[rules-dev] A remarkable flop

[Wolfgang Laun]

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
>
>
>
>
>
>
>
>
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