[rules-users] Calling a rule within a rule

[Swindells, Thomas]

You can't call one rule from another as rule evaluation isn't procedural, when working with rules you are declaring what should happen in different circumstances and it's up to the rules engine to decide how to evaluate it.  One very important thing to remember is that rule evaluation happens in two phases, first all the then parts of the rules are evaluated and matching rules placed on a priority activation stack; only then are the conditions evaluated.  This means in the example you give below rule2 will always be evaluated and could generate a null pointer exception even if the action of rule1 were to terminate the rules execution.

That said there are (at least) three ways you can get the general result you want:
1. Just have a single rule which does both parts of your actions - is there any reason why you want to keep them separate?
2. As you have said just duplicate the conditions, depending on how you are generating your rules this may not be a big issue.
3. Use a customer helper object "ConditionMatched(conditionName)", have the rule with the conditions use insertLogical to insert a new ConditionMatched object with the desired name.  Other rules can then match against the existence of the ConditionMatched object rather than duplicating all the logic.  InsertLogical has the nice property that if the rule which triggered it becomes false then the object is automatically retracted again.

Thomas

> -----Original Message-----
> From: rules-users-bounces at lists.jboss.org [mailto:rules-users-
> bounces at lists.jboss.org] On Behalf Of Dinesh kumar
> Sent: 02 June 2010 02:30
> To: rules-users at lists.jboss.org
> Subject: [rules-users] Calling a rule within a rule
>
>
> Hi All,
>
> I am new to Drools and I need a help.
>
> Is it possible to call a rule within a rule.
> i.e., only when rule1 condition is true , my rule2 should execute
>
> rule1
> when a!=null
> then
> ...
>
> rule2
> when a.toString() !=""
> then
> ...
>
> Though i can write both the conditions in the same rule, its not a feasible
> solution as my business rules are much complicated. I am expecting to write
> something like this..
>
>
> rule1
> when a!=null
> then
>
> when a.toString() !=""
> then
> ...
>
> end
> end
>
> But this is not working out. It would be great if someone can help me out..
> Also I understood that variables cannot be declared in guvnor.. Is this
> true..? If true then what is an alternative. Kindly help me
>
>
>
>
>
>
>
>
> Regards,
> Dinesh
> --
> View this message in context: http://drools-java-rules-
> engine.46999.n3.nabble.com/Calling-a-rule-within-a-rule-tp863409p863409.html
> Sent from the Drools - User mailing list archive at Nabble.com.
> _______________________________________________
> rules-users mailing list
> rules-users at lists.jboss.org
> https://lists.jboss.org/mailman/listinfo/rules-users


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