According to MVEL they’ve added a convenience for checking the “emptiness” of a value (see: http://mvel.codehaus.org/Value+Tests )

 

In MVEL you’d simply write:  myMap == empty

 

This seemed like a MUCH cleaner way to write some of the LHS of the rules, so I thought I’d try it out. 

 

Given the following rule:

rule "Check Empties - Original"

    when        

        $grabBag : MyGrabBag(someMap != null)

        eval ($grabBag.getSomeMap().size() == 0)

    then

end

 

This could be rewritten:

 

rule "Check Empties - Simple"

    when        

        MyGrabBag(someMap == empty)

    then

end

 

The IDE (Eclipse 3.4.2 w/ Drools plugin) seems to recognize “empty”  is a keyword (it makes it bold), but the drools compiler in the IDE  reports:
BuildError: Unable to create restriction '[VariableRestriction: == empty ]' for field 'someMap' in the rule 'Check Empties - Simple'

BuildError: Unable to return Declaration for identifier 'empty'

 

Am I assuming incorrectly that the LHS is fully MVEL compliant?

 

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