Allen,
Both your examples must
work just fine:
$b : Y(running == true)
// attempting to find all
applicants named Bob
$p : Person(firstName == "Bob")
$ap :
Applicant(person == $p)
I can't think about a reason
for them to not work. If they aren't, can you please provide a self contained
test case for us to evaluate?
Answering your question,
in Drools 4, assuming your facts are beans, there should be almost no reason for
you to fall back into an eval. The usage of eval is usually related to the need
for calculation inside a constraint or the call of
functions.
[]s
Edson
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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Edson Tirelli
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