Re: comment on RIF-PRD

Dear Wolfgang,

Wolfgang Laun <wolfgang.laun@gmail.com> wrote on 10/12/2009 13:38:00:
> 
> thank you very much for your reply. to summarize:
>  - your argument against "no-loop" is not well substantiated;

It is, indeed, interesting that Jess and Drools agree that "no-loop" 
should be defined as "inhibit the creation of an activation 
A:<R,F1,F2,...> during the execution of R's consequence". (Note that the 
Jess/Drools definition does not prevent looping should R activate R' and 
then R' activate R.)

However, JRules and other major engines do not agree, and we could not 
find a suitable definition that was shared by all the production rule 
systems we looked at. As a consequence, we made two changes in the 
specification:
* Modify is now a compound action, that can always be replaced by one or 
more Retract followed by an Assert;
* The default conflict resolution strategy rif:forwardChaining takes now 
into account the intermediate states of the fact base after each atomic 
action (rather than only after the complete execution of each rule 
instance).

As a consequence, to answer to your initial question (in [1]), the 
rif:forwardChaining strategy, that is defined as the minimal conflict 
resolution strategy that any RIF-PRD implementation must support, does not 
support the no-loop feature.

However, the RIF dialects are meant to be extended, and we expect that 
more elaborate conflict resolution strategies will be specified. 
Discussions regarding proposed extensions are welcome on the RIF 
developers mailing list (mailto:public-rif-dev@w3.org).

[1] 
http://lists.w3.org/Archives/Public/public-rif-comments/2009Aug/0000.html

>  - I'm still not convinced that "forall" is a well-chosen keyword;

We certainly agree, regarding RIF-PRD alone.

However, there is a non-trivial intersection of production rules and logic 
rules - called RIF Core - where the syntax and semantics of the two kinds 
of rules coincide.

Clearly for Core (and for the dialects that extend it on the logic side, 
like RIF-BLD), use of "forall" makes logical sense. We chose not to 
introduce a different keyword for the non-Core production rules, even if 
"forall" makes less intuitive sense in some cases.

>  - we seem to agree on the importance of being able to define "class
> [relationship] assertions".

The working group has published a first public working draft of a document 
that specifies how RIF can be combined with XML (data and or schema) [2]. 
We would welcome your comments on that draft.

[2] http://www.w3.org/TR/rif-xml-data/

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Cheers,

The RIF WG
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Christian de Sainte Marie

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