Ah, yes, and I'd actually be interested to learn why the indeterminism of
rule firings not only depends on two or more rules matching the fact set but
also on the order and/or number of facts being inserted.
Notice that with three SwitchState facts inserted, id's 1, 2 and 5 match and
fire the "correct" (i.e., expected) rule. Adding a fourth fact with id 4
suddenly fires the match of no. 5 with the "wrong" (i.e., unexpected) rule.
Actually, this means that you cannot rely on tests of rule sets, as you
could if a potentially multiple match between two rules would always first
activate A or always first B.
-W
On 12 December 2010 09:28, Wolfgang Laun <wolfgang.laun at gmail.com> wrote:
> The solution I adopted is to make SwitchState not a subclass of TrackState
> but of ElementState, which means that the field(s) to be inherited from
> TrackState have to be duplicated in SwitchState. (Luckily, here it is only
> one field.)
>
> Adding a type-identifying field goes against the (my) idea of classes for
> identifying categories and using them as an implicit constraint in rules,
> making them more readable. That's one of the benefits of having an OO RBS,
> isn't it?
>
> Anyway, I just wanted to draw people's attention to this hidden trap;
> working around it is no problem once you know it is there.
>
> -W
>
>
>
> On 12 December 2010 05:06, Greg Barton <greg_barton at yahoo.com> wrote:
>
>> Well, seeing as you're already maintaining two parallel type hierarchies,
>> and it's a mismatch between the two that's causing the problem, you might as
>> well use that in the rules.
>>
>> rule "Xyz state update"
>> when
>> $s: XyzState( $id: id, $type: type ... )
>> $e: Xyz( id == $id, type == $type )
>>
>> then
>> //...update $e, retract $s
>> end
>>
>> rule "Xyz state for unknown element"
>> when
>> $s: XyzState( $id: id, $type: type )
>> not Xyz( id == $id, type == $type )
>> then
>> //... diagnostic, retract $s
>> end
>>
>> That oughter do it, and it avoids salience.
>> --- On *Sat, 12/11/10, Wolfgang Laun <wolfgang.laun at gmail.com>* wrote:
>>
>>
>> From: Wolfgang Laun <wolfgang.laun at gmail.com>
>> Subject: [rules-dev] A remarkable flop
>> To: "Rules Dev List" <rules-dev at lists.jboss.org>
>> Date: Saturday, December 11, 2010, 10:23 AM
>>
>>
>>
>> Given classes Switch and Track, and the "natural" class hierarchy
>> representing state changes for these two elements:
>> ElementState
>> TrackState extends ElementState ("free/occupied")
>> SwitchState extends TrackState (adds "left/right/moving")
>> Now we have rules for updating Switch and Track elements, like this one:
>> rule "Xyz state update"
>> when
>> $s: XyzState( $id: id, ... )
>> $e: Xyz( id == $id )
>> then
>> //...update $e, retract $s
>> end
>> and, since we want to catch bad ids, also
>> rule "Xyz state for unknown element"
>> when
>> $s: XyzState( $id: id )
>> not Xyz( id == $id )
>> then
>> //... diagnostic, retract $s
>> end
>>
>> Best practice, wouldn't you say?
>>
>> Testing by inserting a few of SwitchState objects works fine:
>> updated: Switch 5 RIGHT occupied
>> updated: Switch 2 RIGHT
>> updated: Switch 1 RIGHT occupied
>> so everything is allright, wouldn't you say?
>>
>> Add another SwitchState for Switch "4" to the test, and suddenly:
>> updated: Switch 4 RIGHT
>> track state for unknown element 5
>> updated: Switch 2 RIGHT
>> updated: Switch 1 RIGHT occupied
>> What's this?!
>>
>> After some headscratching I realized that the negative rule for the
>> SwitchState's superclass TrackState produces another activation, since, for
>> any Switch element with an id x there clearly isn't a Track element with id
>> x! This activation *occasionally *precedes the activation for the match
>> in the "update" rule.
>>
>> This is annoying. Of course, negative salience for the "not" rules fixes
>> this, but who would have thought that you need it with conditions
>> (seemingly!) describing disjoint situations. And the negative salience is
>> counter-intuitive, since normally you'd perform the check "no such element"
>> *before *permitting any update action.
>>
>> Remarkable.
>> Wolfgang
>>
>>
>>
>>
>>
>>
>>
>>
>> -----Inline Attachment Follows-----
>>
>> _______________________________________________
>> rules-dev mailing list
>> rules-dev at lists.jboss.org<http://mc/compose?to=rules-dev@lists.jboss.org>
>> https://lists.jboss.org/mailman/listinfo/rules-dev
>>
>>
>>
>> _______________________________________________
>> rules-dev mailing list
>> rules-dev at lists.jboss.org
>> https://lists.jboss.org/mailman/listinfo/rules-dev
>>
>>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.jboss.org/pipermail/rules-dev/attachments/20101212/9348627a/attachment-0001.html