COM362 Knowledge Engineering

1
DemonsPattern and Event Based Reasoning

• John MacIntyre
• 0191 515 3778
• john.macintyre_at_sunderland.ac.uk

2
Demons

• Normally related to frame-based reasoning
• Procedures linked to a particular slot in a frame
• Can be simple, single-value procedures, or
  triggered by more complex patterns or events
• Can be used to monitor the KBS for changes

3
Simple Demons

• One of the simplest Demons is the WHEN_REQUESTED
  procedure
• Instances instantiated with defaults values at
  creation time
• The Demon is used to replace these values at run
  time
• Only activates if a frame with the appropriate
  slot value is examined

4
...continued

• Another example - the WHEN_READ procedure
• Activates only when the relevant slot value is
  read
• Actions of the procedure can be to write values
  into other slots, e.g.
• WHEN_READ Physique in Frame Athlete)
• IF current_value IS muscular then
• WRITE large to Appetite

5
...continued

• Another example - the WHEN_WRITTEN procedure
• Activates only when the relevant slot value is
  written, e.g
• WHEN_WRITTEN Physique in Frame Athlete)
• IF new_value IS lean then
• WRITE average to Appetite

6
Handling Uncertaintywith Demons

• Demons can be used to help uncertainty handling
• Actions of the procedure can be to instigate
  actions on the basis of values given or inferred
• WHEN_READ Physique in Frame Athlete)
• IF current_value IS unknown then
• SEND user_dialogue to Enquiry_Form

7
...continued
WHEN_WRITTEN Bearing_1_Temperature in Frame
Plant_Monitor) IF new_value IS hot and
certainty IS gt0.7 then WRITE machine_trip to
Control_System
8
Maintaining Constraints

• Demons can, therefore, be used to maintain
  constraints on slot values
• WHEN_READ and WHEN_WRITTEN can be used to ensure
  appropriate changes in slot values if the value
  of one slot value is either changed or is present
• Sophisticated systems will also trigger these
  Demons with some order of precedence

9
Perspective and Context

• Demons can be used to apply perspective and
  context to changes in the KB
• These can be viewed as conditional dependencies
• Demon commonly used for this purpose is
  WITH_RESPECT_TO
• Example describe the size of a person who is 5ft
  tall!

10
Perspective

• From the perspective of an adult, the value
  should be small
• From the perspective of a small child, the value
  should be tall
• Using WITH_RESPECT_TO
• WITH_RESPECT_TO Size in Frame Person)
• IF type IS adult then
• WRITE small to Size
• ELSE IF type IS small child then
• WRITE tall to Size

11
Context

• We can also use WITH_RESPECT_TO to condition
  values on the context of information
• For example, a particular persons mood is happy
  if in the pub, but grumpy if at work
• WITH_RESPECT_TO Mood in Frame Person
• IF name IS John AND context IS work then
• WRITE grumpy to Mood
• IF name IS John and context IS pub then
• WRITE happy to Mood

12
Inheritance of Demons

• Frame-based systems allow the inheritance of
  values associated with the frames at the top
  level
• Therefore, high level frames should represent
  high level concepts in the domain model
• Demons can also be inherited, which allows for
  the development of procedural semantics in KBS

13
More Complex Demons

• Demons can be used to trigger on more complex
  combinations, events and patterns
• Typical use of such Demons is in natural language
  processing, or other complex multi-variate
  applications
• Allows the application of PATTERN-BASED REASONING

14
Pattern Matching

• It is often the case that each pass through a
  knowledge base will need to interrogate many
  rules
• Forward and backward chaining require the
  inference engine to check each rule interrogated
  to determine whether facts are available to fire
  the rule
• Very slow!

15
An Alternative Approach

• Pattern-based reasoning uses algorithms which
  records the state of patterns of facts at each
  iteration of inferencing
• Then looks only at the changes between each
  iteration - much more efficient
• However, requires a lot of memory to hold the
  state of the patterns of facts!
• Very powerful when dealing with situations which
  need a pattern-based approach

16
Language Processing

• Many applications of KBS in natural language
  processing
• Complex issues such as
• Topic of Concept
• Context
• Directionality
• Procedural approaches often used to cope with
  these problems

17
Conclusions

• Demons are procedures which are usually linked to
  slots in frames
• Only activate when the particular slot is
  examined in some way
• Can be used for maintenance of constraints,
  handling uncertainty, and context
• Good for complex, pattern-based reasoning