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Chapter 11: Classes, Instances, and Message-Handlers

Chapter 11: Classes, Instances, and Message-Handlers. Expert Systems: Principles and Programming, Fourth Edition. The Defclass Construct. Before instances of classes can be created, CLIPS needs to know the list of valid slots for the given class.

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Chapter 11: Classes, Instances, and Message-Handlers

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  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: (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/AroundMessage-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/AroundMessage-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/AroundMessage-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 primaryhandler. • 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 primaryhandler is also subject to being overridden by a subclass. • Unless the overriding class calls the call-next-handler function, the primaryhandler 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-instancesfunction – 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-instancescommand – saves instances to a file • load-instances command – loads in a group of instances stored in a file • bsave- and boad-instancescommand – 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

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