share a variable between rules, use as condition in LHS

[Zorro]

Hi guys,

I am trying to use a way to share a variable at runtime between the rules but without success.

I tried to use a global, but I understand that this is for read-only and I tried to use a fact, but that is always null.

I need to access this "something" in LHS to based my flow on it.

Let me explain what I tried.

The global version

Java code:

a. Loading the rules

   Navigator navigator = new Navigator();

   workingMemory.setGlobal("navigator", navigator);

b. Navigator.java

private String compass;

public String getCompass(){

    return this.compass;

}

public void setCompass(String compass){

    this.compass = compass;

}

....

Drools rules

.......

global Navigator navigator;

rule "Rule1"

when 

       condition 1

then

      navigator.setCompass("west");

end

rule "Rule2"

when 

       eval(navigator.getCompass()=="west")

then

      System.out.println("Go west");

end

Looks like Rule2 never gets executed although "Rule1" is called.

Now the fact version:

Java code:

Loading the rules

   Navigator navigator = new Navigator();

   workingMemory.insert(navigator);

Drools rules

rule "Rule1"

when 

       $navigator: Navigator()

then

      $navigator.setCompass("west");

end

rule "Rule2"

when 

       $navigator: Navigator(compass =="west")

then

      System.out.println("Go west");

end

How can I share at the runtime a variable in LHS between rules?

Thanks,

Z

[Mark Proctor]

Don’t set it as a global, insert it.

The engine will not react to globals.

If you really can’t insert it, for what reason I don’t know, you can do

Navigator() from navigator

Mark

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[Davide Sottara]

Your second version, using facts, seems the right way to do it. However, you can't just call "$navigator.setCompass(..)".. the engine will not be aware of the change and reevaluate the other rules.

You have to do something like modify ( $navigator ) { setCompass(...) }

Beware, though, of rules looping. See the manual for the details on "modify" and/or this blog post for details on how to deal with recursions
http://ilesteban.wordpress.com/2012/11/16/about-drools-and-infinite-execution-loops/

Best
Davide 

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