1. Chapter 11: Classes, Instances, and Message-Handlers
Expert Systems: Principles and Programming, Fourth Edition

2. The Defclass Construct
Before instances of classes can be created, CLIPS needs to know the list of valid slots for the given class.
To provide this information, the defclass construct is used:
(defclass <class-name> [<optional-comment>]
(is-a <superclass-name>)
<slot-definition>*)
Expert Systems: Principles and Programming, Fourth Edition

3. The Defclass Construct
Note that <superclass-name> is the class from which the newly defined class will inherit information.
All user-defined classes ultimately inherit from the system class USER.
A user-defined class will therefore inherit from the USER class or from another user-defined class.
Expert Systems: Principles and Programming, Fourth Edition

4. The Slot Definition The syntax of the <slot-definition> is:
(slot <slot-name> <slot-attribute>* |
(multislot <slot-name> <slot-attribute>*)
type, range, cardinality, allowed-symbols
allowed-strings, allowed-lexemes, allowed-integers
allowed-floats, allowed-numbers, allowed-values
allowed-instance-names, default, and default-dynamic
Expert Systems: Principles and Programming, Fourth Edition

5. Creating Instances
To create an instance of a class, use the make-instance command as follows:
(make-instance [<instance-name-expression>]
of <class-name-expression>
<slot-override>*)
where <slot-override> is:
(<slot-name-expression> <expression>)
Instances belong to the module in which their corresponding defclass is defined.
Expert Systems: Principles and Programming, Fourth Edition

6. System-Defined Message-Handlers
Just like data, procedural information can be attached to classes.
Such procedures are called message-handlers.
User-defined
System-defined – automatically created
Message handlers can be invoked for an instance (object) using the send command.
(send <object-expression>
<message-name-expression> <expression>*)
Expert Systems: Principles and Programming, Fourth Edition

7. System Message-Handlers
For each slot defined in a defclass, CLIPS automatically defines get- and put-slot message-handlers that are used to retrieve and set slot values.
The get-message-handlers have no arguments and return the value of the slot.
The put-message-handlers take zero or more arguments. If not arguments are supplied, the slot is restored to its original default-value.
Expert Systems: Principles and Programming, Fourth Edition

8. System Message-Handlers
Supplying the arguments will set the slot value to those values.
The return value of a put-message-handler is the new value of the slot.
When slots are being watched, an informational message is printed whenever the value of an instance slot is changed.
When instances are watched, an informative message appears when an instance is created/deleted.
Expert Systems: Principles and Programming, Fourth Edition

9. The Definstances Construct
The definstances construct is the equivalent of the deffacts construct.
When a reset command is issued, all instances are sent a delete message.
Then all instances found in the definstances constructs are created.
Expert Systems: Principles and Programming, Fourth Edition

10. Definstances Construct
General format:
(definstances <definstances name> [active]
[<optional comment>]
<instance-definition>*)
Where <instance-definition> is:
([instance-name-expression>] of
<class-name-expression>
<slot-override>*)
Expert Systems: Principles and Programming, Fourth Edition

11. Definstances Construct
By default, pattern matching does not occur for definstances instances until all the slot overrides have been processed.
Several commands exist for manipulating definstances:
list-definstances – displays list of definstances maintained by CLIPS
ppdefinstances – displays text representations of definstances
undefinstances – deletes definstances
get-definstances-list – returns multifield value containing list of definstances
Expert Systems: Principles and Programming, Fourth Edition

12. Classes and Inheritance
One benefit of using COOL is class inheritance.
Classes allow us to share common information among multiple classes w/o duplication of information or inclusion of unnecessary information.
It is possible for a class to redefine a slot that was already defined by one of its superclasses.
Expert Systems: Principles and Programming, Fourth Edition

13. Classes and Inheritance
Definitions:
Subclass – class that inherits directly/indirectly from another class
Superclass – class from which subclass inherits
A single-inheritance class hierarchy is one in which each class has only one direct superclass.
A multiple-inheritance hierarchy (COOL) is where a class may have more than one direct superclass.
Expert Systems: Principles and Programming, Fourth Edition

14. Classes and Inheritance
The source slot attribute allows slot attributes to inherit from superclasses.
The default value for the slot is exclusive – attributes determined by most specific class defining the slot.
Single inheritance – the class having fewest superclasses
Source slot composite, attributes not explicitly defined in most specific class defining slot are taken from next most specific that defines the attribute.
Expert Systems: Principles and Programming, Fourth Edition

15. Classes and Inheritance
It is possible to disable the inheritance of a slot using the propagation slot attribute:
Inherit (default) – slot will be inherited by subclasses
No-inherit – slot will not be inherited by subclasses
It is possible to define classes to be used only for inheritance – abstract classes; instances cannot be created – concrete by default.
The role class attribute specifies whether a class is abstract or concrete.
Expert Systems: Principles and Programming, Fourth Edition

