie Mellon University for his OPS shell The term Rete is Latin for net and describes the software architecture of the pattern matching process It is very difficult to give precise rules that will always improve the efficiency of a program running under the Rete Algorithm However the following should be taken as general guidelines that may help 1 Put the most specific patterns in a rule first Patterns with unbound variables and wildcards should be lower down in the list of rule patterns A control fact should be put first in the patterns 2 Patterns with fewer matching facts should go first to minimize partial matches 3 Patterns that are often retracted and asserted volatile patterns should be put last 78 in the list of patterns As you can see these guidelines are potentially contradictory A non specific pattern may have few matches see guidelines 1 and 2 Where should it go The overall guideline is to minimize changes of the partial matches from one cycle of the Inference Engine to the next This may require much effort by the programmer in watching partial matches An alternative solution is simply to buy a faster computer or an accelerator board This is becoming more attractive since the price of hardware always goes down while the price of human labor always goes up Because CLIPS is designed for portability any code developed on one machine should work on another
87. e companion volume to this book is the CLIPS Reference Manual which does provide a complete comprehensive discussion of all the topics in this book and much more Who Should Read This Book The purpose of the CLIPS User s Guide is to provide an easy to read elementary introduction to expert systems for people with little or no experience with expert systems The CLIPS User s Guide can be used in the classroom or for self teaching The only prerequisite is that you have a basic knowledge of programming in a high level language such as Pascal Ada FORTRAN C OK BASIC if nothing else but we won t vil admit it in public and will disavow this statement if asked How To Use This Book The CLIPS User s Guide is designed for people who want a quick introduction to expert system programming in a hands on manner The examples are of a very general nature Also since learning a new language can be a frustrating experience the writing is in a light humorous style I hope compared to serious minded massive and intimidating college textbooks Hopefully the humor will not offend anyone with a sense of humor For maximum benefit you should type in the example programs in the text as you read through the book By typing in the examples you will see how the programs should work and what error messages occur if you make a mistake The output for the examples is shown or described after each example Finally you should read the correspo
88. e light is color crlf The variable color will be bound to whatever color is matched by the field yellowlblinking yellow The amp also is useful with the For example suppose you want a rule that triggers when the light is not yellow and not red Cdefrule not yeLLlow red Clight color amp red amp yel Low 66 Cprintout t Go since light is color crlf It s Elementary Besides dealing with symbolic facts CLIPS also can perform numeric calculations However you should keep in mind that an expert system language like CLIPS is not primarily designed for number crunching Although the math functions of CLIPS are very powerful they are really meant for modification of numbers that are being reasoned about by the application program Other languages such as FORTRAN are better for number crunching in which little or no symbolic reasoning is being done You ll find the computational capability of CLIPS useful in many applications CLIPS provides basic arithmetic and math functions div max min abs float and integer For more details see the CLIPS Reference Manual Numeric expressions are represented in CLIPS according to the style of LISP In both LISP and CLIPS a numeric expression that customarily would be written as 2 3 must be written in prefix form 2 3 In the prefix form of CLIPS the function precedes the arguments and parentheses must surround the numeric expression The customary way of w
89. e losing money on your stockbrokers advice CLIPS has two functions to help with logical support The dependencies function lists the partial matches from which a pattern entity receives logical support or none if there is no support The second logic function is dependents which lists all pattern entities which receive logical support from a pattern entity The connective constraint uses amp I or Another type of field constraint is 80 called a predicate constraint and is often used for pattern matching of more complex fields The purpose of a predicate constraint is to constrain a field depending on the result of a Boolean expression If the Boolean returns FALSE the constraint is not satisfied and the pattern matching fails You ll find that the predicate constraint is very useful with numeric patterns A predicate function is one which returns a FALSE or a non FALSE value The colon followed by a predicate function is called a predicate constraint The may be preceded by amp I or or may stand by itself as in the pattern fact gt 2 1 It is typically used with the amp connective constraint as amp Predicate Function Check if lt arg gt is evenp lt arg gt even number floatp lt arg gt floating point number integerp lt arg gt integer lexemep lt arg gt symbol or string numberp lt arg gt float or integer oddp lt arg gt odd number pointerp lt arg gt external
90. e of static default the ID values were the same gen1 since the gensym was only evaluated once when the class was defined rather then for every new instance in the case of dynamic default 118 Cardinal Properties The cardinality of a slot refers to one of two types of fields that a slot can hold 1 single field or 2 multifield The term cardinality refers to a count A bound single field slot contains only one field while a bound multifield slot may contain zero or more fields The bound single field slot and the bound multifield slot each contain one value However the one multifield value may have multiple fields in it For example abc isa single multifield value with three fields The empty string is a single field value just as a b c is In contrast an unbound slot has no value As an analogy to single and multifield variables think of a slot as your mailbox Sometimes you may find a single piece of junk mail that doesn t have an envelope Instead an address label has just been stuck on a folded piece of paper addressed to Resident Other times you may find an envelope with multiple ads in it The single piece of junk mail without an envelope is like a single field value while the envelope with multiple ads is like the multifield value If the junk mail distributor slips up and mails you an envelope with nothing inside this corresponds to the empty multifield variable Come to think of it if the junk mail
91. ecial class of quadrilateral parallelogram is a special class of trapezoid rectangle is 135 a special class of parallelogram and square is a special class of rectangle Inheritance can also be used to build up more complex classes However this is not quite as direct in CLIPS As an example the basic class for geometry is a POINT containing a single slot point1 A LINE can be then defined by adding point2 to POINT A TRIANGLE is defined by adding point3 to LINE and so on for QUADRILATERALS PENTAGONS etc Cdefclass POINT Cis a USER CmultislLot point1 Cdefclass LINE Cis a POINT Cmultislot point2 Cdefclass TRIANGLE is a LINE CmultislLot point3 Notice how each class is a specialization of its parent class by inheriting the superclass points and then adding one new point The opposite paradigm is Inheritance by Generalization in which more general classes are built up from simple ones For example a LINE is considered made up of two points A TRIANGLE is made up of three lines AQUADRILATERAL is made up of four lines and so forth This would be a good paradigm for building an object oriented drawing system in which new objects could be built up out of simpler ones There is a subtle but important difference between this example of specialization and generalization In specialization new classes are built up by adding specialized slots which are the same type as superclass slots Thus POINT LINE and TRIANGLE all
92. ed as the first field but may be used as others For example the names of predefined CLIPS functions see Appendix of the CLIPS Reference Manual def constructs and so forth cannot be used A fact consists of one or more fields enclosed in matching left and right parentheses For simplicity we ll only discusss facts in the first seven chapters but most of the discussion of pattern matching applies to objects as well Exceptions are certain functions such as assert and retract which only apply to facts not objects The corresponding ways to handle objects are discussed in chapters 8 12 A fact may be ordered or unordered All the examples you ve seen so far are ordered facts because the order of fields makes a difference For example notice that CLIPS considers these as separate facts although the same values 1 2 and 3 are used in each 5 Ccoordinates 1 2 3 6 Ccoordinates 1 3 2 f f Ordered facts must use field position to define data As an example the ordered fact duck Brian has two fields and so does Brian duck However these are considered as two separate facts by CLIPS because the order of field values is different In contrast the fact duck Brian has only one field because of the concatenating the two values Deftemplate facts described in more detail later are unordered because they use named fields to define data This is analogous to the use of records in Pascal and other languages Multiple fields
93. ee cece ee eee eee eeeeeeeseeeeeeeeeseeeeees 53 Mrz WOndertul sorisa ess sri aad ri tence ANCE NERE dee ov ieee iy ee 53 Ain t No Strings on Meisterin a a a a aoia aii aaah 58 Whats ma Name s isrmnt ee n A hounds val aeea A E havea vane aden iets 59 Chapter 6 Being Functional cececeeeeeeeeeeeeeeeeeseceeeseseeeeeeeeeeeeees 63 Not My Constraint e mae a aa a a a tiaras Aaa a ee eat 63 Be Cautious sina o aa e a E EA a A E Hees O ENa TE AAEE AA 64 And Away We GO prred esceh scan st ccdecectiey dane ssudde kaada aeriana EAN Aa ieri ia ENAA adain is 65 its Blementary ices ac ara SaN ones T aANT a sea a AN ENN A 66 Extensive Arguments cceceee cece ee ee eter centre e eee tees eee ee ee aaae eres aaaeeeeeaaaeeeeeaaaeeeesaaea ees 67 Mixed RESURS aaia esri AEN LENN ATEA bade santas eduer TAERE ENOS VNE 68 Bound Bachelors rassa ana a Aa Aa a a a aaa tetas 69 Doing Your OWn Thing misioa eed a aa edie Seed 71 Other F at resS aneve nee perenne errr per rctrrereertee rire pereree crete re cnererre EEA PARE RaT 72 Chapter 7 How to Be in Control 2 ccecececeeeeeceeeeececeeeceseeeeeeeeeeeeees 75 Let s Start Reading se syede delve ea eee deceives Ava tec ade 75 Being Eficiont osc ected ni ain e bias Mase nein ee ae eae 77 Other Features arrere fesebelfeeeetaty eget PA des tang E asi edeeaty ogee aue nd veetaby clndel deivertaiy gee 78 Chapter 8 Matters of Inheritance 2 0cecececeeeeeeeeeeeceeeeeseeeeeeeeeeeeeeees 83 H
94. eeeeeeeeeaeaaea 118 Other Features s 2 aerial eee ee i ee 119 Chapter 11 Handling Handlers 0 0 ecececeeeeeeeceeeeececeeeseseeeeeeeeeeeeees 121 Your Primitive Selfs i208 aA tas reir das ne neti 121 Make Em Pay Through the NOS6 eceeeseeeeeeeeaeeeeeeaaeeeeeeaaeeeeeeaaeeeeeeaaeeeeeeaaieeees 125 Getting AOUN ai eerie fate Hate iaa iE AE eo ee een ae ee 126 Primary Considerations ccccceeeeeee cece ee enee eect esse ee ae aaeneeeeeeeeeaeaaaaeeeeeeeeeeeaaaaeaes 127 The Power ot Belo arasia eh i a ee en ee aA 129 The Truthtul Dasmi earr este eces tices NAEK E docs eves enced EEE aSk 131 Getihe POI asas i eee erate eana ne Aaeeea aAA a vent aE Oaa eieaa RERA ani 134 Treas ro MAPS a ceshvs tee caus sicker SEa YEE AIENEA sic cela EEEE espace NAINEN AAEE ETEENI EE 140 Other Feat reSam an n a a a a T Bae a nat a A 145 Chapter 12 Questions and Answers c0cececeeeeeeeeeeeeeeeeeeeseeeeeeeeeeneees 147 Object Lessons aea e e dee a eaa Ena AAA EA KA IDOA teen EIA ea sien 147 Objects inthe Database miea sided en es veces ee ee es AES 148 WW TAK SAM ict ezeceinvt E a EE TAE E R E AEA 150 Design DECISIONS a aaee ss id A E AEE EAA AAEREN E TAANE 151 Other Features nan 4 fecpeupacg aevaeethiee sagan sea yet nag daa Ne dae Ea AA ATA NE ee eee 154 Support Informatio Msisi eesin tarie aanta aana aaae riai 155 Readme The first step on the road to wisdom is the admission of ignorance The second step is realizing
95. efault and so it was not necessary to specify it in this example The value of a slot is changed using the put message Notice the dash at the end of put The dash is an essential part of the message syntax since it separates the out from the slot name Only one put is allowed in a Send Thus to change multiple slots or the same slot of many instances you must send multiple messages Instead of doing this manually it s possible to write a function to do multiple sends or use the modify instance function The value of a slot can be set by a slot override in a make instance As an example CLIPS gt Cmake instance Dixie_Duck of DUCK sound quack Cage 2 Dixie_Duck CLIPS gt send Dixie_Duck print Dixie_Duck of DUCK Csound quack Cage 2 CLIPS gt The complementary message to put is get which gets the data from a slot as shown in the following example If a put is successful it returns the new value while if get is successful it returns the appropriate data If the put or get does not succeed an error message will be returned The following examples show how this works CLIPS gt Csend Dorky_Duck put color white No slot color MSGFUN1 No applicable primary message handlers found for put color FALSE CLIPS gt Csend Dorky_Duck get age nil CLIPS gt send Dorky_Duck put age 1 Value put in age 1 CLIPS gt send Dorky_Duck get age Check value is correct 1 CLIPS gt In con
96. envelope is empty have you really received junk mail A multiple facet with keyword multislot is used to store a multifield value as shown in the following example CLIPS gt Cclear CLIPS gt Cdefclass DUCK is a USER CmultisLot sound default quack quack CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck CLIPS gt Csend Dorky_Duck print Dorky_Duck of DUCK Csound quack quack CLIPS gt A multifield value can be accessed using get and put as shown in the following examples which shows how to keep track of quacks CLIPS gt send Dorky_Duck put sound quack1 quack2 quack3 Cquack1 quack2 quack3 CLIPS gt Csend Dorky_Duck get sound Cquack1 quack2 quack3 119 CLIPS gt Standard CLIPS functions such as nth to get the nth field of a multislot value can be used The following example shows how to pick a certain quack CLIPS gt Cnth 1 Csend Dorky_Duck get sound quack1 CLIPS gt Cnth 2 Csend Dorky_Duck get sound quack2 CLIPS gt Cnth 3 Csend Dorky_Duck get sound quack3 CLIPS gt Other Features CLIPS has several multifield slot functions as shown in the following table Function Meaning slot replace Replace the specified range slot insert Insert the specified range slot delete Delete the specified range A multifield slot with no values e g the empty multifield value may be assigned to a slot with a multiple facet Note that there is a difference betwee
97. er Handlers are arranged in a message handler precedence which determines how they are called The process of determining which handlers should be called and in what order is called the message dispatch Every time a message is sent to an object CLIPS arranges the message dispatch for the applicable handlers of that object which can be viewed by the preview send command The applicable handlers are all 146 handlers in all classes along the object s inheritance path that may respond to the type of message Other functions useful with pattern matching objects by rules follow Function object pattern match delay modify instance active modify instance message modify instance active message modify instance Meaning Delay pattern matching of rules until after instances are created modified or deleted Modifies instance using slot overrides Object pattern matching delayed until after modifications Change the values of the instance concurrent with object pattern matching with direct nodify message Change the values of the instance Delay object pattern matching until all slots are changed Change the values of the instance concurrent with object pattern matching using message modify siir Chapter 12 Questions and Answers The best way to learn is by asking yourself questions the best way to be sorry is by answering all of them In this chapter you ll learn how to pattern match on instances One way is with rul
