This document contains brief descriptions of the relationship
between WRL and other selected relevant technologies.
This document is a part of the WRL Submission.
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RDFS [RDFS] is a simple ontology
modeling language based on triples. It allows to express
classes, properties, class hierarchies, property hierarchies, and
domain- and range restrictions. Several proposals for more
expressive Semantic Web and Semantic Web Service descriptions
extend RDFS, however there are difficulties in semantically
layering an ontology language on top of RDFS:
- RDFS allows the use of the language vocabulary as subjects
and objects in the language itself.
- RDFS allows the use of the same identifier to occur at the
same time in place of a class, individual, and property
identifier.
We believe that the number of use cases for the first feature,
namely the use of language constructs in the language itself, is
limited. However, the use of the same identifier as class,
individual and property identifier (also called meta-modeling) is
useful in many cases. WRL does not allow the use of the language
constructs in arbitrary places in an ontology, but does allow
meta-modeling in its Flight and Full variants.
WRL layers on a significant part of RDFS. WRL does not allow the
use of language constructs in the language itself and does not allow
full treatment of blank nodes, because this would require reasoning
with existential information, which is typically not possible in the
context of standard rule languages. WRL provides a significant extension of
RDFS through the possibility of specifying local attributes, range
and cardinality constraints for attributes and attribute features
such as symmetry, transitivity and reflexivity. Furthermore, WRL
provides an expressive rule language which can be used for the
manipulation of RDF data.
The Web Ontology Language OWL [OWL] is
a language for modeling ontologies based on the Description Logic
paradigm. OWL consists of three species, namely OWL Lite, OWL DL
and OWL Full, which are layered according to
increasing expressiveness. OWL Lite is a notational variant of
the Description Logic SHIF(D); OWL DL is a notational
variant of the Description logic SHOIN(D) [Horrocks et
al., 2003]. OWL Full layers both on top of OWL DL and RDFS and
is a direct extension of RDFS.
WRL-Core is a semantic subset of OWL Lite, in the sense that
WRL-Core and a subset of OWL Lite agree on ground
entailment. However, there is a major
difference between ontology modeling in WRL and ontology
modeling in OWL. WRL uses an epistemology which abstracts from the
underlying logical language, whereas OWL directly uses Description
Logics epistemology; WRL separates between conceptual modeling
for the non-expert users and logical modeling for the expert user.
Figure 1: The roles of OWL and WRL in the Semantic Web
Layer Cake
OWL and WRL are two complementary
ontology languages for the Semantic Web, as depicted in Figure
1. (The DL species of) OWL
is based on Description Logics, whereas WRL is based on Deductive
Databases and Logic Programming. WRL and OWL support different use
cases for the Semantic Web. A strong point of OWL is subsumption
reasoning over conceptual
schemas (ontologies), whereas WRL focuses on consistency checking sets of
data (with respect to constraints) and for the specification of and
reasoning with arbitrary rules.
The Semantic Web Rule Language [SWRL]
is an extension of OWL which adds support for Datalog syntax-style rules
over OWL DL ontologies. Instead of arbitrary predicates (as in Datalog), SWRL
allows arbitrary OWL DL descriptions in both the head and the
body of rules, where a unary predicate corresponds to an
OWL class and a binary predicate corresponds to an OWL
property. While a subset of SWRL falls inside Horn Logic, a SWRL
knowledge base easily goes beyond this
fragment, because of the use of classical negation and existentially
quantified variables and
disjunction in the head of the rule. A set of Horn Logic formulae
can be reduced to standard Logic Programming rules; the Horn Logic
formulae and the Logic Programming rules entail exactly the same set of
ground formulae. Consequently, SWRL and standard rule languages
differ in expressiveness. The advantage of common rule languages
which are based on Horn Logic is
the efficient reasoning support which has been developed for certain
reasoning tasks like query answering. By going beyond the Horn
fragment, SWRL loses this advantage.
SWRL is an expressive extension of OWL DL, which add important
features to the standard, but is not a
standard rule language, because OWL DL is not a rule language and
only a subset can be translated to rules. In contrast to SWRL, WRL is
based on standard rule languages in order to benefit from the many
years of research in the areas of Deductive Databases and Logic
Programming.
