From: pat hayes (phayes@ai.uwf.edu)
Date: 06/05/01
DAML+OIL Rules ------------------------------------ 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.) -------------- --------------------------------------------------------------------- IHMC (850)434 8903 home 40 South Alcaniz St. (850)202 4416 office Pensacola, FL 32501 (850)202 4440 fax phayes@ai.uwf.edu http://www.coginst.uwf.edu/~phayes
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