98. ercase letters in the fields of the fact That is the T of The and the Q of Quack are uppercase CLIPS is said to be case sensitive because it distinguishes between uppercase and lowercase letters For example assert the facts duck and Duck and then issue a facts command You ll see that CLIPS allows you to assert duck and Duck as different facts because CLIPS is case sensitive The following example is a more realistic case in which carriage returns are used to improve the readability of a list To see this assert the following fact where carriage returns and spaces are used to put fields at appropriate places on different lines Dashes or minus signs are used intentionally to create single fields so CLIPS will treat items like fudge sauce as a single field CLIPS gt Cclear CLIPS gt assert Cgrocery list ice cream cookies candy fudge sauce lt Fact 0 gt CLIPS gt facts f 0 Cgrocery Llist ice cream cookies candy fudge sauce For a total of 1 fact CLIPS gt As you can see CLIPS replaced the carriage returns and tabs with single spaces While the use of white space in separating the facts is convenient for a person reading a program they are converted to single spaces by CLIPS A Matter of Style It is good rule based programming style to use the first field of a fact to describe the relationship of the following fields When used this way the first field is called a relation The remaining fields of the
99. es Also CLIPS has a number of query functions to match instances In addition control facts and slot daemons can be used for pattern matching Object Lessons One of the new features of Version 6 0 is the ability of rules to pattern match on objects The following example shows how the value of the slot sound is pattern matched by a rule CLIPS gt Cclear CLIPS gt Cdefclass DUCK 1s a USER CmultisLot sound default quack quack CLIPS gt Cmake instance Dorky_Duck of DUCK Dorky_Duck CLIPS gt Cmake instance Dinky_Duck of DUCK Dinky_Duck CLIPS gt Cdefrule find sound duck lt object Cis a DUCK Csound find gt Cprintout t Duck Cinstance name duck crlf CLIPS gt Crun Duck Dinky_Duck says Cquack quack Duck Dorky_Duck says Cquack quack CLIPS gt says find 148 The object pattern conditional element object is followed by the classes and slots to be matched against Following the is a and the slot name can be constraint expressions involving amp and I In addition instance names can be specified for pattern matching The following example shows how only one instance of the DUCK class is matched using the name constraint name of instances Note that name is a reserved word and cannot be used as a slot name CLIPS gt Cdefrule find sound duck lt object Cis a DUCK Csound find Cname Dorky_Duck gt Cprintout t Duck Cinstance name duck says find
100. es behave as concrete classes Thus by default any class inheriting from USER is a concrete class and does not need to be declared as is done in the following commands If a class has role abstract no direct instances of it can be made Abstract classes are defined for inheritance purposes only For example an abstract class called PERSON could be defined whose properties such as name address age height weight and so on are inherited by concrete classes MAN and WOMAN A direct instance of MAN could be a man person called Harold and a direct instance of WOMAN is a woman person called Henrietta CLIPS gt Cdefclass DUCK is a USER role concrete CLIPS gt Cdescribe class DUCK OK K K K OK K K K K K K K K OK K K K K K K K K K K K K K K K OK K K K K K K K K K K K K K K K K K KKK KK KK K K KKK KK KKK Concrete direct instances of this class can be created Reactive direct instances of this class can match defrule patterns Direct Superclasses USER Inheritance Precedence DUCK USER OBJECT Direct Subclasses Recognized message handlers init primary in class USER delete primary in class USER create primary in class USER print primary in class USER direct modify primary in class USER message modify primary in class USER direct duplicate primary in class USER message duplicate primary in class USER KK K K K K K K K K KK K K K K K K K KK K K K KK K K K K K K K K K K K K K K K K K K K K K K K K K K K K K K k K K kK K C
101. es that do completely different things goes against the Principle of Least Astonishment Another example of polymorphism could be defined using a message handler for If a message for is sent to strings or symbols i e LEXEME objects they will be concatenated because of a handler defined for the LEXEME class If the is sent to ordinary numbers the result is addition because of the predefined system handler for addition of numbers If the is sent to complex numbers defined as a subclass of USER called COMPLEX a handler defined for the COMPLEX class will do complex number addition Thus the does related types of operations that matches our intuition and does not surprise us In addition to the predefined handlers such as print you may define your own handlers Let s start by writing a handler to add numbers through messages As you can see from Fig 1 7 of chapter 1 the class NUMBER has subclasses INTEGER and FLOAT Since these are predefined classes it would seen natural to do numeric calculations by sending messages to numbers Let s try it as follows CLIPS gt Cclear CLIPS gt Csend 1 2 MSGFUN1 No applicable primary message handLlers found for FALSE CLIPS gt Well as you can see this example didn t work The reason why is implied in the error message Let s check it out by obtaining more information about the INTEGER class since that was the target object of the print message 123 CLIPS
102. ey are the same The term dummy argument is sometimes called a parameter in other books Although each action may have returned values from function calls within the action these are blocked by the deffunction from being returned to the user The deffunction will only return the value of the ast action lt actionK gt This action may be a function a variable or a constant The following is an example of how a deffunction is defined to calculate the hypotenuse and then used in a rule Even if the variable names in the rule are the same as the dummy arguments there s no conflict That s why they re dummy because they don t mean anything CLIPS gt Cclear CLIPS gt Cdeffunction hypotenuse name C a b dummy arguments CsqrtC a a C b b 3 action CLIPS gt Cdefrule calculate hypotenuse dimensions base height gt Cprintout t Hypotenuse Chypotenuse base height crlf CLIPS gt Cassert Cdimensions 3 4 lt Fact Q gt CLIPS gt Crun Hypotenuse 5 Q CLIPS gt Deffunctions may be used with multifield values as the following example shows CLIPS gt Cclear CLIPS gt Cdeffunction count arg Clength arg CLIPS gt Ccount 1 2 3 a duck quacks 6 CLIPS gt Other Features Other useful functions follow For more information see the CLIPS Reference Manual Function Meaning round Round toward closest integer If exactly between two integers rounds toward negative infinity integer Truncate
103. fact are used for specific values An example is grocery list a2 4A ice cream cookies candy fudge sauce The dashes are used to make multiple words fit in a single field Good documentation is even more important in an expert system than in languages such as Pascal C Ada etc because the rules of an expert system are not generally executed in a sequential manner CLIPS aids the programmer in writing descriptive facts like this by means of deftemplates Another example of related facts is duck horse and cow It s better style to refer to them as Canimal is duck Canimal is horse Canimal is cow or as the single fact Canimals duck horse cow since the relation animal is or animals describes their relation and so provides some documentation to the person reading the code The explicit relations animal is and animals make more sense to a person than the implicit meaning of duck horse and cow While this example is simple enough that anyone can figure out the implicit relations it is an easy trap to fall into to write facts in which the relationship is not so obvious In fact it s much easier to make something more complicated than easy since people are more impressed by complexity than simplicity Getting Spaced Out Since spaces are used to separate multiple fields it follows that spaces cannot simply be included in facts For example CLIPS gt Cclear CLIPS gt Cassert Canimal is waLrus lt Fact
104. fast The send function is the heart of OOP operation since it is the only proper way for objects to communicate According to the principle of object encapsulation one object should only be allowed to access another object s data by sending a message For example if someone wants to know what you had for breakfast they ll generally ask you i e send a message An impolite alternative would be to yank open your mouth and peer down your throat If the principle of object encapsulation is not followed any object can fool around with the private parts of other objects with potentially disastrous results One useful application of send is to print information about an object So far all the examples of objects that you have seen have no structure However just as deftemplate gives structure to a rule pattern the slots give an object structure For both 109 deftemplate and objects a slot is a named location in which data can be stored However unlike deftemplate slots objects obtain their slots from classes and classes use inheritance Thus the information in object slots can be effectively inherited by objects of subclasses An unbound slot is one that has no values assigned All slots must be bound As a simple example let s make an object with slots to hold personal information and then send messages to it The following commands will first set up the CLIPS environment with the appropriate constructs The slots named sound
105. fault dynamic 115 default dynamic 117 default facet 115 default strategy 33 defclass 87 defensive programming 141 deffacts 33 deffunction 71 defgenerics 152 defglobal 80 define template 53 definstances 34 106 defmethods 152 deftemplate default values 54 deftemplates 7 14 delete 107 delete 73 delete instance 145 delimited 8 dependencies 79 dependents 79 depth strategy 33 direct instances 97 120 direct subclass 89 direct superclass precedence list 88 distributed action 151 documentation 115 do for all instances 151 do for instance 151 double precision 10 dribble 35 dribble off 35 dribble on 35 dummy arguments 71 dynamic 41 dynamic binding 84 153 dynamic get 129 dynamic put 129 E edge 96 encapsulation 84 EOF 82 equal sign 67 eval 82 evaluation 82 evenp 80 exclusive facet 120 exit 2 expert systems 1 explicit method 153 explode 73 exponentiation 67 Extended Math 68 external functions 79 external address 8 F facet expression 116 facets 109 fact 11 fact identifier 3 fact address 8 44 fact index 3 45 fact list 1 facts 1 facts command 3 field 6 field constraint 64 find all instances 151 fires 24 float 8 10 102 floating point 10 floatp 80 format 72 forward declaration 154 forward declared 137 function 23 160 G generic 152 generic dispatch 154 generic
106. go crlf 37 The following shows how matches is used Enter the commands as shown Notice that watch facts is turned on This is a good idea when you are asserting facts manually since it gives you an opportunity to check the spelling of facts CLIPS gt Cwatch facts CLIPS gt assert work done gt f Q Cwork done lt Fact Q gt CLIPS gt Cmatches take a vacation Matches for Pattern 1 f 0 Matches for Pattern 2 None Matches for Pattern 3 None Partial matches for CEs 1 2 CE is conditional element None Partial matches for CEs 1 3 None Activations None CLIPS gt The fact with fact identifier f 0 matches the first pattern or conditional element in the rule and is reported by matches Given that a rule has N patterns the term partial matches refers to any set of matches of the first N 1 patterns with facts That is the partial matches begin with the first pattern in a rule and end with any pattern up to but not including the last Nth pattern As soon as one partial match cannot be made CLIPS does not check any further For example a rule with four patterns would have partial matches of the first and second patterns and also of the first second and third patterns If all N patterns match the rule will be activated Other Features Some additional commands are useful with deffacts For example the command list deffacts will list the names of currently loaded deffacts in CLIPS Another useful
107. gt CLIPS gt Cassert animal is walrus FALSE CLIPS gt Cassert animal is walrus FALSE CLIPS gt facts f Canimal is walrus For a total of 1 fact CLIPS gt Only one fact animal is walrus is asserted since CLIPS ignores white space and considers all these facts equivalent Thus CLIPS responds with a FALSE when you try to enter the last two duplicate facts CLIPS normally does not allow duplicate facts to be entered unless you change the set fact duplicate setting If you want to include spaces in a fact you must use double quotes For example CLIPS gt Cclear CLIPS gt assert Canimal is duck lt Fact Q gt CLIPS gt Cassert Canimal is duck lt Fact 1 gt CLIPS gt Cassert Canimal is duck lt Fact 2 gt CLIPS gt Cassert Canimal is duck lt Fact 3 gt CLIPS gt facts f Canimal is duck f 1 Canimal is duck f 2 Canimal is duck f 3 Canimal is duck For a total of 4 facts CLIPS gt Note that the spaces make each of these facts different to CLIPS although the meaning is the same to a person What if you want to include the double quotes in a field The correct way to put double quotes in a fact is with the backslash as the following example shows CLIPS gt Cclear CLIPS gt Cassert Csingle quote duck lt Fact Q gt CLIPS gt assert Cdouble quote duck lt Fact 1 gt CLIPS gt facts f 0 Csingle quote duck f 1 Cdouble
108. gt Cdescribe class INTEGER OK K K K K K K K K K K K K OK K K K K K K K K K K K K K KK K K K K K K K K K K K K K KK KK KK KK K K KKK KKK KKK KKK Abstract direct instances of this class cannot be created Direct Superclasses NUMBER Inheritance Precedence INTEGER NUMBER PRIMITIVE OBJECT Direct Subclasses 9K 2K K 2K K K 26 K K K OK K K K OK K K OK K K K K 2K K K K K K K K K K K K K K K K kK K OK K K OK K K OK OK K K OK K K OK OK KOK k kK k k k kK CLIPS gt The problem is that the INTEGER class has no handler for In fact it has no handlers at all since none are listed As you recall the USER class and its subclasses always have print delete and other handlers automatically defined by CLIPS If we want to send print messages to an object like 1 of the INTEGER class we ll have to make our own print handler Before writing this handler let s answer a question that you may have concerning how INTEGER can have instances Since INTEGER is an abstract class you may wonder how it can have instances such as 1 2 3 etc Although you cannot make direct instances of an abstract class you can make use of existing instances For the case of the predefined system class INTEGER all the integers up to the maximum allowed are available as objects Likewise strings and symbols are available for the system defined abstract classes STRING and SYMBOL and so forth for the other predefined classes Shown following is the definition of a handler
109. gt Cretract Q CLIPS gt facts CLIPS gt To retract a fact you must specify the fact index You can also retract multiple facts at once as shown by the following CLIPS gt Cclear CLIPS gt Cassert Canimal is duck lt Fact Q gt CLIPS gt Cassert Canimal sound quack lt Fact 1 gt CLIPS gt assert The duck says Quack lt Fact 2 gt CLIPS gt Cretract 2 CLIPS gt facts f 1 CanimaL sound quack For a total of 1 fact CLIPS gt To retract multiple facts just list the fact id numbers in the retract command You can just use retract to retract all the facts where the indicates all CLIPS gt Cclear CLIPS gt Cassert Canimal is duck lt Fact Q gt CLIPS gt assert Canimal sound quack lt Fact 1 gt CLIPS gt assert The duck says Quack lt Fact 2 gt CLIPS gt facts 18 f 0 Canimal is duck f 1 Canimal sound quack f 2 The duck says Quack For a total of 3 facts CLIPS gt Cretract CLIPS gt facts CLIPS gt Watch that Fact CLIPS provides several commands to help you debug programs One command allows you to continuously watch facts being asserted and retracted This is more convenient than having to type in a facts command over and over again and trying to figure out what s changed in the fact list To start watching facts enter the command watch facts as shown in the following example CLIPS gt Cclear CLIPS gt Cwatch facts C
