4.19. Implementing Inference and TruthMaintenance

Procedure 4.1. Task

1. Open a set of rules. In this example, a buss pass issuing system will be used:

   rule "Issue Child Bus Pass" when
     $p : Person( age < 16 )
   then
     insert(new ChildBusPass( $p ) );
   end
    
   rule "Issue Adult Bus Pass" when
     $p : Person( age >= 16 )
   then
     insert(new AdultBusPass( $p ) );
   end

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2. Insert the fact insertLogical and add the terms you wish to be inferred.

   rule "Infer Child" when
     $p : Person( age < 16 )
   then
       insertLogical( new IsChild( $p ) )
   end
   rule "Infer Adult" when
       $p : Person( age >= 16 )
   then
       insertLogical( new IsAdult( $p ) )
   end

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   The fact has been logically inserted. This fact is dependent on the truth of the "when" clause. It means that when the rule becomes false the fact is automatically retracted. This works particularly well as the two rules are mutually exclusive. In the above rules, the IsChild fact is inserted if the child is under 16. It is then automatically retracted if the person is over 16 and the IsAdult fact is inserted.
3. Insert the code to issue the passes. These can also be logically inserted as the TMS supports chaining of logical insertions for a cascading set of retracts.

   rule "Issue Child Bus Pass" when
       $p : Person( )
       IsChild( person == $p )
   then
       insertLogical(new ChildBusPass( $p ) );
   end
    
   rule "Issue Adult Bus Pass" when
       $p : Person( age >= 16 )
       IsAdult( person =$p )
   then
       insertLogical(new AdultBusPass( $p ) );
   end

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   Now when the person changes from being 15 to 16, not only is the IsChild fact automatically retracted, so is the person's ChildBusPass fact.
4. Insert the 'not' conditional element to handle notifications. (In this situation, a request for the returning of the pass.) When the TMS automatically retracts the ChildBusPass object, this rule triggers and sends a request to the person:

   rule "Return ChildBusPass Request "when
     $p : Person( )
          not( ChildBusPass( person == $p ) )
   then
       requestChildBusPass( $p );
   end

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