[rules-users] Insert new facts in WorkingMemory event handler

[Bruno Freudensprung]

Hi Michael,

Thanks for you answer! Following you suggestion I've taken a look at at 
"logical insertions" and on one hand I have to say that it looks like 
what I need, but on the other hand it is not clear to me how to 
"implement it".

If you have some time to take a look, here are the objects in my session:
- one T object (something that can be considered as a "type" object)
- many I objects (something that can be considered as "instance" 
objects), I objects have one "type" field beeing a T (there is no link 
from T to Is)

Those facts are inserted into the session and I have the impression that 
we are not speaking here of logical insertions: they have to be here, it 
is the input data the user has to write rules on.
The user can decide to retract the T object: in this case all I objects 
have to be retracted as well.
The user can decide to retract some I objects: when all of them have 
been retracted, the T object should be retracted as well.

The first "solution" I imagined was to have high-salience rules:

rule "1"
when
       t : T
       not I(type == t)
then
       retract t

rule "2"
when
       i : I(t : type)
       not Type (this == t)
then
       retract i

The other solution I imagined (that I have not tried) was to retract 
facts using a WorkingMemoryEventHandler.
However as I am a Drools' absolute beginner (not only to Drools, but to 
the rule paradigm itself), I was wondering what is the "most natural 
way" for you guys to implement that behavior :-).

Bruno.


Michael Anstis a écrit :
> Would the other retractions be automatically provided for by Drools' 
> Truth Maintenance?
>
> Otherwise, passing a StatefulKnowledgeSession to a 
> WorkingMemoryEventHandler subclass and invoking it's methods should, 
> IMO, not be an issue.
>
> Have you tried it and experienced issues?
>
> On 6 December 2010 09:32, Bruno Freudensprung 
> <bruno.freudensprung at temis.com <mailto:bruno.freudensprung at temis.com>> 
> wrote:
>
>     Hi everyone,
>
>     Do you know if it is possible to insert/modify/retract facts in a
>     working memory event handler?
>
>     Here is my use case actually: I want my users to be able to write
>     rules
>     like "When ... Then retract X" without having to bother with the
>     consequences of  retracting X on the other objects of the sessions
>     (some
>     of them might be retracted as well). These consequences can be
>     expressed
>     by rules, but I would like them to be kind of "part of the framework"
>     and it would be easier for users if those rules were implicitly
>     defined.
>
>     Many thanks in advance for any hint,
>     Best regards,
>
>     Bruno.
>
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