flex Expert System Toolkit - LPIS

3. Forward Chaining 36
The basic cycle of chosing a rule and then firing that rule remains, but with
some elaborations.
1. In the simple model, the engine fires the first rule whose if part is satisfiable. A
more sophisticated approach is to go through all the currently available rules,
and check which ones are satisfiable. This forms a conflict set. Some algorithm
is then used to select a rule to fire from this conflict set (this is called conflict
resolution).
2. In the simple model, the engine terminates only when no more rules can be
selected. In the extended model the user can specify an alternative criterion (a
Prolog call) for early termination. The engine will terminate as soon as the call
succeeds, regardless of whether there are more rules which could be fired.
3. The extended model also allows for the handling of the misfiring of rules. A rule
is said to misfire if its conditions are satisfied but its actions fail for some reason.
A recovery corresponds to the specified misfire program succeeding, in which
case the cycle completes and the engine carries on. Otherwise an error
condition is raised.
4. The final extension to the simple model concerns the rule agenda, from which
rule selections are made during each and every cycle of the engine. This agenda
can be updated at the end of each cycle according to which rule was fired
during that cycle. For example, removing that rule from the agenda means that
rules can only be fired at most once during that particular forward chaining
session. Other possibilities include re-ordering the list of rules thus making it
more likely that certain rules will be fired next time.
Ruleset
Rules are grouped together into rulesets, and forward chaining is started
using the KSL keywords invoke ruleset. This provides a construct for
forming rules into stratified rule-bases and governing the forward-chaining
engine in terms of rule selection method and agenda updating.
For example, to move a piece in a game we might have a ruleset called
make_move containing rules for possible moves.
ruleset make_move
contains corner_move, edge_move, centre_move .
action move ;
invoke ruleset make_move .
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