Chapter 11: Classes, Instances, and Message-Handlers

1
Chapter 11Classes, 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 ltclass-namegt ltoptional-commentgt
• (is-a ltsuperclass-namegt)
• ltslot-definitiongt)

3
The Defclass Construct

• Note that ltsuperclass-namegt 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.

4
The Slot Definition

• The syntax of the ltslot-definitiongt is
• (slot ltslot-namegt ltslot-attributegt
• (multislot ltslot-namegt ltslot-attributegt)
• type, range, cardinality, allowed-symbols
• allowed-strings, allowed-lexemes,
  allowed-integers
• allowed-floats, allowed-numbers, allowed-values
• allowed-instance-names, default, and
  default-dynamic

5
Creating Instances

• To create an instance of a class, use the
  make-instance command as follows
• (make-instance ltinstance-name-expressiongt
• of ltclass-name-expressiongt
• ltslot-overridegt)
• where ltslot-overridegt is
• (ltslot-name-expressiongt ltexpressiongt)
• Instances belong to the module in which their
  corresponding defclass is defined.

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 ltobject-expressiongt
• ltmessage-name-expressiongt ltexpressiongt)

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.

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.

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.

10
Definstances Construct

• General format
• (definstances ltdefinstances namegt active
• ltoptional commentgt
• ltinstance-definitiongt)
• Where ltinstance-definitiongt is
• (instance-name-expressiongt of
• ltclass-name-expressiongt
• ltslot-overridegt)

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

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.

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.

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.

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.

16
Commands to Manipulate Defclasses

1. list-defclasses displays the current list of
   defclasses maintained by CLIPS
2. browse-classes displays the inheritance
   relationships between a class and its subclasses
3. ppdefclass displays the text representation of
   a defclass
4. undefclass deletes a defclass

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.

18
Object Pattern

• General format
• (object ltattribute-constraintgt)
• where ltattribute-constraintgt is
• (is-a ltconstraintgt)
• (name ltconstraintgt)
• (ltslot-namegt ltconstraintgt)
• and ltconstraintgt is the same as the pattern slot
  constraints that are used in deftemplate patterns.

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.

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.

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 ltclass-namegt ltmessage-namegt
• lthandler-typegt
• ltoptional-commentgt
• (ltregular-parametergt
• ltwildcard-parametergt)
• ltexpressiongt)

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
  ltexpressiongt, 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.

23
Slot Shorthand References

• A shorthand mechanism allows one to access the
  slots of the instances bound to the ?self
  variable.
• The expression ?selfltslot-namegt can be used
  to retrieve the value of a slot.
• Similarly,
• (bind ?selfslot-name ltexpressiongt)
• 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.

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.

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

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.

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 dont 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.

28
Before/After/AroundMessage-Handlers

1. Primary handler (default) typically the main
   handler for responding to a message, overrides /
   shadows primary message-handler for same message
   inherited from a superclass.
2. Before/After handlers invoked before and after
   the primary handler, respectively.
3. Around handlers must explicitly invoke the
   other handler types.

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.

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.

31
Handler Execution Order

• If an existing classs 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.

32
Instance Creation, Initialization, and Deletion
Message-Handlers

1. create -- is called after an instance is created
   but before any default values or slot overrides
   have been applied.
2. init is called after slot overrides have been
   processed to set any remaining slot values that
   were not overridden to their default values
3. delete either explicitly called to delete an
   instance or automatically called when you call
   make-instance and specify instance name of
   existing instance.

33
Modifying / Duplicating Instances

1. modify-instance slot values are changed
   directly by the direct-modify message-handler w/o
   invoking message passing.
2. message-modify-instance same as 1 but uses
   message-passing to change slot values.
3. active-modify
4. active-message-modify

34
Commands for Duplicating Instances

1. duplicate-instance
2. message-duplicate-instance
3. active-duplicate-instance
4. active-message-modify-instance

35
Instance Set Query Functions

1. 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.
2. Find-instance query function returns a
   multifield value containing the first instance
   set satisfying the query, then the multifield
   value will be empty.
3. find-all-instances function returns a
   multifield value containing all instance sets
   satisfying the query.
4. Do-for-instance, do-for-all-instances, and
   delayed-do-for-all allow actions on the instance
   sets satisfying a query.

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

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.

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.

39
Loading and Saving Instances

1. save-instances command saves instances to a
   file
2. load-instances command loads in a group of
   instances stored in a file
3. bsave- and boad-instances command similar to 1
   and 2, except binary format is used

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.

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.

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.