flex Expert System Toolkit - LPIS

9. Flex Predicates 128
forward_chain( +Selection,+Misfire,+Termination,+Update,+Agenda )
Initiate a forward chaining session with the given selection algorithm and
rule agenda.
Selection is the name of the algorithm for selecting rules during each
cycle of the forward chaining engine. There are three built-in algorithms:
fcfs
crss
crss(T)
First come first served.
Conflict resolution scoring system.
Conflict resolution with a threshold value.
In addition, a user-defined algorithm can be nominated. For example,
my_selector/3 randomly selects a rule from the agenda.
my_selector( Agenda, Rule, Action) :-
length( Agenda, N ),
Posn is irand( N ) + 1,
mem( Agenda, [Posn], Rule ),
isa_rule( Rule, _, Action, _, _ ) .
If the Selection algorithm fails then the call to forward_chain/5 will
terminate.
Misfire is the name of a program which is executed whenever the action
associated with the selected rule fails.
There are three built-in misfire programs :
true
fail
misfire
Ignore misfires.
The original call to forward_chain/5 will also fail.
Report the misfire and abort the process.
In addition, the name of a user-defined program can be nominated, such as
the following mf/1.
mf( Name ) :-
write( 'Rule has misfired ' - Name ), nl.
Termination is a Prolog call which is tested during each and every cycle
of the forward chaining engine. As soon as the Termination criterion
succeeds the call to forward_chain/5 will terminate.
In particular, if Termination is the atom fail, then the call to
forward_chain/5 will only terminate when a situation arises in which no
rules can be selected.
Agenda and Update together specify the rule agenda, and how it changes
after each cycle of the forward chaining engine.
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