110. gument nor can it be bound to a different value The active instance is the instance to which the message was sent In our example all the predefined classes such as NUMBER INTEGER and FLOAT are all subclasses of the PRIMITIVE class This is in contrast to USER which is the other main subclass of OBJECT As another example lets write a handler to concatenate strings symbols or both CLIPS gt Cdefmessage handler LEXEME arg Argument of handler Csym cat self arg Function concatenation of handler CLIPS gt send Dorky_ Duck SYMBOL STRING Dorky_Duck SYMBOL result CLIPS gt Notice that the handler is defined for the LEXEME class since that is a superclass of both SYMBOL and STRING In this case the handler returns a SYMBOL since sym cat is used This example also illustrates why brackets may be necessary in a Send As shown in this example the message goes to the SYMBOL Dorky_ without brackets With brackets the message goes to an object Dorky_ of a user defined class Here we assume that Dorky_ could be an object of a user defined class such as DUCK 125 Make Em Pay Through the Nose The real utility of handlers is with subclasses of USER since you can define instances of these classes To see how handlers work in this case let s first set up the environment as follows CLIPS gt Cclear CLIPS gt Cdefclass DUCKLING Cis a USER Cslot sound default quack Cslot age visibility pub
111. h the gt function because it expects NUMBER types as arguments However it s easy to define a defgeneric which will overload the gt to accept STRING types as well as NUMBER types For example if the arguments of gt are of type STRING the defgeneric will do a string comparison letter by letter starting from the left until the ASCII codes differ In contrast if the arguments of gt are of type NUMBER the system compares the sign and magnitude of the numbers The user defined gt for STRING types is an explicit method while a system defined or user defined external function such as gt for NUMBER type is an implicit method The technique of overloading a function name so that the method which implements it is not known until run time is another example of dynamic binding Any object reference of name or address may be bound at run time in CLIPS to functions through dynamic binding also Some languages such as Ada have a more restrictive type of overloading in which the function name must be known at compile time rather than at run time The run time dynamic binding is the least restrictive since methods can be created during execution by the build statement However you should be careful in using build since dynamically creating constructs is often hard to debug Also the resulting code may be difficult to verify and validate since you ll have to stop execution to examine the code Dynamic binding is a characteristics of
112. have point type slots In contrast generalization builds up using new types of slots defined for each class Class POINT has a point type slot LINE has two point type slots TRIANGLE has three line type slots QUADRILATERAL has four line type slots and so on Generalization is good for synthesis which means a building up The opposite of synthesis is analysis which means taking apart or a simplification The model for analysis is specialization Fig 10 1 illustrates one inheritance scheme for polygons in which classes are built by inheritance In this case the link between classes would be is made of Thus a LINE is made of POINT A POLYGON is made of LINE and so forth 136 POLYGON IRREGULAR REGULAR POLYGON POLYGON EQUILATERAL TRIANGLE SOU ARE Fig 10 1 Polygon Hierarchy Showing a Few Regular Polygon Classes EQUILATERAL PENTAGON Links such as is made of can be simulated in CLIPS by appropriate slot definitions even though only is a links are supported in Version 6 0 As an example of generalization let s build up a LINE class as a generalization of a POINT class The POINT class will provide instances that have a position In order to make this example realistic we ll assume an arbitrary number of dimensions by defining the position as multiple Thus a one dimensional point will have one value in the position slot a two dimensional point will have two values and so forth The definition of the
113. ialized since non definstances will simply be deleted Also slot overrides may be used in initialize instance as the last example shows Two predicate functions are designed for use with the access facets Both these predicate functions return an error message if the specified slot or instance does not exist The slot writablep is a predicate function which returns TRUE if a slot is writable and FALSE if it is not The slot initablep predicate function returns TRUE if the slot is initializable and FALSE if it is not The term initializable means that it is not read only The inherit facet which is the default specifies that indirect instances of a class may inherit this slot from the class As you recall the indirect instances of a class are the instances of its subclasses while the direct instances are those defined specifically for the class The indirect instances of a class are direct instances of the subclasses in which they are defined For example Dorky_Duck is a direct instance of DUCK and an indirect instance of USER which is the superclass of DUCK The no inherit facet specifies that only a direct instance has the class slot It s important to realize that the no inherit facet only prohibits inheritance from the class and not from its superclasses This means that an instance may still inherit from superclasses of the no inherit class The composite facet facet states that facets which are not explicitly defined in the highest
114. ikely candidates CLIPS gt Cunwatch all CLIPS gt Creset CLIPS gt Crun The ideal duck is Dicky List is Dicky Dorky Dinky CLIPS gt Notice the assertion change list yes in the move to front rule Without this assertion the print list rule would always fire on the original list This assertion is an example of a control fact made to control the firing of another rule Control facts are very important in controlling the activation of certain rules and you should study this example carefully to understand why it s used Another method of control is modules as discussed in the CLIPS Reference Manual The move to front rule removes the old list and asserts the new list If the old list was not retracted two activations would be on the agenda for the print list rule but only one would fire Only one will fire because the print list rule removes the control fact required for the other activation of the same rule You would not know in advance which one would fire so the old list might be printed instead of the new list Chapter 5 Doing It Up In Style Style today gone tomorrow In this chapter you will learn about a keyword called deftemplate which stands for define template This feature can aid you in writing rules whose patterns have a well defined structure Mr Wonderful Deftemplate is analogous to a record structure in Pascal That is the deftemplate defines a group of related fields in a pattern similar to the way
115. in which a Pascal record is a group of related data A deftemplate is a list of named fields called slots Deftemplate allows access by name rather than by specifying the order of fields Deftemplate contributes to good style in expert systems programs and is a valuable tool of software engineering A slot is a named single slot or multislot A single slot or simply slot contains exactly one field while a multislot contains zero or more fields Any number of single or multislot slots may be used in a deftemplate To write a slot give the field name attribute followed by the field value Note that a multislot slot with one value is strictly not the same as a single slot slot As an analogy think of a cupboard the multislot that may contain dishes A cupboard with one dish is not the same as a dish single slot However the value of a single slot slot or variable may match a multislot slot or multislot variable that has one field As an example of a deftemplate relation consider the attributes of a duck who might be considered a good matrimonial prospect Attributes Value name Dopey Wonderful assets rich age 99 A deftemplate may be defined for the relation prospect as follows where white space and comments are used for readability and explanation Cdeftemplate prospect j name of deftemplate relation vital information o0ptional comment in quotes slot name name of field Ctype STRING type of field Cdefault DERIVE
116. ince the modify has retracted it and asserted f 2 CLIPS gt facts f Cinitial fact f 2 prospect name Dopey Wonderful assets poor Cage 99 For a total of 2 facts CLIPS gt The make bad buys rule is activated by a rich prospect as specified by the assets slot This rule changes the assets to poor using the modify action Notice that the slot assets can be accessed by name Without a deftemplate it would be necessary to enumerate all the fields by single variables or by using a wildcard which is less efficient The purpose of the poor prospect rule is simply to print out the poor prospects thus Seia demonstrating that the make bad investments rule did indeed modify the assets Chapter 6 Being Functional Functionality is the inverse of style In this chapter you will learn more powerful functions for matching patterns and some that are very useful with multifield variables You also will learn how numeric calculations are done Not My Constraint Let s reconsider the problem of designing an expert system to help a robot cross a street One rule that you would have follows Cdefrule green Light Clight green Cprintout t Walk crlf Another rule would cover the case of a red light Cdefrule red light Clight red Cprintout t Don t walk crlf A third rule would cover the case in which a walk sign said not to walk This would take precedence over a green light Cdefrule walk sign Cwalk sign says dont
117. ines the methods order of listing Higher precedence methods are listed before lower precedence methods The highest precedence method will also be tried first by CLIPS A shadowed method is one in which one method must be called by another The process by which CLIPS picks the method with highest precedence is called the generic dispatch For more information see the CLIPS Reference Manual 155 Support Information Questions and Information Inquiries related to the use or installation of CLIPS can be sent via electronic mail to clips ghg net The URL for the CLIPS Web page is http www ghg net clips CLIPS html Usenet users can also find information and post questions about CLIPS to the comp ai shells news group The CLIPS Frequently Asked Questions FAQ list can be found at the URL http www ghg net clips CLIPS FAQ CLIPS List Server An electronic conferencing facility soonsored by Distributed Computing Systems http Awww discomsys com is also available to CLIPS users Subscribers to this facility may send questions observations answers editorials etc in the form of electronic mail to the conference All subscribers will have a copy of these messages reflected back to them at their respective electronic mail addresses To subscribe send a single line message to clips equest discomsys com containing the word subscribe The subject field is ignored but the address found in the Reply Reply to or
118. ing in the multislot version so CLIPS treats the name as two independent fields Dopey and Wonderful What s in a Name Deftemplate greatly simplifies accessing a specific field in a pattern because the desired field can be identified by its slot name The modify action can be used to retract and assert a new fact in one action by specifying one or more template slots to be modified As an example consider the following rules which show what happens when duck bachelor Dopey Wonderful loses all his fish buying Donald Duck posters and banana fishsplits for his new duckette Dixie CLIPS gt Cundefrule CLIPS gt Cdefrule make bad buys prospect lt prospect Cname name Cassets rich Cage months gt Cprintout t Prospect name crlf Note not name eich crlf months months old crlf crlf modify prospect assets poor CLIPS gt Cdefrule poor prospect prospect lt prospect Cname name Cassets poor Cage months gt Cprintout t Ex prospect name crlf Note not name poor crlf months months old crlf crlf CLIPS gt Cdeffacts duck bachelor prospect name Dopey Wonderful Cassets rich Cage 99 CLIPS gt Creset CLIPS gt Crun Prospect Dopey Wonderful rich 99 months old Ex prospect Dopey Wonderful poor 99 months old CLIPS gt If you do a facts command as follows you ll see that the f 1 fact corresponding to prospect assets rich age 99 name Dopey Wonderful is gone s
119. ingless The importance of printout is in its side effect of output In general functions may have nested arguments if appropriate to your desired effect Before a file can be accessed for reading or writing it must be opened using the open function The number of files that can be opened at once is dependent on your operating system and hardware When you no longer need to access a file you should close it with the close function Unless a file is closed there is no guarantee that the information written to it will be saved The logical name of a file is how CLIPS identifies the file The logical name is a global name by which CLIPS knows this file in al rules Although the logical name could be identical to the filename you may want to use something different Another advantage of a logical name is that you can easily substitute a different filename without making major program changes The function to read data from a file is the familiar read or readline The only new 82 thing that you have to do is to specify the logical name from which to read as the argument of read or readline To read more than one field you must use a loop Even with readline a loop is necessary to read multiple lines A loop can be written by having one rule trigger another or with a while loop The loop should not try to read past the end of file or the operating system will issue an error message To help prevent this CLIPS returns an EO
120. inish If you accidentally enter a semicolon in top level just type in a closing parenthesis and carriage return CLIPS will respond with an error message and the CLIPS prompt will reappear This is one of the few approved occasions in life in which it s necessary to do something wrong to get something right As you read through this manual you will learn the special meanings of the characters above With the exception of the amp I and you may use the others as described However it may be confusing to someone reading your program and trying to understand what the program is doing In general it s best to avoid using these characters in symbols unless you have some good reason for using them The following are examples of symbols duck duck1 duck_soup duck soup duck1 1_soup soup d 7 A The second type of field is the string A string must begin and end with double quotes The double quotes are part of the field Zero or more characters of any kind can appear between the double quotes Some examples of strings follow duck duck1 duck soup duck soup duck soup is good The third and fourth types of field are numeric fields A field which represents a 10 number which can be either an integer or floating point type field A floating point type is commonly referred to simply as a float All numbers in CLIPS are treated as long integers or double precision floats Numbers without a decimal poi
