Re: [rules-users] "from" keyword does not actually check types?
Hi Jevon,
That is a discussion we had for some time (whether "from" should check
return type or not for matching purposes). The only reason it was not
implemented so far is that it would possibly cause "silent failures".
Example:
Person( name == "bob" ) from $people
If $people returns an Alien() and the check is in place, the rule will
not match and the user might not notice the reason, as the way it is today,
it will raise a ClassCastException.
Although, in my opinion, the advantages of the check surpass the "silent
failure" problem and we should add this check, specially because sometimes
you have a collection of objects that share a common superclass, but with
different actual subclasses instances, and you want to operate only on a
give subclass.
Anyway, I will talk with Mark and possibly add this to Drools 5.1.
Regarding your keyword usage issue, I added a comment to it:
https://jira.jboss.org/jira/browse/JBRULES-2218
Regards,
Edson
2010/2/24 Jevon Wright <jevon(a)jevon.org>
Hi,
I am using Drools 4.0.7, and I had a DRL rule similar to the following:
rule "..."
when
other : Foo( )
x : Bar( ) from other.property
then
...
end
An unexpected bug occured. In some situations, other.property would _not_
be of type Bar, yet Drools would still think that it was. As a result I was
getting some unusual exceptions being thrown: "org.mvel.CompileException:
unable to resolve property: ..."
If I changed the rule to:
x : Bar( ) from other.property
eval ( x instanceof Bar )
It would instead throw a ClassCastException (i.e. "Baz cannot be cast into
Bar").
If, instead, I changed the rule to:
x : Bar( other.property == x )
It would start working correctly, and 'x' would only be of type Bar.
Is this known behaviour? Is it expected? If so, I would appreciate it if
the 4.0.7 documentation was updated to reflect that "from" actually ignores
the return type of properties. It took me a couple of hours to detect and
fix this bug.
I would try these rules in Drools 5.x, but I'm still waiting for the
blocking JBRULES-2218 to be resolved.
Cheers
Jevon
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Edson Tirelli
JBoss Drools Core Development
JBoss by Red Hat @ www.jboss.com
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