Copyright © 2005 Retained by the Authors
This paper discusses the interoperability between RDF and RuleML regarding webizing, typing, blank nodes, two negations, as well as F-logic/TRIPLE rules and SWRL rules.
This document is a position paper as required for participation to the W3C Workshop on Rule Languages for Interoperability.
It represents the opinion of its authors.
1 Introduction
2 RDFizing RuleML
2.1 RDF Triples and Rules as RuleML
Sublanguages
2.2 RDF-Style Webizing of RuleML
2.3 RDF Types and RDF Schema in RuleML
2.4 Blank Nodes as Skolem Constants
3 Two Kinds of Negation in RDF/RuleML
4 F-logic And TRIPLE Rules in RDF/RuleML
5 SWRL Rules in RDF/RuleML
6 Conclusions
7 References
Keeping positional Horn logic as one of its sublanguages, and even adding FOL, RuleML has been developed increasingly closer towards RDF, with an eye also on F-logic and Desription logic. This is already reflected by RuleML/RDF tools and a RuleML/N3 alignment. The newest RuleML release as described here will further simplify RDF interoperability regarding webizing, typing, and blank nodes. It also integrates two negations, as required for the Semantic Web. On this basis, F-logic/TRIPLE rules and SWRL rules are being introduced.
At the RDF Interest Group face to face meeting in Feb 2001 the Rule Markup Initiative was presented with a focus on RDF Relationships and DTD Modularization. This section shows the RDFizing of RuleML, using the new RuleML XML syntax.
Before Object-Oriented (OO) RuleML became available, the family of
sublanguages of RuleML had to use (subject,predicate,object)
triples for representing RDF as ground (variablefree) binary atoms
predicate(subject,object)
, where predicate
and
subject
needed to be webized and object
was
optionally webized. RDF rules could then be represented by allowing variables
inside binary atoms.
As will be illustrated in section 2.2, the slots of OO RuleML have permitted to express 'subject-centered' RDF descriptions without duplicating a subject for every triple it participates in. These descriptions are anchored in the Web through URIs. To integrate RDF more deeply with other aspects of RuleML, and to reduce the number of sublanguages, the binary atoms are no longer distinguished in the newest RuleML.
RuleML allows an individual constant (Ind
) to contain web
labels and web references, leading to the following RDF correspondence:
The subject of an RDF triple or the about
attribute in an rdf:Description
becomes a RuleML role
tag called oid
(object identifier) containing an
Ind
with a wlab
(web label) attribute.
An RDF predicate or property becomes the first
child of a RuleML slot
, which is an Ind
with a
wref
(web reference) attribute.
An RDF object becomes the second child of a RuleML
slot
, which for an RDF literal is a RuleML Data
element and for an RDF resource
again is a
wref
-attributed Ind
.
Illustrating the general correspondence (omitting namespace declarations):
<rdf:Description about="http://www.w3.org"> <dc:title>World Wide Web Consortium</dc:title> <eg:director rdf:resource="http://www.w3.org/People/Berners-Lee"/> </rdf:Description>
<Atom> <oid><Ind wlab="http://www.w3.org"/></oid> <slot><Ind wref="dc:title"/><Data>World Wide Web Consortium</Data></slot> <slot><Ind wref="eg:director"/><Ind wref="http://www.w3.org/People/Berners-Lee"/></slot> </Atom>
The rdf:type
of a resource in RuleML is regarded as the
relation of its describing atom. Again via a web reference it points
to an RDF Schema definition.
Illustrating the type-relation correspondence (omitting namespace declarations):
<rdf:Description about="http://www.w3.org"> <rdf:type resource="http://schema.org/StandardsBody"/> <dc:title>World Wide Web Consortium</dc:title> <eg:director rdf:resource="http://www.w3.org/People/Berners-Lee"/> </rdf:Description>
<Atom> <oid><Ind wlab="http://www.w3.org"/></oid> <opr><Rel wref="http://schema.org/StandardsBody"/></opr> <slot><Ind wref="dc:title"/><Data>World Wide Web Consortium</Data></slot> <slot><Ind wref="eg:director"/><Ind wref="http://www.w3.org/People/Berners-Lee"/></slot> </Atom>
For multiple types of a resource, an atom with multiple relations can be used, which provides a useful shortcut even in the absence of an oid.
Blank Nodes have recently been introduced to RuleML for complete RDFizing:
Skolem constants are marked up as <Skolem/>
(anonymous)
or <Skolem>
...</Skolem>
(named), and
adopt the Yang/Kifer model theory [5].
While the previous section showed how RuleML caught up with RDF expressiveness, the current section discusses RuleML sublanguages also proposed as RDF extensions.
In his visionary article [2], Tim Berners-Lee identified two fundamental issues for the Semantic Web:
It is disappointing, therefore, that neither in RDF nor in OWL these issues have been addressed so far. RDF does not even have any kind of negation, while OWL has a limited form of negation only. Neither the current RDF nor OWL are able to deal with Closed-World assumptions, nor do they have any feature for handling inconsistency. In [3], Tim Berners-Lee says: "We remove the centralized concepts of absolute truth, total knowledge, and total provability, and see what we can do with limited knowledge". This is exactly what has been done in RuleML by providing two kinds of negation on the basis of partial logic. Unlike classical logic, which is the basis of OWL, partial logic allows to handle truth value gaps (partiality) and truth value clashes (inconsistency).
RuleML includes the computational construct of negation-as-failure (NAF) for expressing local completeness (Closed-World) assumptions. And it includes a more conservative negation connective (NEG), also called strong negation, which corresponds to Kleene negation in 3-valued logic or to Boolean negation, depending on the semantic nature of the predicate beeing negated. With these computational logic facilities, RuleML supports more real-world knowledge representation tasks than RDF and OWL, which are based on the highly idealized classical (2-valued) logic.