121. is similar to find instance except that it performs a single distributed action when the query is satisfied The do for all instances function is similar to the do for instance except that it performs an action for every instance set that satisfies the query Design Decisions In contrast to rules which are only activated when their patterns are satisfied 152 deffunctions are explicitly called and then executed Just because a rule is activated does not mean it will be executed Deffunctions are completely procedural in nature because once called by name their code is executed in a procedural manner statement by statement Also no pattern matching involving constraints is used ina deffunction to decide if its actions should be executed Instead any arguments that match the number expected by the deffunction argument list will satisfy the deffunction and cause its actions to be executed The basic idea of deffunctions as named procedural code is carried to a much greater degree with defgenerics and the defmethods that describe their implementation A defgeneric is like a deffunction but much more powerful because it can do different tasks depending on its argument constraints and types The ability of a generic function to perform different actions depending on the classes of its arguments is called overloading the function name By proper use of operator overloading it s possible to write code that is more readable and reusable For
122. ist is ordered means that the position in the list is significant A multifield is a sequence of fields each of which may have a value The examples of duck Brian and Brian duck are multifield facts If a field has no value the special symbol nil which means nothing may be used for an empty field as a placeholder For example duck nil would mean that the killer duck bagged no trophies today Note that the nilis necessary to indicate a placeholder even if it has no value For example think of a field as analogous to a mailbox There s a big difference between an 8 empty mailbox and no mailbox at all Without the nil the fact becomes a single field fact duck If a rule depends on two fields it will not work with only one field as you ll see later There are a number of different types of fields available float integer symbol string external address fact address instance name and instance address The type of each field is determined by the type of value stored in the field In an unnamed field the type is determined implicitly by what type you put in the field In deftemplates you can explicitity declare the type of value that a field can contain The use of explicit types enforces the concepts of software engineering which is a discipline of programming to produce quality software A symbol is one type of field that starts with a printable ASCII character and is followed optionally by zero or more printable char
123. ition CLIPS gt send Triangle1 find Line 1 1 2 CLIPS gt 145 Notice that the create function was used to combine both point values into a single multifield value 1 0 0 2 which was then returned Other Features A number of other functions are useful with handlers Some of these follow Function Meaning undefmessage handler Deletes a specified handler list defmessage handlers Lists the handlers delete instance Operates on the active instance message handler existp Returns TRUE if handler exists else FALSE The grouping functions group COOL items into a multifield variable Function Meaning get defmessage handler list Groups the class names message names and types direct or inherited class superclasses Groups all superclass names direct or inherited class subclasses Groups all subclass names direct or inherited class slots Groups all slot names explicitly defined or inherited slot existp Returns TRUE if class slot exists else FALSE slot facets Groups the specified slot facet values of a class slot sources Groups the slot names of classes which contribute to a slot in the specified class The preview send function is useful in debugging since it displays the sequence of all handlers that potentially may be involved in processing a message The reason for the term potentially is that shadowed handlers will not be executed if the shadower does not use call next handler or override next handl
124. l as asserting the facts in deffacts statements The reset has a further advantage compared to a clear command in that reset doesn t get rid of all the rules The reset leaves your rules intact Like clear it removes all activated rules from the agenda and also removes all old facts from the fact list Giving a reset command is a recommended way to start off program execution especially if the program has been run before and the fact list is cluttered with old facts In summary the reset does three things for facts 1 It removes existing facts from the fact list which may remove activated rules from the agenda 2 It asserts initial fact 3 It asserts facts from existing deffacts statements Actually the reset also does corresponding operations on objects It deletes instances creates initial object and asserts instances from definstances 35 Selective Elimination The undeffacts command excises a deffacts from asserting facts by eliminating the deffacts from memory For example CLIPS gt Cundeffacts walk CLIPS gt Creset CLIPS gt facts f Cinitial fact For a total of 1 fact CLIPS gt This example demonstrates how the deffacts walk has been excised To restore a deffacts statement after an undeffacts command you must enter the deffacts statement again You can even get rid of initial fact with undeffacts In addition to facts CLIPS also allows you to eliminate rules selective
125. lass architecture and how messages will be sent between objects Thus actions in customary procedural programs are performed explicitly while the actions are performed implicitlty in OOP In either case the end result is the same However the OOP system can be more easily verified validated and maintained The proper use of classes is summarized in the following three rules Rules of Class Etiquette 1 The class hierarchy should be in specialized logical increments using is a links 2 A class is unnecessary if it has only one instance 3 Aclass should not be named for an instance and vice versa The first rule discourages the creation of a single class for your application If a single class is adequate then you probably don t need OOP By creating classes in increments you can more easily verify validate and maintain your code In addition incremental class hierarchies can be easily put in class libraries to greatly facilitate the creation of new code This concept of class libraries is analogous to subroutine libraries for actions Only is a links can be used since this is the only relationship that Version 12 6 0 of CLIPS supports The second rule encourages the idea that classes are intended as a template to produce multiple objects of the same kind Of course you can start out with zero or one instance However if you ll never need more than one instance in a class you should consider modifying its superclass to acco
126. lic CLIPS gt Cdefclass DUCK is a DUCKLING Cslot sound default quack CLIPS gt Cdefinstances DUCKY_OBJECTS Dorky_Duck of DUCK Cage 2 CDinky_Duck of DUCKLING Cage 1 CLIPS gt Creset CLIPS gt As a simple example let s write a handler that will print out the slots of the active instance We can make use of the ppinstance function to print out the slots of the active instance This function does not return a value and is used only for its side effect of printing to the standard device Also it can only be used from within a handler since only there is the active instance known Shown following is a USER defined handler called print slots that prints out the slots of the active instance using ppinstance CLIPS gt Cdefmessage handler USER print sLlots Cppinstance CLIPS gt Csend Dorky_Duck print slots Dorky_Duck of DUCK Cage 2 Csound quack CLIPS gt Although the handler could be defined just for DUCK in this case a handler defined for USER will be called for all subclasses of USER not just DUCKLING and DUCK Thus the handler print slots will work for all subclasses that we may define of USER 126 Of course it s possible to get carried away and define all your message handlers as USER handlers However it s good style and improves efficiency to define handlers as close as possible to the class or classes for which they are intended Efficiency is improved because CLIPS does not have to
127. ltiple inheritance there appears to be multiple bosses of equal authority which are the direct superclasses For the case of classes arranged in a tree i e single inheritance the inheritance is simply all classes along the inheritance path back to OBJECT The inheritance path in a tree is just the shortest path from a class back to OBJECT This concept of inheritance also applies to a subgraph of a graph which is a tree For example the inheritance precedence of the subgraph UPPIE SUPPIE MUPPIE YUPPIE and PUPPIE is a tree since each of them has only one parent Thus the inheritance precedence of each of them is the shortest sequence of links back to OBJECT For example the inheritance precedence of PUPPIE is PUPPIE YUPPIE UPPIE USER and OBJECT Fig 1 8 is another example of a graph Notice that some nodes such as rhombus have more than one parent QUADRILATERAL TRAPEZOID KITE 4 EN PARALLELOGRAM ISOSCELES N TRAPEZOID RHOMBUS RECTANGLE T I Fig 1 8 The Hoa Graph For simplicity we will only discuss single inheritance in this book For more details on multiple inheritance see the CLIPS Reference Manual 99 Other Features Some other functions useful with classes are shown following Function Meaning ppdefclass Pretty print the defclass internal structure undefclass Eliminate class describe class Additional information about classes class abstractp Predicate function returns TRUE if the class is ab
128. ly by using the undefrule Watch It You can watch rules firing and watch activations on the agenda The watch statistics prints information about the number of rules fired run time rules per second mean number of facts maximum number of facts mean number of activations and maximum number of activations The statistics information may be useful in tuning up an expert system to optimize its speed Another command called watch compilations shows information when rules are being loaded The watch all command will watch everything Printing of watch information to the screen or to disk with the dribble command will slow down your program somewhat because CLIPS uses more time to print or to save to disk The dribble on command will store everything entered in the Dialog Window to a disk file until the dribble off command is entered This is convenient in providing a permanent record of everything that happens These commands are as follows Cdribble on lt filename gt Cdribble off lt filename gt Another useful debugging command is run which takes an optional argument of the Ie number of rule firings For example a run 21 command would tell CLIPS to run the program and then stop after 21 rule firings A run 1 command allows you to step through a program firing one rule at a time The step command is equivalent to run 1 Just like many other programming languages CLIPS also gives you the capability of setting breakpoint
129. mmand You ll see just the original f 1 duck This shows that CLIPS will not accept a duplicate entry of a fact However there is an override command set fact duplication which will allow duplicate fact entry Of course you can put in other different facts For example assert a quack fact and then issue a facts command You ll see CLIPS gt Cassert Cquack lt Fact 2 gt CLIPS gt facts f Cinitial fact f 1 Cduck f 2 Cquack For a total of 3 facts CLIPS gt Notice that the quack fact is now in the fact list Facts may be removed or retracted When a fact is retracted the other facts do not have their indices changed and so there may be missing fact indices As an analogy when a football player leaves a team and is not replaced the jersey numbers of the other players are not all adjusted because of the missing number unless they really hate the guy s guts and want to forget he ever played for them Clearing Up the Facts The clear command removes all facts from memory as shown by the following CLIPS gt facts f Cinitial fact f 1 Cduck f 2 Cquack For a total of 3 facts CLIPS gt Cclear CLIPS gt and all facts in the fact list will be removed The clear command essentially restores CLIPS to its original startup state It clears the memory of CLIPS and resets the fact identifier to zero To see this assert animal is duck then check the fact list Notice that animal is duck has a f
130. mmand in this example should be CLIPS gt facts f Cduck For a total of 1 fact CLIPS gt The term f Ois the fact identifier assigned to the fact by CLIPS Every fact inserted into the fact list is assigned a unique fact identifier starting with the letter f and followed by an integer called the fact index On starting up CLIPS and after certain 4 commands such as clear and reset to be discussed in more detail later the fact index will be set to zero and then incremented by one as each new fact is asserted The reset command will also insert a fact initial fact as f 0 This fact is often used for convenience to initially activate rules Shown following is what happens when a reset is done first CLIPS gt Creset CLIPS gt facts f Cinitial fact For a total of 1 fact CLIPS gt Cassert Cduck lt Fact 1 gt CLIPS gt facts f Cinitial fact f 1 Cduck For a total of 2 facts CLIPS gt Notice that the fact index is lt Fact 1 gt after the duck fact is asserted because there are now two facts in working memory and the duck has index 1 What happens if you try to put a second duck into the fact list Let s try it and see Assert a new duck then issue a facts command as follows CLIPS gt Cassert Cduck FALSE CLIPS gt facts f Cinitial fact f 1 Cduck For a total of 2 facts CLIPS gt The FALSE message is returned by CLIPS to indicate that it was not possible to perform this co
131. modate the instance rather than defining a new subclass If all your classes only have one instance it is probable that your application is simply not well suited to OOP and that coding in a procedural language may be best The third rule means that classes should not be named after instances and vice versa to eliminate confusion Other Features 113 There are a number of useful predicate functions for slots If you use these predicate functions to test for appropriate values to functions your program will be more robust against crashes In general if a function does not return TRUE it returns FALSE Function class slot existp slot existp slot boundp instance address instance name instancep instance addressp instance namep instance existp list definstances ppdefinstances watch instances unwatch instances save instances load instances undefinstances Meaning Returns TRUE if the class slot exists Returns TRUE if the instance slot exists Returns TRUE if the specified slot has a value Returns the machine address at which the specified instance is stored Returns the name given an address and vice versa Returns TRUE if its argument is an instance Returns TRUE if its argument is an instance address Returns TRUE if its argument is an instance name Returns TRUE if instance exists Lists all the definstances Pretty prints the definstance Allows you to watch instances being created and deleted Turns off watching
132. n a slot with an empty multifield value and an unbound slot If you think of an empty multifield value as analogous to an empty bus you can see there is a difference between no people unbound slot and a bus with no people empty multifield value A storage facet defines one of two places that a slot value is stored 1 in an instance or 2 in the class A slot value stored in the instance is called local because the value is only known to the instance Thus different instances may exist that have different local slot values In contrast a slot value stored in a class is called shared because it is the same for all instances of the class A local value is specified by the local facet which is the default for a slot A shared value is specified by the shared facet and all instances with this slot type will have their slot value automatically changed if one changes An access facet defines one of three types of access for a slot as summarized in the following table 120 Another way to set the instance values is with the initialize instance function An initialize instance can be called at any time to reset the default values and retain values in non default slots A reset can be thought of as a cold initialization since all values in non default slots are cleared while a initializeinstance can be considered a warm nitialization since non default values are retained Of course only definstances can be cold init