16. Commands to Manipulate Defclasses
list-defclasses – displays the current list of defclasses maintained by CLIPS
browse-classes – displays the inheritance relationships between a class and its subclasses
ppdefclass – displays the text representation of a defclass
undefclass – deletes a defclass
Expert Systems: Principles and Programming, Fourth Edition

17. Object Pattern Matching
Single object pattern can match instances from several classes.
Changes to slot values that are not specified in an object pattern do not retrigger the rule to which the pattern belongs.
Changes to slot values that are not specified in an object pattern within a logical conditional element do not remove logical support provided by the associated rule.
Expert Systems: Principles and Programming, Fourth Edition

18. Object Pattern General format: where <attribute-constraint> is:
(object <attribute-constraint>*)
where <attribute-constraint> is:
(is-a <constraint>) |
(name <constraint>) |
(<slot-name> <constraint>*)
and <constraint> is the same as the pattern slot constraints that are used in deftemplate patterns.
Expert Systems: Principles and Programming, Fourth Edition

19. Object Patterns
One difference between object patterns and fact patterns is that only those object patterns that explicitly match on a slot are affected when the slot value of an instance is changed.
It is possible to force a slot or a class not to participate in pattern matching using the pattern-match attribute.
Object patterns and instance creation can be used in conjunction with the logical conditional element just as facts and facts patterns can.
Expert Systems: Principles and Programming, Fourth Edition

20. Object Patterns
Changes in instance slots do not affect the logical support for a fact or instance if the slot was not referenced in an object pattern within the logical conditional element.
When using object patterns in a rule, CLIPS will sometimes use the initial-object/fact pattern. If so, an initial-fact pattern is added if the pattern preceding the insertion position is a fact pattern – if an object pattern an initial-object pattern is inserted.
Expert Systems: Principles and Programming, Fourth Edition

21. User-Defined Message-Handlers
In addition to print, delete, put, and get-system-defined message handlers, COOL automatically defines for each class, you can define your own message-handlers using defmessage-handler.
General format:
(defmessage-handler <class-name> <message-name>
[<handler-type>]
[<optional-comment>]
(<regular-parameter>*
[<wildcard-parameter>])
<expression>*)
Expert Systems: Principles and Programming, Fourth Edition

22. User-Defined Message-Handlers
By default, a message-handler is a primary message-handler.
Each class has its own set of message-handlers.
The body of the message-handler, represented by <expression>*, behaves like the body of a deffunction.
The bind function can be used to bind local variables and the last expression evaluated in the body is the value returned.
Expert Systems: Principles and Programming, Fourth Edition

23. Slot Shorthand References
A shorthand mechanism allows one to access the slots of the instances bound to the ?self variable.
The expression: ?self:<slot-name> can be used to retrieve the value of a slot.
Similarly,
(bind ?self:slot-name <expression>*)
can be used to set a slot value.
Both bypass message-passing and directly manipulate slots and can only be used for slots directly defined by the class.
Expert Systems: Principles and Programming, Fourth Edition

24. Encapsulation COOL supports object encapsulation
Hiding the details of the class
Limiting access to the class via a well-defined interface – message-handlers defined for the class
If the visibility slot attribute is set to private (default), the slot can only be directly accessed by message-handlers of class defining it; if public, slot can be directly accessed by subclasses and superclasses defining it.
Expert Systems: Principles and Programming, Fourth Edition

25. Watching Messages & Message Handlers
If message-handlers or messages are watched with watch, informational messages are printed when a message-handler begins or ends execution.
Defmessage-handler commands
list-defmessage-handlers – displays current list of defmessage-handlers maintained by CLIPS
ppdefmessage-handler – displays text representation of defmessage-handler
undefmessage-handler – deletes a defmessage-handler
get-defmessage-handler – returns multifield value containing list of defmessage-handlers for class
Expert Systems: Principles and Programming, Fourth Edition

26. Slot Access and Handler Creation
The access and create-accessor slot attributes control the access of slots.
Access attribute restricts the type of access allowed to slots – read-write, read-only, initialize only.
The create-accessor attribute controls the automatic creation of the get- and put-handlers for class slots – read-write, read-only, write-only, and none.
Expert Systems: Principles and Programming, Fourth Edition

27. Before/After/Around Message-Handlers
When an existing class does not meet your needs and may depend on other code to maintain its behavior – unfamiliar code or code you don’t want to modify.
To get around this, you can define a new class that will inherit whatever behavior you want from the existing class.
Message-handler can be one of four types: primary, before/after, and around.
Expert Systems: Principles and Programming, Fourth Edition