SWSL-Rules, part of [SWSL] is a rule
language proposal for Semantic Web Services, which can also be used
for the Semantic Web. SWSL-Rules is
closely related to WRL. Both languages are largely based on
F-logic and they mostly share the logical expression syntax.
However, the WRL and SWSL-Rules initiatives have pursued
complementary goals. WRL was developed with more
focus on the end user, which resulted in a "conceptual syntax" for
top-level descriptions of ontologies, which we believe might make
the specifications easier to read. More
attention was paid in WRL to the issue of OWL compatibility. To this end, we
defined WRL-Core as a subset of both OWL and WRL, which will
serve as a common ground for ontology interoperability. In
contrast, the initiative around SWSL-Rules focused particularly on extending the functionality of
their rule-based language. In particular, SWSL-Rules supports
meta-reasoning with its HiLog and reification extensions. It also
supports prioritized defaults and classical negation by
incorporating Courteous Logic Programming.
WRL is strongly related to SWSL-Rules, which has also
been proposed as a rule language for the Semantic Web. We expect
that both approaches will converge in a possible working
group..
The Web Service Modeling Language WSML [WSML] is a language for the modeling of various
aspects related to Semantic Web Services. The language is based
on the formalisms coming from the areas of Description Logics and
Logic Programming. WSML defines in total 5 language variants.
WSML-DL is based on Description Logics and WSML-Flight and
WSML-Rule are based on rules languages (Deductive Databases and
Logic Programming). WSML-Core marks the intersection of
Description Logics and Logic Programming, based on DLP [Grosof et al.,
2003]. It is furthermore envisioned in WSML to have a
variant, WSML-Full, which unifies Description Logics and Logic
Programming, but this unifying language is still an open research issue.
WRL extracts the rule-based variants of WSML, namely
WSML-Flight and WSML-Rule, as well as the basic inter-operation
layer with Description Logics, namely WSML-Core, leaving out the
Web Service-specific elements, such as Goals, Web Services and
Mediators. WRL inherits from WSML the conceptual syntax for the
specification of ontologies and the logical expression syntax for
the specification of rules as part of an Ontology.
The work is funded by the European Commission under the
projects DIP, Knowledge Web, InfraWebs, SEKT, SWWS, ASG and
Esperonto; by Science Foundation Ireland under the DERI-Lion
project; by the FIT-IT (Forschung, Innovation, Technologie -
Informationstechnologie) under the projects RW² and TSC; by the German Federal Ministry of Education and Research (BMBF) under the SmartWeb project.
The editor would like to thank to all the members of the
WSML working group for
their advice and input into this document.
[Grosof et al.,
2003] B. N. Grosof, I. Horrocks, R. Volz, and S.
Decker. Description logic programs: Combining logic programs with
description logic. In Proc. of the Twelfth International
World Wide Web Conference (WWW 2003), pages 48-57. ACM,
2003.
[Horrocks et al., 2003] I.
Horrocks, P. F. Patel-Schneider, and F. van Harmelen. From SHIQ
and RDF to OWL: The making of a web ontology language.
Journal of Web Semantics, 1(1):7-26, 2003.
[OWL] M.
Dean, G. Schreiber, (Eds.). OWL Web Ontology Language
Reference, W3C Recommendation, 10 February 2004. http://www.w3.org/TR/2004/REC-owl-ref-20040210/.
[RDFS] D. Brickley and R. V. Guha.
RDF vocabulary description language 1.0: RDF schema. W3C
Recommendation 10 February 2004. http://www.w3.org/TR/rdf-schema/.
[SWRL] I.
Horrocks, P.F. Patel-Schneider, H. Boley, S. Tabet, B. Grosof, M.
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RuleML, 2004. http://www.w3.org/submissions/2004/SUBM-SWRL-20040521/
[SWSL] S.
Battle, A. Bernstein, H. Boley, B. Grosof, M. Gruninger, R. Hull,
M. Kifer, D. Martin, S. McIlraith, D. McGuinness, J. Su, and S.
Tabet: Semantic Web Services Language (SWSL), 2005.
http://www.daml.org/services/swsf/1.0/swsl/
[WSML] The
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