133. namic binding These terms will be explained in detail as you read through this book CLIPS supports all five of these features The term abstract means that we are not concerned with nonessential details An abstract description of a complex system is a simplified description that concentrates on relevant information for a specific purpose Thus the system is represented by a simpler easier to understand model As a familiar example when certain people drive cars they utilize an abstract model of driving that consists of two items the steering wheel and the accelerator That is these people are not concerned with the hundreds of 85 components that make up an automobile nor the theory of internal combustion engines traffic laws and so forth Knowing only how to use the steering wheel and accelerator is their abstract model of driving One of the five fundamental features of OOP is inheritance Classes are arranged in a hierarchy with the most general classes at the top and the more specialized classes below This allows new classes to be easily defined as specialized refinements or modifications of existing classes The use of inheritance can greatly speed up software development and increase reliability because new software does not need to be created from scratch each time a new program is made OOP makes it easy to utilize reusable code OOP programmers often make use of object libraries consisting of hundreds or thousands of o
134. nd then assert this bound variable on the RHS For example enter Cdefrule make quack Cduck sound sound Cassert Csound is sound Now assert duck sound quack then run the program Check the facts and you ll see that the rule has produced sound is quack because the variable sound was bound to quack Of course you also can use a variable more than once For example enter the following Be sure to do a reset and assert duck sound quack again Cdefrule make quack Cduck sound sound Cassert Csound is sound sound When the rule fires it will produce sound is quack quack since the variable sound is used twice What the Duck Said Variables also are used commonly in printing output as in CdefrulLe make quack Cduck sound sound Cprintout t The duck said sound crlf Do a reset enter this rule and assert the fact and then run to find out what the duck said How would you modify the rule to put double quotes around quack in the output More than one variable may be used in a pattern as the following example shows CLIPS gt Cclear CLIPS gt Cdefrule whodunit Cduckshoot hunter who gt Cprintout t hunter shot who crlf CLIPS gt Creset CLIPS gt assert Cduckshoot Brian duck lt Fact 1 gt CLIPS gt Crun Brian shot duck Duck dinner tonight CLIPS gt assert Cduckshoot duck Brian lt Fact 2 gt CLIPS gt Crun duck shot Brian Brian dinner tonight CLIPS gt Cassert C
135. nding material in the CLIPS Reference Manual as you cover each chapter in the CLIPS User s Guide Like any other programming language you will only learn programming in CLIPS by writing programs in it To really learn expert system programming you should pick a problem of interest and write it in CLIPS Acknowledgments greatly appreciate the advice and reviews of this book by many people Thanks to Gary Riley Chris Culbert Brian Donnell Bryan Dulock Steven Lewis Ann Baker Steve Mueller Stephen Baudendistel Yen Huynh Ted Leibfried Robert Allen Jim Wescott Marsha Renals Pratibha Boloor Terry Feagin and Jack Aldridge Special thanks to Bob Savely for supporting the development of CLIPS Chapter 1 Just the Facts If you ignore the facts you ll never worry about being wrong This chapter introduces the basic concepts of an expert system You ll see how to insert and remove facts in CLIPS If you are using a Macintosh or the Windows version of CLIPS for the IBM PC or compatible you can select most commands by the mouse instead of typing them in The arrow keys on the keyboard will also move the cursor and allow selection of menu items Introduction CLIPS is a type of computer language designed for writing applications called expert systems An expert system is a program which is specifically intended to model human expertise or knowledge In contrast common programs such as payroll programs word processors spreadsheets
136. normally are separated by white space consisting of one or more spaces tabs carriage returns or linefeeds For example enter the following examples as shown and you ll see that each stored fact is the same CLIPS gt Cclear CLIPS gt assert The duck says Quack a lt Fact Q gt CLIPS gt Cfacts f 0 The duck says Quack For a total of 1 fact CLIPS gt Cclear CLIPS gt Cassert The duck says Quack lt Fact gt CLIPS gt facts f 0 The duck says Quack For a total of 1 fact CLIPS gt Carriage returns may also be used to improve readability In the following example a carriage return is typed after every field and the asserted fact is the same as before when the fact was entered on one line CLIPS gt Cclear CLIPS gt Cassert The duck says Quack lt Fact gt CLIPS gt facts f 0 The duck says Quack For a total of 1 fact CLIPS gt However be careful if you insert a carriage return inside of a string as the following example shows CLIPS gt Cassert The duck says CLIPS gt facts f 0 The duck says Quack f 1 The duck says Quack For a total of 2 facts 3 CLIPS gt As you can see the carriage return embedded in the double quotes was output with the string to put the closing double quote on the next line This is important because CLIPS considers fact f 0 as distinct from fact f 1 Notice also that CLIPS preserved the uppercase and low
137. nt are treated as integers unless they are outside integer range The range is machine dependent on the number of bits N used to represent the integer as follows 2N 1 2N 1 1 For 32 bit long integers this corresponds to a range of numbers 2 147 483 648 2 147 483 647 As some examples of numbers assert the following data where the last number is in exponential notation and uses the e or E for the power of ten CLIPS gt Cclear CLIPS gt facts CLIPS gt Cassert Cnumber 1 lt Fact Q gt CLIPS gt Cassert x 1 5 lt Fact 1 gt CLIPS gt assert y 1 lt Fact 2 gt CLIPS gt Cassert z 65 lt Fact 3 gt CLIPS gt Cassert Cdistance 3 5e5 lt Fact 4 gt CLIPS gt Cassert Ccoordinates 1 2 3 lt Fact 5 gt CLIPS gt Cassert Ccoordinates 1 3 2 lt Fact 6 gt CLIPS gt facts f Cnumber 1 x 1 5 Cy 1 Cz 65 Cdistance 350000 0 Ccoordinates 1 2 3 Ccoordinates 1 3 2 or a total of 7 facts OuUBRWNE f f f f f f F CLIPS gt As you can see CLIPS prints the number entered in exponential notation as 350000 0 because it converts from power of ten format to floating point if the number is small enough Notice that each number must start with a symbol such as number x y etc Before CLIPS version 6 0 it was possible to enter only a number as a fact However now a symbol is required as the first field Also certain reserved words used by CLIPS cannot be us
138. nterpreted by CLIPS Two other useful commands allow you to save and load facts using a file These are save facts and load facts The save facts will save all the facts in the fact list to a file while load facts will load in the facts from a file into the fact list The batch command allows you to execute commands from a file as if they were typed in at the top level Another useful command provides an interface to your operating system The system command allows the execution of operating system commands or executables within CLIPS For more information on all these topics see the CLIPS Reference Manual Chapter 3 Adding Details It s not the big picture that is the problem it s the details In the first two chapters you learned the fundamentals of CLIPS Now you will see how to build on that foundation to create more powerful programs Stop And Go Until now you ve only seen the simplest type of program consisting of just one rule However expert systems consisting of only one rule are not very useful Practical expert systems may consist of hundreds or thousands of rules Let s now take a look at an application requiring multiple rules Suppose you wanted to write an expert system to determine how a mobile robot should respond to a traffic light It is best to write this type of problem using multiple rules For example the rules for the red and green light situations can be written as follows Cdefrule red light Clight
139. of DUCKLING Dix1e_Duck of DUCK For a total of 4 instances CLIPS gt The Power of Belief The self parameter is useful because it can be used to read a slot value in the form seLlf lt sLot name gt However it cannot be used to write a slot value The self notation is more efficient than sending messages but can only be used from within a handler on its active instance In contrast the dynamic get and dynamic put functions can be used from within a handler to read and write a slot value of the active instance Although you can use messages from inside a handler such as the following it s not efficient Csend self dynamic get lt slot gt Csend self dynamic put lt slot gt As an example of dynamic get let s change our example as follows CLIPS gt Cdefmessage handler USER print age primary Cprintout t Starting to print crlf Age Cdynamic get age crlf Finished printing crlf CLIPS gt Csend Dorky_Duck print age Starting to print 130 Age 2 Finished printing CLIPS gt The self age can only be used in a class and its subclasses which inherit the slot age The self lt slot name gt is evaluated in a static way through inheritance This means that if a subclass redefines a slot a superclass message handler will fail if it tries to directly access the slot using self lt slot name gt In contrast the dynamic get and dynamic put can be used by super
140. of a field as a picture frame The frame can hold a picture perhaps a picture of your pet duck For those of you who are curious what a picture of a ay ae quack looks like it could be 1 a photo of an oscilloscope trace of a duck saying quack where the signal input comes from a microphone or 2 for those of you who are more scientifically inclined a Fast Fourier Transform of the quack signal or 3 a TV huckster selling a miracle cure for wrinkles losing weight etc Named placeholders are only used with deftemplates described in more detail in chapter 5 The duck fact has a single unnamed placeholder for the value duck This is an example of a single field fact A field is a placeholer for a value As an analogy to fields think of dishes fields for holding food values The order of unnamed fields is significant For example if a fact was defined Brian duck and interpreted by a rule as the hunter Brian shot a duck then the fact Cduck Brian would mean that the hunter duck shot a Brian In contrast the order of named fields is not significant as you ll see later with deftemplate Actually it is good software engineering to start the fact with a relation that describes the fields A better fact would be hunter game duck Brian to imply that the first field is the hunter and the second field is the game A few definitions are now necessary A listis a group of items with no implied order Saying that a l
141. of the matching patterns is the activation If either the rule or the pattern entity or both change the activation is removed An activation may also be removed by a command or an action of another rule that fired before and removed the conditions necessary for activation The Inference Engine sorts the activations according to their salience This sorting process is called conflict resolution because it eliminates the conflict of deciding which rule should fired next CLIPS executes the RHS of the rule with the highest salience on the agenda and removes the activation This execution is called firing the rule in analogy with the firing of a neuron A neuron emits a voltage pulse when an appropriate stimulus is applied After a neuron fires it undergoes refraction and cannot fire again for a certain period of time Without refraction neurons would just keep firing over and over again on exactly the same stimulus Without refraction expert systems always would be caught in trivial loops That is as soon as a rule fired it would keep firing on that same fact over and over again In the real world the stimulus that caused the firing eventually would disappear For example a real duck might swim away or get a job in the movies However in the computer world once data is stored it stays there until explicitly removed or the power is turned off The following example shows activations and firing of a rule Notice that the watch commands are use
142. ology for trying to solve a problem Today there are many programming paradigms such as OOP procedural rule based and connectionist The term artificial neural systems is a modern synonym for the older term connectionist Traditional programming is procedural because it emphasizes algorithms or procedures in solving problems Many languages have been developed to support this procedural paradigm such as Pascal C Ada FORTRAN and BASIC These languages have also been adapted for object oriented design OOD by either adding extensions or imposing a design methodology on the programmers In contrast new languages have been developed to provide OOP which is not the same as OOD You can do OOD in any language even assembly language CLIPS provides three paradigms rules objects and procedures You will learn more about the objects in the CLIPS Object Oriented Language COOL which is integrated 84 with the rule and procedural based paradigms of CLIPS CLIPS supports the procedural paradigm through generic functions deffunctions and user defined external functions Depending on the application you can use rules objects procedures or a combination Rather than imposing a single paradigm on the user our philosophy is that a variety of specialized tools a multi paradigm approach is better than trying to force everyone to use a single general purpose tool As an analogy while you could use a hammer and nails for fastening every
143. owto be Objective ysi sr i cheesy aes eee eke ceca eeed aera Ava ec aed 83 The Glass Stuffs sect ees ie a eared a earn Bae a ns vei eas 84 How the UPPIES Get TheirS cccccccceeeeeeeeeee cece aa eaeeeeeeeeeeeeaaaaaaeeeeeeeeeaaaaaaneeeeeeees 86 The Illegitimate YUKKIE c cceececeeeeeee cece eeeeeeeeee TAEA PINATAAS AEAN 91 SHOW ME pran inen nan N a a AN e cord aA N N A cts 93 Other FeatUreS eina aaea aaa aaa aa ee aaa a aa aaa araa AEren 99 Chapter 9 Meaningful Messages 2 ececeeeeeeeeeeeeeeeseseeeseseeeeeeeeeeneees 101 The Birds and the B S c cceeeecceeeeeeceeeneeeeeeeee ee eeeeee ee AAN AASA EA VA Sa 101 DOrky DUCK Aasra a Sessa a N aa aa cou Ga ba cuadne V awards N a eal 103 Much Ado About Instances merecidu reeet rie ainke ie riena ies eae a sivas 106 The Disappearing DUSK cresian ar Aar esana ntn aare sevens EE AO ENEA AER En ANE ENKAR 107 What Did You Have For Breakfast ssuesssseerrssssrrrreernrnnennrnnsnnnnnnantnunnnnnunnnnnnnnnnene 108 Class Etiquette 1 svete he ie a aa seed an es eee een Ns eee 111 Other Features orendi kegen eeni boone ves ia xed E Naa aai ote vee Giay cen EPEE E Saet 113 Chapter 10 Fascinating Facets scececeeeeeeeeeeeeeeeseseeeeeseseeeeeseeneees 115 A Slot N med Defa lti nsns oea enee iona veils nithesatyat ceived becsan iv DEO te deeiess 115 Cardinal Properties 00ccceeeeeee eee ence eeeeeeee ee aaa atten sees esse aaa eeeeeeeeaeaaaa
144. pattern is animal is duck where the fields are animal is and duck CLIPS attempts to match the patterns of rules against facts in 24 the fact list If all the patterns of a rule match facts the rule is activated and put on the agenda The agenda is a collection of activations which are those rules which match pattern entities Zero or more activations may be on the agenda The symbol gt that follows the patterns in a rule is called an arrow The arrow represents the beginning of the THEN part of an IF THEN rule and may be read as implies The last part of a rule is the list of zero or more actions that will be executed when the rule fires In our example the one action is to assert the fact sound is quack The term fires means that CLIPS has selected a certain rule for execution from the agenda A program will cease execution when no activations are on the agenda When multiple activations are on the agenda CLIPS automatically determines which activation is appropriate to fire CLIPS orders the activations on the agenda in terms of increasing priority or salience The part of the rule before the arrow is called the left hand side LHS and the part of the rule after the arrow is called the right hand side RHS If you enter a reset command CLIPS memory is cleared and the special fact initialfact is always put into the fact list with fact identifier f O If no patterns are specified the pattern initial fact