28. Before/After/Around Message-Handlers
Primary handler – (default) typically the main handler for responding to a message, overrides / shadows primary message-handler for same message inherited from a superclass.
Before/After handlers – invoked before and after the primary handler, respectively.
Around handlers – must explicitly invoke the other handler types.
Expert Systems: Principles and Programming, Fourth Edition

29. Before/After/Around Message-Handlers
#2 and #3 handlers of superclasses are now shadowed by subclass definition.
It is possible to override the arguments passed to a message-handler by using the override-next-handler command.
Expert Systems: Principles and Programming, Fourth Edition

30. Handler Execution Order
With all the available techniques to modify class behaviors, which is the best approach?
A class can slightly modify the behavior of a superclass using before and after handler w/o overriding the primary handler.
Subclass cannot prevent the execution of a before or after handler unless they terminate the message – preventing the execution of all before, after, and primary handlers.
Expert Systems: Principles and Programming, Fourth Edition

31. Handler Execution Order
If an existing class’s behavior is modified by redefining a new class and then overriding the primary handler, the primary handler is also subject to being overridden by a subclass.
Unless the overriding class calls the call-next-handler function, the primary handler will not be executed.
See page 670 of the text for steps.
Expert Systems: Principles and Programming, Fourth Edition

32. Instance Creation, Initialization, and Deletion Message-Handlers
create -- is called after an instance is created but before any default values or slot overrides have been applied.
init – is called after slot overrides have been processed to set any remaining slot values that were not overridden to their default values
delete – either explicitly called to delete an instance or automatically called when you call make-instance and specify instance name of existing instance.
Expert Systems: Principles and Programming, Fourth Edition

33. Modifying / Duplicating Instances
modify-instance – slot values are changed directly by the direct-modify message-handler w/o invoking message passing.
message-modify-instance – same as #1 but uses message-passing to change slot values.
active-modify
active-message-modify
Expert Systems: Principles and Programming, Fourth Edition

34. Commands for Duplicating Instances
duplicate-instance
message-duplicate-instance
active-duplicate-instance
active-message-modify-instance
Expert Systems: Principles and Programming, Fourth Edition

35. Instance Set Query Functions
Any-instancep function – if a set of instances is found that satisfy the query, then the any-instancep function returns the symbol TRUE; otherwise FALSE.
Find-instance query function – returns a multifield value containing the first instance set satisfying the query, then the multifield value will be empty.
find-all-instances function – returns a multifield value containing all instance sets satisfying the query.
Do-for-instance, do-for-all-instances, and delayed-do-for-all allow actions on the instance sets satisfying a query.
Expert Systems: Principles and Programming, Fourth Edition

36. Multiple Inheritance Specifying:
Single inheritance – a single class specified in the is-a attribute
Multiple inheritance – specify more than one class in the is-a attribute
Expert Systems: Principles and Programming, Fourth Edition

37. Multiple Inheritance Conflicts
The most practical examples involve cases where the superclass from which the class is inheriting do not share slots or message-handlers – no conflicts occur here.
In simple cases where the classes specified in the is-a attribute do not share common user-defined superclasses, the order in which the classes are specified determines the precedence when there are multiple definitions of the same slot or message-handler.
Expert Systems: Principles and Programming, Fourth Edition

38. Defclasses and Defmodules
In a similar manner to other constructs, defclass constructs can be imported and exported by modules.
The export and import statements previously discussed which export or import all constructs, also apply to defclasses.
Explicit specifying which defclasses are exported or imported is possible.
Expert Systems: Principles and Programming, Fourth Edition

39. Loading and Saving Instances
save-instances command – saves instances to a file
load-instances command – loads in a group of instances stored in a file
bsave- and boad-instances command – similar to #1 and #2, except binary format is used
Expert Systems: Principles and Programming, Fourth Edition

40. Summary
This chapter introduced the CLIPS Object-Oriented Language (COOL)
Instances (objects) are another data representation provided by CLIPS.
Instance attributes are specified using the defclass construct.
Procedural code is implemented using the defmessage-handler construct.
Inheritance allows classes to make use of slots and message-handlers associated with another class.
Expert Systems: Principles and Programming, Fourth Edition

41. Summary COOL supports single- and multiple- inheritance.
In addition to the slot attributes provided with deftemplates, several additional slot attributes are also supported by defclasses.
Several predefined system message-handlers for creating, initializing, printing, and deleting instances are available.
User-defined message-handlers can also be created.
Expert Systems: Principles and Programming, Fourth Edition

42. Summary
Message-handlers are invoked by sending an instance a message name along with associated arguments via the send command
Object pattern matching provides several capabilities not found with fact pattern matching.
Finally, COOL provides several instance set query functions that allow direct queries on sets of instances satisfying a specified set of conditions.
Expert Systems: Principles and Programming, Fourth Edition