flex Expert System Toolkit - LPIS

3. Forward Chaining 29
3. Forward Chaining and Rules
In this chapter we shall describe the rules and inference engine of the
forward chaining system and contrast it with backward chaining. Rules are
really the life-blood of expert systems technology, and provide elegant,
expressive and intuitive means of expressing knowledge.
A simple rule may be, 'if it is raining then carry an umbrella'. Then given
the fact that 'it is raining', we can infer or derive that we should 'carry an
umbrella'. Facts can be thought of as degenerate cases of rules, i.e. rules
without any pre-conditions, or rules whose pre-condition part is always true.
Facts may be stored in a local database, retrieved from some external
database, obtained through user question and answer interaction, stored as
global variables, or derived from other rules.
Another rule may be 'if it is raining then the ground is wet'. Now, given
the same fact that 'it is raining', we can also infer or derive that 'the ground
is wet', and store this as a fact, or, if ground is an attribute of some frame,
set its value to wet.
The collection of all facts is often referred to as the fact base. In rule-based
programming we use an inference engine to match rules against facts to
produce new facts which means we can then use new rules, until we reach
some final state.
Trying to prove the pre-conditions of forward chaining rules may well
involve some backward chaining evaluation of sub-goals, i.e. there may be a
backward chaining program for determining the current state of the
weather. The integration of the two inference mechanisms can be as simple
or as complicated as required. Notice, in the absence of information,
execution fails and conditions in effect are deemed false. This is known as
the closed world assumption.
We shall use the following propositional example to show the difference
between backward chaining inferences and forward chaining inferences.
A if B and C .
B if D .
C .
D .
The first two statements are rules, and the second two statements are
axioms (or facts).
Forward Chaining
Forward chaining inference is very much akin to the way in which
mathematical proofs are presented (but not necessarily arrived at). Using
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