145. pect Dopey Wonderful rich 99 months old CLIPS gt Notice that the default value of rich was used for Dopey since the assets field was not specified in the assert command If the assets field is given a specific value such as poor the specified value for assets of poor overrides the default value of rich as shown in the following example about Dopey s penurious nephew CLIPS gt Creset CLIPS gt assert prospect name Dopey Notwonderful Cassets poor Cage 95 lt Fact 1 gt CLIPS gt Crun Prospect Dopey Notwonderful poor 95 months old CLIPS gt A deftemplate pattern may be used just like any ordinary pattern For example the following rule will eliminate undesirable prospects CLIPS gt Cundefrule matrimonial_candidate CLIPS gt Cdefrule bye bye bad prospect lt prospect Cassets poor Cname name gt Cretract bad prospect printout t bye bye name crlf CLIPS gt Creset CLIPS gt Cassert Cprospect Cname Dopey Wonderful Cassets rich lt Fact 1 gt CLIPS gt assert prospect name Dopey Notwonderful Cassets poor lt Fact 2 gt CLIPS gt Crun bye bye Dopey Notwonderful CLIPS gt Ain t No Strings on Me Notice that only single fields were used for the patterns in the examples so far That is the field values for name assets and age were all single values In some types of rules you may want multiple fields Deftemplate allows the use of multiple values ina multislot
146. pey is now happily married duck crlf Cretract duck CLIPS gt Crun Dopey is now happily married lt Fact gt CLIPS gt facts CLIPS gt Notice that the printout prints the factindex of duck lt Fact O gt since the left arrow bound the address of the fact to duck Also there is no fact bachelor Dopey because it has been retracted 45 Variables can be used to pick up a fact value at the same time as an address as shown in the following example For convenience a deffacts has also been defined CLIPS gt Cclear CLIPS gt Cdefrule marriage duck lt Cbachelor name gt Cprintout t name Cretract duck CLIPS gt Cdeffacts good prospects bachelor Dopey bachelor Dorky bachelor Dicky CLIPS gt Creset CLIPS gt Crun Dicky 1S now happily married Dorky 1S now happily married Dopey 1S now happily married CLIPS gt 1S now happily married crlf Notice how the rule fired on all facts that matched the pattern bachelor name CLIPS also has a function called fact index which can be used to return the fact index of a fact address It s Not Important Instead of binding a field value to a variable the presence of a nonempty field can be detected alone using a wildcard For example suppose you re running a dating service for ducks and a duckette asserts that she only dates ducks whose first name is Richard Actually two criteria are in this specification since there is an implication th
147. phism is one of the five generally accepted features of a true OOP language For example the same type of message Send lt instance name gt print may have different actions depending on the class of the object which receives it In addition to egg of and color DUCK objects print out sound ID and age while DUCKLING objects only print out sound and age In languages without polymorphism you would have to define one function send egg print for egg types and another function send duckling print for the duckling type However in OOP no matter how many classes are defined the same message send lt instance name gt print will print the object slots This greatly improves the efficiency of program development since you do not have to define new functions for every new type 202 Polymorphism can be carried to extremes by having the same message do completely different things For example a print message handler could be defined that would print objects of a certain class and delete objects of another class Another extreme possibility would be not having the print handler print anything Instead it would delete objects of one class save objects of another class add objects of another class and so forth This extreme use of polymorphism would create a programmer s Tower of Babel and make it very hard to understand the code since everything would be run time dependent Defining message handlers of the same name in different class
148. precedence class take their facets from the next higher precedence class If the facet is not explicitly set in the next higher precedence class and it is composite too CLIPS tries the next higher and so on until the facet is defined or there are no more classes If the next higher class is not composite CLIPS does not check further The opposite to the composite facet is the exclusive facet which is the default For more information see the CLIPS Reference Manual S49 Chapter 11 Handling Handlers There are two steps in learning how to use a shovel 1 finding which part is the handle and 2 what to do with the handle Handlers are essential in OOP because they support object encapsulation The only proper way that objects can respond to messages is by having an appropriate handler to receive the message and take appropriate action In this chapter you ll learn the how messages are interpreted by objects You ll see how to modify existing message handlers and how to write your own Your Primitive Self So far you ve learned about the static structure of classes through the inheritance hierarchy and slots However the dynamic part of an object is determined by its message handlers or handlers for short which receive messages and performs actions in response The handlers are responsible for the dynamic properties of an object which determine its behavior You ve already used one handler many times the print in a send Polymor
149. pter 3 Adding DetallS ive siiiiiecccweiinierteceeediaiiesweeeeiwe ie kanemadsbennnawed 31 Stop ANd GO lessens sepnut 31 Fake AWAK meeer n iA E ERREA AARE T yaa becca EEO ERAR a day boca EO 32 A Question of Strategy 0 ccccecececcee ee ee cece ee ee eee te rest ee ee ae ea tntnnt antt ee rnnr nnanaa nenn nrnna nenn nn 32 Gimme Dettacts 0c c cecce din inde aint al Wien anid iti eee aaa 33 Selective Elimination 0 cccccecceccecceeee cece cece ae aa eee ee eeee ease aaaaaaeeeeeeeeaeaaaaaaneeeeeeeaeaaea 35 Wath Wtlscocca detec co ace inte sober Gea bch dads gente ge 35 A Good Mateh a a ra E pets he a oh ite a oo dete Mi A anaa Aaa pete eSa 36 Other Features 0 atch in aod Maa heed ek a eee 37 Chapter 4 Variable Interests c cceceeeeeeeeeeeeeeeeeeeeseeeeeeseeeeeeeeeneees 41 Let s Get Variables ccs cs niet nnn ete ae thet nee Add Rees eae 41 Be ASS Mive a a a aa n e E Pa aa pa a shady a E A TE Ea En Ee EE ea ni Eens 42 W atthe Duck Said sei ecssendiveieledtebinlseceleivss deoteedevtds eaidebsabdaaed tunes sactedaveseladtaioids 43 The Happy Bachelor rei iee iison poen ek donar enaa nadaa ees Decsain akan pecan Queaes 44 Its Not IMportant scedsvied Aa eA a EA ea E ANRA A EEIN KA OEREN A eens 45 Going Wild area a a E a a ea N e a AN O AAR EE AES 47 The ldeal Bachek anrison ainiaan dadaa 49 The Lucky DUCK visne enii a coos ech tints ce NA AA cen ae awe ec UA AREN A 51 Chapter 5 Doing It Up In Style 0c cc cc
150. re the commands to add the subclasses shown in Fig 1 6 93 CLIPS gt Cclear CLIPS gt Cdefclass UPPIE Cis a USER CLIPS gt Cdefclass CHILD Cis a USER CLIPS gt Cdefclass SUPPIE is a UPPIE CLIPS gt Cdefclass MUPPIE is a UPPIE CLIPS gt Cdefclass YUPPIE is a UPPIE CLIPS gt Cdefclass PUPPIE is a YUPPIE CLIPS gt Cdefclass YUKKIE Cis a YUPPIE CHILD The order that classes are defined must be such that a class is defined before its subclasses Thus Cdefclass CHILD Cis a USER must be entered before Cdefclass YUKKIE Cis a YUPPIE CHILD CLIPS will issue an error message if you try to enter the YUKKIE class before the CHILD class Notice the left to right order that SUPPIE MUPPIE and YUPPIE are drawn in Fig 1 6 This corresponds to the order that these classes are entered into CLIPS and is the convention that we will follow You can also see why CHILD is drawn to the right of UPPIE since it was entered after the UPPIE class In Fig 1 6 notice that the link YUKKIE YUPPIE is drawn to the left of the link YUKKIE CHILD Another convention that we will follow is to write the direct superclasses from left to right in the precedence list according to their left to right order drawn in a graph The ordering YUPPIE CHILD in the precedence list of YUKKIE is done to satisfy this convention Show Me CLIPS provides a number of functions to show information about classes such as predicate functions to
151. red printout t Stop crlf Cdefrule green Light Clight green Cprintout t Go crlf After the rules have been entered into CLIPS assert a fact light red and run You ll see Stop printed Now assert a light green fact and run You should see Go printed Zgor Take a Walk If you think about it other possibilities beside the simple red green and yellow cases exist Some traffic lights also have a green arrow for protected left turns Some have a hand that lights up to indicate whether a person can walk or not Some have signs that say walk or don t walk So depending on whether our robot is walking or driving it may have to pay attention to different signs The information about walking or driving must be asserted in addition to information about the status of the light Rules can be made to cover these conditions but they must have more than one pattern For example suppose we want a rule to fire if the robot is walking and if the walk sign says walk A rule could be written as follows Cdefrule take a waLk Cstatus walking Cwalk sign walk Cprintout t Go crlf The above rule has two patterns Both patterns must be satisfied by facts in the factiist for the rule to fire To see how this works enter the rule and then assert the facts status walking and walk sign walk When you run the program will print out Go since both patterns are satisfied and the rule is fired You can have any number of patte
152. required to cause the execution of rules Originally CLIPS had capabilities to represent only rules and facts However the enhancements of Version 6 0 allow rules to match objects as well as facts Also objects may be used without rules by sending messages and so the inference engine is no longer necessary if you use only objects In chapters 1 through 7 we ll discuss the facts and rules of CLIPS The object features of CLIPS are covered in chapters 8 through 12 The Beginning and the End To begin CLIPS just enter the appropriate run command for your system You should see the CLIPS prompt appear as follows CLIPS gt At this point you can start entering commands directly into CLIPS The mode in which you are entering direct commands is called the top level If you have a window version of CLIPS you can just select the command using the mouse or arrow keys rather than typing it in Please refer to the CLIPS Reference Manual for a discussion of the commands supported under windows For simplicity and uniformity in this book we ll assume the commands are typed in The normal mode of leaving CLIPS is with the exit command Just type Cexit in response to the CLIPS prompt and then press the carriage return key Making a List As with other programming languages CLIPS recognizes certain keywords For example if you want to put data in the fact list you can use the assert command Zgaga As an example of assert enter the follo
153. riting numeric expressions is called infix form because the math functions are fixed in between the arguments Functions can be used on the LHS and the RHS For example the following shows how the arithmetic operation of addition is used on the RHS of a rule to assert a fact containing the sum of two numbers x and y Note that the comments are in infix notation for your information only since infix cannot be evaluated by CLIPS CLIPS gt Cclear CLIPS gt Cdefrule addition Cnumbers x y gt Cassert Canswer pLus x y Add x y CLIPS gt Cassert Cnumbers 2 3 lt Fact Q gt CLIPS gt Crun CLIPS gt facts f Cnumbers 2 3 f 1 Canswer pLus 5 For a total of 2 facts CLIPS gt 7 ae A function can be used on the LHS if an equal sign is used to tell CLIPS to evaluate the following expression rather than use it literally for pattern matching The following example shows how the hypotenuse is calculated on the LHS and used to pattern match against some stock items The exponentiation function is used to square the x and y values The first argument of exponentiation is the number which is to be raised to the power of the second argument CLIPS gt Cclear CLIPS gt Cdeffacts database Cstock A 2 0 Cstock B 5 0 Cstock C 7 0 CLIPS gt Cdefrule addition Cnumbers x y Cstock ID Csqrt C x 2 C y 2 Hypotenuse gt printout t Stock ID ID crlf CLIPS gt Creset CLIPS gt Cassert
154. rns or actions in a rule The important point to realize is that the rule is placed on the agenda only if a the patterns are satisfied by facts This type of restriction is called a logical AND conditional element CE in reference to the AND relation of Boolean logic An AND relation is said to be true only if all its conditions are true Because the patterns are of the logical AND type the rule will not fire if only one of the patterns is satisfied All facts must be present before the LHS of a rule is satisfied and the rule is placed on the agenda A Question of Strategy The word strategy was originally a military term for the planning and operations of warfare Today the term strategy is commonly used in business because business is Zaga war to refer to the high level plans of an organization in achieving its goals e g Make a lot of money by selling more greasy hamburgers than anyone else in the world In expert systems one use of the term strategy is in conflict resolution of activations Now you might say Well I ll just design my expert system so that only one rule can possibly be activated at one time Then there is no need for conflict resolution The good news is that if you succeed conflict resolution is indeed unnecessary The bad news is that this success proves that your application can be well represented by a sequential program So you should have coded it in Ada C or Pascal in the first place and not bothered wri
155. s A breakpoint is simply an indicator to CLIPS to stop execution just prior to executing a specified rule A breakpoint is set by the set break command The remove break command will remove a breakpoint that has been set The show breaks will list all the rules which have breakpoints set The syntax of these rules for the argument lt rulename gt is shown following Cset break lt rulename gt Cremove break lt rulename gt Cshow breaks A Good Match You may encounter a situation in which you are certain a rule should be activated but isn t While it is possible that this is due to a bug in CLIPS it s not very likely because of the great skill of the people who programmed CLIPS NOTE PAID COMMERCIAL ANNOUNCEMENT FOR THE DEVELOPERS In most cases the problem occurs because of the way that you wrote the rule As an aid to debugging CLIPS has a command called matches that can tell you which patterns in a rule match facts Patterns which do not match prevent the rule from becoming activated One common reason that a pattern won t match a fact results from misspelling an element in the pattern or in the assertion of the fact The argument of matches is the name of the rule to be checked for matches To see how matches works first clear then enter the following rule Cdefrule take a vacation Cwork done Conditional element 1 Cmoney plenty Conditional element 2 Creservations made Conditional element 3 Cprintout t Let s
