Roman Schindlauer:
Answer-Set Programming for the Semantic Web.
Abstract
This thesis makes a contribution to the research efforts of
integrating rule-based inference methods with current knowledge
representation formalisms in the Semantic Web. Ontology languages such as
OWL and RDF Schema seem to be widely accepted and successfully used for
semantically enriching knowledge on the Web and thus prepare it for
machinereadability. However, these languages are of restricted
expressivity if it comes to inferring new from existing knowledge. On the
other side, rule formalisms have a long tradition in logic programming,
being a common and intuitive tool for problem specifications. It is
evident that the Semantic Web needs a powerful rule language complementing
its ontology formalisms in order to facilitate sophisticated reasoning
tasks. Ontology languages commonly derive from Description Logics. As a
fragment of first-order logic, their semantics diverge significantly from
logic programming languages like Datalog and its various descendants -
especially if we consider the powerful category of non-monotonic logic
programming. In order to overcome this gap, different approaches have been
presented how to combine Description Logics with rules, varying in the
degree of integration.
Answer-set programming (ASP) is one of the most prominent and successful
semantics for non-monotonic logic programs. The specific treatment of
default negation under ASP allows for the generation of multiple models
for a single program, which in this respect can be seen as the encoding of
a problem specication. Highly efficient reasoners for ASP are available,
each extending the core language by various sophisticated features such as
aggregates or weak constraints.
In the first part of this thesis, we propose a combination of logic
programming under the answer-set semantics with the description logics
SHIF(D) and SHOIN(D), which underly the Web ontology languages OWL Lite
and OWL DL, respectively. This combination allows for building rules on
top of ontologies but also, to a limited extent, building ontologies on
top of rules. We introduce description logic programs (dl-programs), which
consist of a description logic knowledge base L and a finite set of
description logic rules (dl-rules) P. Such rules are similar to usual
rules in logic programs with negation as failure, but may also contain
queries to L, possibly default-negated, in their bodies. We show that
consistent stratied dl-programs can be associated with a unique minimal
Herbrand model that is characterized through iterative least Herbrand
models. We then define strong and weak answer-set semantics which both
properly generalize answer sets of ordinary normal logic programs, based
on a reduction to the least model semantics of positive dl-programs and to
the answer-set semantics of ordinary logic programs respectively. We also
present a definition of the well-founded semantics for dl-programs, based
on a generalization of the notion of unfounded sets. We then give fixpoint
characterizations for the (unique) minimal Herbrand model semantics of
positive and stratified dl-programs as well as for the wellfounded
semantics, and show how to compute these models by finite fixpoint
iterations. Furthermore, we give a precise picture of the complexity of
deciding answer set existence for a dl-program, and of brave, cautious,
and well-founded reasoning. We lay out possible applications of
dl-programs and present the implementation of a prototype reasoner.
In the second part of the thesis, we generalize our approach to
hex-programs, which are nonmonotonic logic programs under the answer-set
semantics admitting higher-order atoms as well as external
atoms. Higher-order features are widely acknowledged as useful for
performing meta-reasoning, among other tasks. Furthermore, the possibility
to exchange knowledge with external sources in a fully declarative
framework such as ASP is particularly important in view of applications in
the Semantic Web area. Through external atoms, hex-programs can model some
important extensions to ASP, and are a useful KR tool for expressing
various applications. We define syntax and semantics of hex-programs and
show how they can be deployed in the context of the Semantic Web. We give
a picture of the computation method of hex-programs based on the theory of
splitting sets, followed by a discussion on complexity. Then, the
implementation of a prototype reasoner for hex-programs is outlined, along
with a description how to extend this framework by custom
modules. Eventually, we show the usability and versatility of hex-programs
and our prototype implementation on the basis of concrete, real-world
scenarios.
URL:
http://rewerse.net/publications/rewerse-publications.html#REWERSE-RP-2006-163
@phdthesis{REWERSE-RP-2006-163, author = {Roman Schindlauer}, title = {Answer-Set Programming for the Semantic Web}, school = {Institute of Computer Science, LMU, Munich}, year = {2006}, note = {PhD Thesis, Institute of Information Systems, Vienna University of Technology, December 2006}, type = {{Dissertation/Ph.D. thesis}}, url = {http://rewerse.net/publications/rewerse-publications.html#REWERSE-RP-2006-163} }