deSkolemizing to get Horn-expressivity with RDF rules

From: Sandro Hawke (
Date: 11/12/02

  • Next message: Benjamin Grosof: "Re: deSkolemizing to get Horn-expressivity with RDF rules"
    I said I'd write something to the list about this.  
    There's a trick to making RDF rules be Horn-expressive instead of
    merely datalog-expressive with only a slight change in the rules
    language.  We use this trick in cwm/n3, and I used it in some
    unpublished pre-w3c work.  I've mentioned it before (in proposals I
    wouldn't exactly support any more) [1] [2], but I still haven't come
    up with any proper references.
    The idea is this: if an RDF rule is a pair of RDF graphs (the
    body/antecedent and the head/consequent), with certain nodes/arcs in
    the graph marked as variables (just like in DQL), then we only have
    datalog rules.  If, however, we also allow nodes/arcs in the
    consequent to be marked as existential variables in the scope of the
    consequent, we have Horn expressivity.  
    To see why this is so, think about how Skolemizing turns existentials
    into Skolem function terms, and then work it backwards, turning function
    terms into existentials and "deSkolem" predicates.
    This is important to RDF/DAML-Rules, because without this trick,
    people working with RDF/DAML (not having function terms) are likely to
    be stuck unnecessarily datalog.  RDF rules need not be any less
    expressive than Horn rules.
        -- sandro

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