156. s you may write external functions or user defined functions in C Ada or other procedural languages and link to CLIPS These external functions are then used as you would any predefined function CLIPS also gives you the capability of specifying an explicit and conditional element an or conditional element and a not conditional element on the LHS The absence of a fact is specified as a pattern on the LHS using the not conditional element The alteration of our information to conform to reality is called truth maintenance That is we try to maintain the state of our minds to contain only true information so as to minimize conflicts with the real world While people can do this fairly easily practice makes perfect it s difficult for computers because they don t normally know which pattern entities are logically dependent on other pattern entities CLIPS has a feature to support truth maintenance which will internally tag those pattern entities which are logically dependent on others If these other pattern entities are retracted CLIPS will automatically retract the logically dependent ones The logical conditional element uses the keyword logical around a pattern to indicate that the matching pattern entities provide logical support to the assertions on the RHS Although the logical support works for assertions it does not reassert retracted facts The moral is if you lose something due to erroneous information you can t get it back lik
157. s Dopey as in the following bachelor Dopey Another interesting possibility occurs if Dopey must be the first name but only those Dopeys with two or three names are acceptable One way of solving this problem is to write two rules For example 47 Cdefrule eligible bachelor Dopey gt Cprintout t Date Dopey crlf Cdefrule eligible three names bachelor Dopey gt printout t Date Dopey crlf Enter and run this and you ll see that Dopeys with both two and three names are printed Of course if you don t want anonymous dates you need to bind the Dopey names with a variable and print them out Going Wild Rather than writing separate rules to handle each field it s much easier to use the multifield wildcard This is a dollar sign followed by a question mark and represents zero or more fields Notice how this contrasts with the single field wildcard which must match exactly one field The two rules for dates can now be written in a single rule as follows CLIPS gt Cclear CLIPS gt Cdefrule dating ducks bachelor Dopey gt Cprintout t Date Dopey crlf CLIPS gt Cdeffacts duck bachelor Dicky bachelor Dopey bachelor Dopey Mallard bachelor Dinky Dopey bachelor Dopey Dinky Mallard CLIPS gt Creset CLIPS gt Crun Date Dopey Date Dopey Date Dopey CLIPS gt 48 Wildcards have another important use because they can be attached to a symbolic field to create a
158. s and FALSE otherwise The combination of instance name followed by the one or more class restrictions is called an instance set member template The query functions may generally be used like any other function in CLIPS Shown following are the Rules of Scope which describe the restrictions on the use of variables in a query function The term scope means the section of code where a variable is visible or known The term reference means the name or address by which a variable is accessed In other words the scope of a variable means where the variable can be referenced e g bound or printed Anything which is not prohibited by these rules is allowed such as nesting Also the term query refers to the query portion of the query function not the query function itself There are two steps involved in trying to satisfy a query First CLIPS generates all the possible instance sets that match the instance set template Second the boolean instance set query is applied to all the instance sets to see which ones if any satisfy the query Instance sets are generated by a simple permutation of the members in a template where the rightmost members are varied first Note that a permutation is not the same as a combination because order matters in a permutation but notina combination The function find all instances returns a multifield value of all instances which satisfy the query or an empty multifield value for none The do for instance query function
159. s shown in the following example CLIPS gt Cdefmessage handler DUCK 1lie about age around change bind old age self age Cif Cnext handlerp then Coverride next handler change 2 Divide age in half bind new age self age Cif lt gt old age new age then Cprintout t Dorky_Duck is lying crlf Dorky_Duck is Lying crlf He s really old age crlf CLIPS gt Cmake instance Dorky_Duck of DUCK Cage 3 Dorky_Duck CLIPS gt send Dorky_Duck lie about age 1 Starting to print I am only 2 5 Finished printing Dorky_Duck is Lying Dorky_Duck is Lying He s really 3 CLIPS gt t s important to keep in mind that the return value of call next handler and override next handler is that of the shadowed handlers Shown following are the rules of message dispatch However note the following prerequisite to any message handling There must be at least one applicable primary handler 134 Rules of Message handler Precedence All the around handlers start execution Then the before primary and after handlers start and finish followed by completion of the arounds execution The around before and primary handlers are called in order of highest precedence class to lowest The after handlers are called from lowest precedence class to highest Each around handler must explicitly call the next shadowed handler Higher precedence primaries must explicitly call lower prece
160. s the arguments For example CLIPS gt G 2 2 both integer arguments give integer 4 s resuLt CLIPS gt G 2 0 2 0 both floating point arguments give 4 0 floating point result CLIPS gt G 2 2 0 mixed arguments give float result 4 0 Notice that in the last case of mixed arguments CLIPS converts the result to standard 69 double precision floating point type You can explicitly convert one type to another by using the float and integer functions as demonstrated in the following examples CLIPS gt float 2 2 convert integer to float 4 0 CLIPS gt integer 2 0 2 0 3 convert float to integer 4 Parentheses are used to explicitly specify the order of expression evaluation if desired In the example of x y z the customary infix way to evaluate it is to multiply y by z and then add the result to x However in CLIPS you must write the precedence explicitly if you want this order of evaluation as follows Cdefrule mixed calc Cnumbers x y z Cassert answer x y z 3 y z x In this rule the expression in the innermost parentheses is evaluated first so y is multiplied by z The result is added to x Bound Bachelors The analog to assigning a value to a variable on the LHS by pattern matching is binding a value to a variable on the RHS using the bind function It s convenient to bind variables on the RHS if the same values will be repeatedly used As a simple example in
161. s the decimal part of a number format Formats output list deffunctions List all deffunctions ppdeffunction Pretty print undeffunction length nth member subsetp delete explode subseq replace 73 Deletes a deffunction if it is not currently executing and not referred to elsewhere Specifying for lt name gt deletes all Number of fields or the number of characters in a string or symbol Specified field if exists else nil Number of the field if literal or variable exists else FALSE Returns TRUE if a multifield value is a subsetp of another multifield value else FALSE Given a field number deletes the value in the field Each string element is returned as part of a new multifield value Returns a specified range of fields Replaces a specified value 75 Chapter 7 How to Be in Control When you re young you re controlled by the world when you re older you should control the world Up to this point you ve been learning the basic syntax of CLIPS Now you ll see how to apply the syntax you ve learned to more powerful and complex programs You ll also learn some new syntax for input and see how to compare values and generate loops Let s Start Reading Besides matching a pattern a rule can get information in another way CLIPS can read the information that you type from the keyboard using the read function The following example shows how read is used to input data Note that no extra crlf is
162. ses directly except as literals The readline function is used to read multiple values until terminated by a carriage return This function reads in data as a string In order to assert the readline data an assert string function is used to assert the nonstring fact just as input by readline A top level example of assert string follows CLIPS gt Cclear CLIPS gt Cassert string primary color is red lt Fact gt CLIPS gt facts f 0 Cprimary color is red For a total of 1 fact CLIPS gt Notice that the argument of assert string must be a string The following shows how to assert a fact of multiple fields from readline FT CLIPS gt Cclear CLIPS gt Cdefrule test readline Cinitial fact gt Cprintout t Enter input crlf bind string Creadline Cassert string C str cat C string CLIPS gt Creset CLIPS gt Crun Enter input primary color is red CLIPS gt facts f Cinitial fact f 1 primary color is red For a total of 2 facts CLIPS gt Since assert string requires parentheses around the string to be asserted the str cat function is used to put them around string Both read and readline also can be used to read information from a file by specifying the logical name of the file as the argument For more information see the CLIPS Reference Manual Being Efficient CLIPS is a rule based language that uses a very efficient pattern matching algorithm called th
163. so primarily intended for procedural knowledge The five generally accepted features of object oriented programming are supported classes message handlers abstraction encapsulation inheritance polymorphism Rules may pattern match on objects and facts You can develop software using only rules only objects or a mixture of objects and rules CLIPS has also been designed for full integration with other languages such as C and Ada In fact CLIPS is an acronym for C Language Integrated Production System Rules and objects form an integrated system too since rules can pattern match on facts and objects In addition to being used as a stand alone tool CLIPS can be called from a procedural language perform its function and then return control back to the calling program Likewise procedural code can be defined as external functions and called from CLIPS When the external code completes execution control returns to CLIPS If you are already familiar with object oriented programming in other languages such as C Smalltalk Object C or Turbo Pascal you know the advantages of objects in developing software If you are not familiar with object oriented programming you will find that CLIPS is an excellent tool for learning this new concept in software development What This Book is About The CLIPS User s Guide is an introductory tutorial on the basic features of CLIPS It is not intended to be a comprehensive discussion of the entire tool Th
164. st Dopey last crlf CLIPS gt Creset CLIPS gt Crun Date Dopey Dinky Mallard Date Dinky Dopey Date Dopey Mallard Date Dopey 49 CLIPS gt The pattern matches anynames that have a Dopey anywhere in them Single and multifield wildcards can be combined For example the pattern Cbachelor Dopey means that the first and last names can be anything and that the name just prior to the last must be Dopey This pattern also requires that the matching fact will have at least four fields since the matches zero or more fields and all the others must match exactly four Although multifield variables can be essential for pattern matching in many cases their overuse can cause much inefficiency because of increased memory requirements and slower execution As a general rule of style you should use only when you don t know the length of fields Do not use simply as a typing convenience The Ideal Bachelor Variables used in patterns have an important and useful property which can be stated as follows The first time a variable is bound it retains that value only within the rule both on the LHS and also on the RHS unless changed on the RHS For example in the rule below Cdefrule bound Cnumber 1 num Cnumber 2 num gt If there are some facts 50 f Cnumber 1 Q f 1 Cnumber 2 Q f 2 Cnumber 1 1 f 3 Cnumber 2 1 then the rule can only be activated
165. stract For more information on these functions and topics mentioned in this chapter see the CLIPS Reference Manual 101 Chapter 9 Meaningful Messages It s always better to please your superiors than your subordinates Bowen In this chapter you ll learn more about classes and objects called instances You will see how to specify the attributes of classes using slots and how to send messages to objects The Birds and the Bees In Chapter 1 you learned the basic ideas of inheritance The reason that inheritance is so important in OOP is that inheritance allows the easy construction of customized software By customized software we don t mean that the software is built from scratch Rather it s more like taking a mass produced item and modifying it for a special application The mass produced item can be considered a product of a software factory that quickly economically and reliably produces items of a general type that are meant to be easily customized The heart of the OOP paradigm is the creation of a class hierarchy to quickly easily and reliably produce software Generally this software is a modification of existing software so that programmers are not always reinventing the loop Although in the past people have tried to provide reusable code through such mechanisms as subroutine libraries the pure OOP paradigm in languages such as Smalltalk carries the concept of reusable code to its logical conclusion by tr
166. t as you can see by checking the fact index The name of a variable or variable identifier is always written by a question mark followed by a symbol that is the name of the variable The general format is lt varitabLe name gt Global variables to be described in more detail later have a slightly different syntax Just as in other programming languages variable names should be meaningful for good style Some examples of valid variable names follow x noun coLlor sensor valve ducks eaten Before a variable can be used it should be assigned a value As an example of a case where a value is not assigned try to enter the following and CLIPS will respond with the error message shown 42 CLIPS gt Cclear CLIPS gt Cunwatch all CLIPS gt Cdefrule test Cinitial fact asserted by a reset command gt Cprintout t x crlf PRCCODE3 Undefined variable x referenced in RHS of defrule ERROR Cdefrule MAIN test Cinittal fact gt Cprintout t x crlf CLIPS gt CLIPS gives an error message when it cannot find a value bound to x The term bound means the assignment of a value to a variable Only global variables are bound in all rules All other variables are only bound within a rule Before and after a rule fires nonglobal variables are not bound and so CLIPS will give an error message if you try to query a nonbound variable Be Assertive One common use of variables is to match a value on the LHS a
167. tances Performs an action for every instance set which satisfies a query as they are found delayed doforallinstances Groups all instance sets which satisfy a query and then iterates an action over this group Instance set Query Functions I ll Take Any The any instancep function is a predicate function that returns TRUE if there is an instance matching the pattern and FALSE otherwise Shown following is an example of this query function used with the SENSOR and APPLIANCE classes and instances The query function determines if there is anyinstance in the SENSOR class CLIPS gt Creset CLIPS gt Cinstances inittal object of INITIAL OBJECT T1 of SENSOR T2 of SENSOR S1 of SENSOR W1 of SENSOR FR of APPLIANCE WP of APPLIANCE SA of APPLIANCE For a total of 8 instances CLIPS gt Cany instancep CC ins SENSOR TRUE 151 TRUE Function returns TRUE because there is a SENSOR instance CLIPS gt Cany instancep CC ins DUCK TRUE Evaluation error Bad PRNTUTIL1 Unable to find class DUCK No DUCK class CLIPS gt The basic format of a query function involves an instance set template to specifies the instances and their classes an instance set query as the boolean condition that the instances must satisfy and an action to specifies the single action to be taken Multiple actions can be included by grouping them with the progn function The predicate function class existp returns TRUE if the class exist
