[RIF][RAF] Production rules and Horn rules

> - Roadmap
>
> ACTION: Frank to make his point about semantics of production rules on
> the mailing list
>
> ACTION: Harold to explain technically the basis
> for interoperation between PR and Horn rules
 
Basic interoperation could be permitted on top of scopes (modules) of sets
of facts used 'read-only' by Horn rules and 'read-write' by production rules.
 
Production rules were formally explored in Active Database Systems,
e.g. by Jennifer Widom and colleagues:
http://infolab.stanford.edu/~widom/pubs.html
 
J. Widom and S.J. Finkelstein. Set-Oriented Production Rules in Relational Database Systems <http://infolab.stanford.edu/pub/papers/rule-language.ps> . Proceedings of the ACM SIGMOD International Conference on Management of Data, pages 259-270, Atlantic City, New Jersey, May 1990. 
http://citeseer.ist.psu.edu/cache/papers/cs/558/http:zSzzSzwww-db.stanford.eduzSzpubzSzpaperszSzrule-language.pdf/widom90setoriented.pdf
 
J. Widom. A Denotational Semantics for the Starburst Production Rule Language <http://infolab.stanford.edu/pub/papers/denotational.ps> . ACM SIGMOD Record, 21(3):4-9, September 1992.
http://citeseer.ist.psu.edu/cache/papers/cs/558/http:zSzzSzwww-db.stanford.eduzSzpubzSzpaperszSzdenotational.pdf/widom92denotational.pdf

The interoperation of Horn rules and production rules themselves could start
with an expressive intersection of these two classes of rules. This may consist
of an appropriate subset of -- syntactically sugared -- Horn rules (e.g., slotted
Datalog) that also is a -- small -- subset of production rules (e.g., just having
'look-up' conditions and 'assert' actions). It is called 'pure production rules' in
the Preliminary Design Roadmap, and moved to Phase 1, #4 for early studies:
http://lists.w3.org/Archives/Public/public-rif-wg/2006Feb/0255.html
(In the Charter, production rules were only in Phase 2, 3.2.5. Actions.)
 
A benefit is that any extension we consider for the Phase 1 Horn conditions
(e.g., NAF queries or built-in calls) could already be assessed for its merits
in conditions of  'full production rules' in Phase 2. A cross-fertilization may
take place between (Web) extensions of the two kinds of rules.
 
Frank's concern that the intersection is too small etc. should be carefully
considered, first, by working on a taxonomy of desirable RIF actions.
 
-- Harold