2008/4/24 Bagwell, Allen F <afbagwe@sandia.gov>:

I'm learning how to use Drools, but eval presents a problem to me when I am composing rules. I've read the manual and have a basic understanding of what it is for (very useful in certain situations!), but is there a better explanation of when it cannot be avoided?

For example when writing the LHS portion of a rule, I've run into a problem where literal restrictions alone are fine for one object but inadequate for another.  Like so:

$a : X(temp < 100)

$b : Y(running == true)

In both cases class X and class Y have standard Java bean set up for these fields with appropriate getters and setters, and the corresponding inserted facts see these fields being updated from time to time. But whereas the first fact causes rule activation when the temp field meets the rule critiera, the second will never work unless it is re-written as:

$b : Y()
eval ($b.isRunning() == true)


I've encountered a similar problem with in-line evals where Drools will accept an LHS like this:

// attempting to find all applicants named Bob
$p : Person(firstName == "Bob")
$ap : Applicant(person == $p)

but it will never cause activation unless you re-write it like this:

$p : Person(firstName == "Bob")
$ap : Applicant( eval(person == $p) )

despite the fact that the Applicant object's person field once set never changes.

So is there a more definitive explanation as to why one is forced to use eval when you would think that simple literal restrictions would be enough?  I've read in other posts that "eval is evil" and to best avoid it unless necessary, but this is perplexing me because I haven't yet discovered the way to think about rule composition that prevents eval use from appearing to be arbitrary. Mostly it's been write what I think *should* work and if it does then great, it if doesn't then keep adding evals until it does. For the record, I'm using the java dialect if that makes a difference.

Thanks!

-Allen





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