168. test whether a class is a superclass or subclass of another 94 The superclassp function returns TRUE if lt class1 gt is a superclass of lt class2 gt and FALSE otherwise The subclassp function returns TRUE if lt class1 gt is a subclass of lt class2 gt and FALSE otherwise The general form of both functions is function lt class1 gt lt class2 gt For example CLIPS gt superclassp UPPIE YUPPIE TRUE CLIPS gt Csuperclassp YUPPIE UPPIE FALSE CLIPS gt Csubclassp YUPPIE UPPIE TRUE CLIPS gt Csubclassp UPPIE YUPPIE FALSE CLIPS gt Now let s check to see if CLIPS accepted all these new classes One way of doing this is with the list defclasses command Following is the output of the command CLIPS gt Clist defclasses FLOAT INTEGER SYMBOL STRING MULTIFIELD EXTERNAL ADDRESS FACT ADDRESS INSTANCE ADDRESS INSTANCE NAME OBJECT PRIMITIVE NUMBER LEXEME ADDRESS INSTANCE USER INITIAL OBJECT UPPIE CHILD 95 SUPPIE MUPPIE YUPPIE PUPPIE YUKKIE For a total of 24 defclasses CLIPS gt Notice that the list defclasses command does not indicate the hierarchical class structure That is list defclasses does not indicate which classes are subclasses or superclasses of others If you look down this list you will see all the user defined classes that you entered UPPIE CHILD YUPPIE MUPPIE SUPPIE PUPPIE and YUKKIE In addition to the predefined system classes OBJECT and
169. that you don t have to blab it to the world This section was formerly called the Preface but since nobody read it renamed it to a more conventional title that computers users are conditioned to obey Another suggestion was to call this the Don t Readme section but since people today believe everything they read was afraid they really wouldn t read it The purpose of a Preface oops excuse me a Readme is to provide metaknowledge about the knowledge contained in a book The term metaknowledge means knowledge about the knowledge So this description of the Readme is actually metametaknowledge If you re either confused or intrigued at this point go ahead and read this book anyway because need all the readers can get What Is CLIPS CLIPS is an expert system tool developed by the Software Technology Branch STB NASA Lyndon B Johnson Space Center Since its first release in 1986 CLIPS has undergone continual refinement and improvement It is now used by thousands of people around the world The Internet news group comp ai shells often has discussions of CLIPS CLIPS is designed to facilitate the development of software to model human knowledge or expertise There are three ways to represent knowledge in CLIPS Rules which are primarily intended for heuristic knowledge based on experience Deffunctions and generic functions which are primarily intended for procedural knowledge vi Object oriented programming al
170. thing there are cases in which other fasteners are preferred For example imagine fastening your pants with a hammer and nails instead of a zipper NOTE if anyone does use a hammer and nails on their pants please contact the Guiness Book of World Records The Class Stuff In OOP a class is a template which describes the common characteristics or attributes of objects Note that this use of the term template is not the same as a deftemplate as described in an earlier chapter Here the word template is used in the sense of a tool that is used to build objects having common attributes As analogies a straightedge is a template for drawing straight lines while a cookie cutter is a curvacious template Classes of objects are arranged in a hierarchy or in a graph to describe the relationships of objects in a system Each class is an abstraction of a real world system or some other logical system that we are trying to model For example one abstract model of a real world system might be an automobile Another abstract model of a logical system could be financial instruments such as stocks and bonds or complex numbers The term abstraction refers to 1 the abstract description of a real world object or other system that we are trying to model or 2 the process of representing a system in terms of classes Abstraction is one of the five generally accepted features of a true OOP language The others are inheritance encapsulation polymorphism and dy
171. ting it as an expert system CLIPS offers seven different modes of conflict resolution depth breadth LEX MEA complexity simplicity and random It s difficult to say that one is clearly better than another without considering the specific application Even then it may be difficult to judge which is best For more information on the details of these strategies see the CLIPS Reference Manual The depth strategy is the standard default strategy of CLIPS The default setting is automatically set when CLIPS is first started Afterwards you can change the default setting In the depth strategy new activations are placed on the agenda after activations with higher salience but before activations with equal or lower salience All this simply means is that the agenda is ordered from highest to lowest salience In this book all discussions and examples will assume depth strategy Now that all these different optional settings are available be sure that before you run an expert system developed by someone else that your settings are the same as theirs Otherwise you may find the operation is inefficient or even incorrect In fact it s a good idea to explicitly encode all the settings in any system that you develop so that it will be configured properly Gimme Deffacts As you work with CLIPS you may become tired of typing in the same assertions from the top level If you are going to use the same assertions every time a program is run you c
172. trast to the put message the get message returns the value of a slot Since the value of get is returned it can be used by another function assigned to a variable and so forth In contrast a value that is printed out cannot be used by another function 111 assigned to a variable and so forth because the value goes to the standard output device One way of getting around this problem is to print out to a file and then read in the data from the file While this is not an elegant solution it does work Another way is to write your own print message handler that also returns values A very important point about slots is that you cannot modify the slots of a class by adding slots deleting slots or changing the characteristics of slots The only way to change a class is to 1 delete all instances of the class and 2 use a defclass with the same class name and the desired slots This situation is analogous to modifying a rule by loading in a new rule with the same name Class Etiquette Now that you ve learned aqbout slots and instances it s time to discuss class etiquette The term etiquette refers to a set of guidelines for doing something In contrast to standard procedural programming the OOP paradigm is class oriented Each object is intrinsically related to a class and that class is part of a class hierarchy Rather than concentrating on actions first and foremost the OOP programmer considers the overall class hierarchy or c
173. ty in using the name This may occur in the Send function and so brackets are used in a send In case of doubt use brackets since it doesn t hurt Some of the different types of objects in CLIPS are indicated in the following table Object Class Dorky_Duck SYMBOL Dorky_Duck STRING 1 0 FLOAT 1 INTEGER 1 1 0 Dorky_Duck Dorky_Duck MULTIFIELD lt Pointer OO0AB12CD gt EXTERNAL ADDRESS Dorky_Duck DUCK The classes of SYMBOL STRING FLOAT INTEGER and MULTIFIELD have the same names as those you are familiar with from rule based programming in CLIPS These are called primitive object types because they are provided by CLIPS and automatically maintained as needed These primitive types are mainly provided for use in generic functions Two compound classes are NUMBER which is FLOAT or INTEGER and LEXEME which is SYMBOL or STRING The compound classes are provided for convenience if the type of number or type of characters doesn t matter In contrast user defined object types are those that you define through 103 user defined classes If you refer back to the predefined CLIPS classes of Fig 1 7 in chapter 1 you ll see that primitive and user defined classes are the top4evel division of classes in CLIPS Two functions convert a symbol to an instance name and vice versa The symbol toinstance name converts a symbol to an instance name as shown by the following CLIPS gt clear Get rid of any old classes
174. type water of APPLIANCE of APPLIANCE of APPLIANCE Cloc basement Creading 0 OK Cmin Q Cmax Q Capp WP type furnace Cloc basement Csetting low low or high Cstatus on type water_pump Cloc basement setting fixed Cstatus off Ctype smoke_alarm Cloc basement Csetting fixed Cstatus off Suppose the following questions or queries are asked What are all the objects in the database How are all the objects arranged What are the relationships between objects What are all the devices What are all the sensors What are all the appliances Which sensor is connected to which appliance Are there any sensors whose type is temperature What sensors of type temperature have a reading between min and the max An even more basic query is whether or not there are any sensors 150 present The query system of COOL is a set of six functions that may be used for pattern matching an instance set and performing actions An instance set is a set of instances such as the instances of SENSOR The instances in the instance set may come from multiple classes that do not have to be related In other words the classes do not have to be from the same inheritance path The following table from the CLIPS Reference Manual summarizes the predefined query functions that may be used for instance set access Function Purpose S Performs an action for the first instance set which satisfies a query do for all ins
175. ubclasses of USER Primary Type Class Role init Initialize an instance delete Delete an instance print Print the object direct modify Directly modifies slots message modify Modifies slots using put messages direct duplicate Duplicates instance without put message message duplicate Duplicates an instance using messages Predefined USER Message handlers of Primary Type These primary handlers are predefined and cannot be modified unless you change the source code of CLIPS However you may define the before after and around handler types for these primaries You ve already seen an example of changing the before and after handler types for the USER print handler Now let s look at some examples of defining the before and after handler types for the init primary handler CLIPS gt Cdefmessage handler USER init before C Cprintout t Starting to make instance crlf CLIPS gt Cdefmessage handler USER init after Cprintout t Finished making instance crlf CLIPS gt Creset Starting to make instance Finished making instance 129 Starting to make instance Finished making instance Starting to make instance Finished making instance CLIPS gt Cmake instance Dixie_Duck of DUCK Cage 1 Starting to make instance Finished making instance Dixie_Duck CLIPS gt Cinstances inittal object of INITIAL OBJECT Dorky_Duck of DUCK Dinky_Duck
176. umber of left and right parentheses always must match in a statement The same rule is shown following with comments added to match the parts of the rule Also shown is the optional rule header comment in quotes Here comes the quack There can be only one rule header comment and it must be placed after the rule name and before the first pattern Although we re only discussing pattern matching against facts now more generally a pattern can be matched against a pattern entity A pattern entity is either a fact or an instance of a user defined class Pattern matching on objects will be discussed later CLIPS tries to match the pattern of the rule against a pattern entity Of course white space consisting of spaces tabs and carriage returns may be used to separate the elements of a rule to improve readability Other comments begin with a semicolon and continue until the carriage return key is pressed to terminate a line Comments are ignored by CLIPS Cdefrule duck Here comes the quack Rule header Canimal is duck Pattern gt THEN arrow Cassert Csound is quack Action Only one rule name can exist at one time in CLIPS 23 Entering the same rule name in this case duck will replace any existing rule with that name That is while there can be many rules in CLIPS there can be only one rule which is named duck This is analogous to other programming languages in which only one procedure name can be used to uniquel
177. wing right after the CLIPS prompt as shown CLIPS gt Cassert Cduck Here the assert command takes duck as its argument Be sure to always press the carriage return key to send the line to CLIPS You will see the response lt Fact 0 gt which indicates CLIPS has stored the duck fact in the fact list and given it the identifier 0 The angle brackets are used as a delimiter in CLIPS to surround the name of an item CLIPS will automatically name facts using a sequentially increasing number and list the highest fact index when one or more facts is asserted Notice that the assert command and its duck argument are surrounded by parentheses Like many other expert system languages CLIPS has a LISP like syntax which uses parentheses as delimiters Although CLIPS is not written in LISP the style of LISP has influenced the development of CLIPS And Checking It Twice Suppose you want to see what s in the fact list If your version of CLIPS supports windows you may just select the appropriate command from the menu Alternatively you can enter commands from the keyboard In the following we ll describe the keyboard commands since the window selections are self explanatory The keyboard command to see facts is with the facts command Enter facts in response to the CLIPS prompt and CLIPS will respond with a list of facts in the fact list Be sure to put parentheses around the command or CLIPS will not accept it The result of the facts co
178. y identify a procedure The general syntax of a rule is shown following CdefruLe rule_name optional_comment Cpattern_1 Left Hand Side CLHS Cpattern_2 of the rule consisting of elements before the gt Cpattern_N gt Caction_1 Right Hand Side CRHS Caction_2 of the rule consisting of elements after the gt Caction_M the Last balances the opening C to the Left of defrule Be sure all your parentheses balance or you will get error messages The entire rule must be surrounded by parentheses Each of the rule patterns and actions must be surrounded by parentheses An action is actually a function which typically has no return value but performs some useful action such as an assert or retract For example an action might be assert duck Here the function name is assert and its argument is duck Notice that we don t want any return value such as a number Instead we want the fact duck to be asserted A function in CLIPS is a piece of executable code identified by a specific name which returns a useful value or performs a useful side effect such as printout A rule often has multiple patterns and actions The number of patterns and actions do not have to be equal which is why different indices N and M were chosen for the rule patterns and actions Zero or more patterns may be written after the rule header Each pattern consists of one or more fields In the duck rule the
179. ying to build all the software in a system as reusable code In Smalltalk everything is an object even the classes In CLIPS instances of the primitive types such as NUMBER SYMBOL STRING and so forth as well as user defined instances are objects Classes are not objects in CLIPS For example the NUMBER 1 the SYMBOL Duck the STRING Duck and the user defined instance Duck are all objects The OOP paradigm is quite different from the subroutine library approach in which 102 bits and pieces of subroutine code may or may not be used depending on the programmer s whim The OOP paradigm encourages and supports modular code the message handlers that can be easily modified and maintained This feature of code maintainability is playing an increasingly important role as the size and cost of systems increase A class in OOP is like a software factory which has the design information about an object In other words a class is like a template which can be used to produce identical objects that are the instances of the class The classic analogy is that a class is like the blueprint of a cow and the object that produces milk such as Elsie is the instance The general syntax of an instance name is simply a symbol surrounded by brackets as follows lt name gt The brackets are not actually part of the instance name which is a symbol like Elsie Brackets are used to surround an instance name if there is danger of ambigui

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