The following is an example of a rule using both kinds of negation. Here, the predicate "requiresService" does not allow Closed-World inference, while the predicate "is AssignedToRentalContract" does allow it, and, hence, the former is negated with NEG and the latter with NAF.
<Implies> <body>
<And> <Atom> <Rel>RentalCar</Rel> <Var>Car</Var> </Atom> <Neg> <Atom> <Rel>requiresService</Rel> <Var>Car</Var> </Atom> </Neg> <Naf> <Atom> <Rel>isAssignedToRentalContract</Rel> <Var>Car</Var> </Atom> </Naf> </And> </body>
<head>
<Atom>
<Rel>isAvailable</Rel>
<Var>Car</Var>
</Atom>
</head> </Implies>
These semantics are reflected by RuleML sublanguage schemas with NAF, NEG, and both. RDF can be extended by adding both forms of negation, too, as has been shown in [1].
With the advent of OO RuleML [4], molecules, the most fundamental feature of F-logic (and its early RDF-compliant version, SiLRI) and TRIPLE, became representable.
For TRIPLE's model (or view) feature, which allows RDF statements
to be "@
"-associated with a name
(similar to contexts in Cwm or
named graphs), a RuleML module
role
(on Atom
, Implies
, etc.) is proposed in analogy to the
oid
role, which can contain an Ind
ividual.
The following example shows the proposed RuleML serialization for a TRIPLE molecule about W3C in a module context (omitting namespace declarations):
'http://www.w3.org':''[rdf:type -> s:StandardsBody; dc:title -> "World Wide Web Consortium"; eg:director -> p:'Berners-Lee']@c:mod1. <Atom> <module><Ind wref="c:mod1"/></module> <oid><Ind wlab="http://www.w3.org"/></oid> <opr><Rel wref="s:StandardsBody"/></opr> <slot><Ind wref="dc:title"/><Data>World Wide Web Consortium</Data></slot> <slot><Ind wref="eg:director"/><Ind wref="p:Berners-Lee"/></slot> </Atom>
Paramterized TRIPLE
views, which proved to be very useful for representing
ontology mappings, semantics definitions, etc., can
then be serialized in RuleML with a module
role that contains
a C
omplex term
.
TRIPLE/RuleML rules can be built from such Atom
s in
the head and body, and also employ the explicit quantifiers Forall
and Exists
introduced in FOL RuleML.
In the SWRL submission to W3C an RDF Schema for SWRL (swrl.rdf) was published to provide a partial description of the syntax of SWRL. A corresponding RuleML version has recently been provided as swrl.ruleml (with a KQML-like top-level performative 'Assert' ).
SWRL rules in OWL RDF/XML syntax are interoperable with an RDFized RuleML version, where, e.g., the IndividualPropertyAtom element corresponds to a normal Atom element with a swrl:IndividualPropertyAtom attribute.
Illustrating SWRL Example 6.1-1 in RDFized RuleML (omitting namespace declarations):
<Implies> <oid><Ind wlab="http://www.w3.org/Submission/2004/SUBM-SWRL-20040521/#Example6.1-1"/></oid>
<body>
<And>
<Atom>
<opr><Rel wref="swrl:IndividualPropertyAtom"/></opr>
<slot><Ind wref="swrl:propertyPredicate"/><Ind wref="eg:hasParent"/></slot>
<slot><Ind wref="swrl:argument1"/><Var wref="eg:x1"/></slot>
<slot><Ind wref="swrl:argument2"/><Var wref="eg:x2"/></slot>
</Atom>
<Atom>
<opr><Rel wref="swrl:IndividualPropertyAtom"/></opr>
<slot><Ind wref="swrl:propertyPredicate"/><Ind wref="eg:hasSibling"/></slot>
<slot><Ind wref="swrl:argument1"/><Var wref="eg:x2"/></slot>
<slot><Ind wref="swrl:argument2"/><Var wref="eg:x3"/></slot>
</Atom>
<Atom>
<opr><Rel wref="swrl:IndividualPropertyAtom"/></opr>
<slot><Ind wref="swrl:propertyPredicate"/><Ind wref="eg:hasSex"/></slot>
<slot><Ind wref="swrl:argument1"/><Var wref="eg:x3"/></slot>
<slot><Ind wref="swrl:argument2"/><Ind wref="eg:male"/></slot>
</Atom>
</And>
</body>
<head>
<Atom>
<opr><Rel wref="swrl:IndividualPropertyAtom"/></opr>
<slot><Ind wref="swrl:propertyPredicate"/><Ind wref="eg:hasUncle"/></slot>
<slot><Ind wref="swrl:argument1"/><Var wref="eg:x1"/></slot>
<slot><Ind wref="swrl:argument2"/><Var wref="eg:x3"/></slot>
</Atom>
</head> </Implies>
This paper demonstrates the high level of RDF/RuleML interoperability achieved along with extensions in close reach from RDF and RuleML.
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2. T. Berners-Lee. Design issues - architectual and philosophical points. Personal notes, 1998. Available at http://www.w3.org/DesignIssues/.
3. T. Berners-Lee. What the semantic web can represent. Personal notes, 1998. Available at http://www.w3.org/DesignIssues/RDFnot.html.
4. H. Boley, B. Grosof, M. Kifer, M. Sintek, S. Tabet, G. Wagner. Object-Oriented RuleML, 2004. Available at http://www.ruleml.org/indoo.
5. G. Yang and M. Kifer. Reasoning about Anonymous Resources and Meta Statements on the Semantic Web. Journal of Data Semantics, 2004.