[JBoss JIRA] (DROOLS-365) Rule attribute for adding inverse of consequence as rule condition

Saturday, 7 December 2013

    [
https://issues.jboss.org/browse/DROOLS-365?page=com.atlassian.jira.plugin...
] 

Davide Sottara commented on DROOLS-365:
---------------------------------------

I see your use case. Here is another possible way to do it. If you like it, we may work on
getting it into master by default.
We need the notion of "Undercutting", borrowed from non-monotonic logic. This is
the core idea, to be expanded on as needed.
Imagine you have two rules potentially triggered by the same object(s), and you want one
to take precedence over the other.
Use an annotation like @Undercuts in the first, passing the name of the other rule as an
argument.

{code}
rule "handle all Bars of type second"
@Undercuts( others )
  when
    $bar : Bar( type == 'second', $total : total )
  then
    System.out.println( "Second bars " + $total );
end

rule "others"
  when
    $bar : Bar( $total : total )
  then
    System.out.println( "Other bars " + $total );
end
{code}

In the main, enable the declarative agenda:
{code}
knowledgeBaseConfiguration.setOption( DeclarativeAgendaOption.ENABLED );
{code}

And finally add a "utility" rule. This rule is generic, but its purpose is to
enable the desired functionality:

{code}
rule "Undercut"
salience 9999   // should have max priority
when
    $m : Match( $handles : factHandles )
    $v : Match( rule.name == $m.Undercuts, factHandles == $handles )
then
    System.out.println( "Activation of rule " + $m.getRule().getName() + "
overrides " + $v.getRule().getName() + " for tuple " + $handles );
    kcontext.cancelMatch( $v );
end
{code}

Let me know if this would be enough for your concrete use case.
Davide

...
 Rule attribute for adding inverse of consequence as rule condition
 ------------------------------------------------------------------

                 Key: DROOLS-365
                 URL: https://issues.jboss.org/browse/DROOLS-365
             Project: Drools
          Issue Type: Feature Request
      Security Level: Public(Everyone can see) 
    Affects Versions: 5.5.0.Final
            Reporter: Adar Dembo
            Assignee: Mark Proctor

 I have quite a few rules that look like this:
 {noformat}
 rule "if foo then bar"
   when
     Foo($a : a, $b : b)
     not Bar(a == $a, b == $b)
   then
     insert(new Bar($a, $b));
 end
 {noformat}
 This is a fairly common pattern for me: a rule should not activate if the facts generated
by its consequence were already asserted. It's a pattern I use extensively to generate
linear (i.e. non-repeating) workflows.
 Ideally I'd be able to express it as follows:
 {noformat}
 rule "if foo then bar"
 condition-on-inverse-of-consequence
   when
     Foo($a : a, $b : b)
   then
     insert(new Bar($a, $b));
 end
 {noformat}
 And {{condition-on-inverse-of-consequence}} would cause Drools to generate the right kind
of condition, which would be the inverse of the fact changes made by the consequence.
 I think {{lock-on-active}} is similar but harsher: it prevents the rule from firing a
second time altogether, regardless of input. At least, that's based on my reading of
the docs. In my example, that would be equivalent to  {{not Bar()}} condition in the rule,
which is stricter than {{not Bar(a == $a, b == $b)}}. 
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