From: pat hayes ([email protected])
Date: 06/05/01
DAML+OIL Rules
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There are two somewhat different ideas of what a rules langauge
should be like/for, and Pat and Stefan have been thrashing them out
and arguing about the distinctions between them. This attempts to be
a summary of what we talked about.
One idea (call this RULES4DAML) is that the rules should be thought
of as Horn clauses which preserve validity wrt DAML semantics or
maybe some extension of current DAML semantics. This seems likely to
lead to languages similar to CARIN (Rousset and Levy, 1996). (which
Pat was ignorant of when he wrote the earlier strawman proposal, and
which he agrees needs careful study.)
Another idea (call this GENRULES) is a more general-purpose tool
which is designed to make it easier to write operations on RDF
Kbases. This would also use Horn-clause form, but for pragmatic
rather than DAML-semantic reasons (ease of rule specification, good
assertional programming practice, etc..). This would be immediately
useful and be applicable to any piece of RDF, and in particular could
be used to implement RULES4DAML, but also (for example) the kind of
transformations into circle-and-arrows graphical forms that Dan
Connolly mentioned a few days ago.
GENRULES would be simpler to specify than RULES4DAML. In an ideal
world, it might be possible to specify RULES4DAML as a sublanguage of
GENRULES (they both would be Horn clause languages), but in the
immediate short term they should probably be developed seperately.
Source Identifiers and Skolem Functions
-----------------------------------------------------------
Since RDF data is distributed and different RDF source have different
properties
it is necessary to be able distinguish between RDF data coming from different
sources. Source or Context identifiers as a building block in the
DAML rule language
allow to based inferencing based on different sources of RDF data.
A slight generalization of Context identifiers to skolem functions allows to
construct parameterized RDF models.
Negations and monotonicity
-------------------------------------
RULES4DAML should probably have a strictly monotonic semantics. (?)
On the other hand, an effective Horn logic programming language (like
GENRULES wants to be) may well want to work with a closed-world
assumption and negation by failure, for efficiency reasons. It could
also then utilize the the Lloyd-Topor transformation which allows to
have expressive (FOL) rule bodies which can be compiled to ordinary
horn rules with a CWA based negation.
All of which suggests we need a way to temporarily operate with a CWA
and report the results to a larger context in a monotonic framework.
Source identifiers as context labels may provide a way to do this
cleanly. (This connects with some recent discussions on RDFLogic from
Lynn, Sandro and others, BTW.)
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