SWRL 0.6 A Semantic Web Rule Language Combining OWL and RuleML

1
SWRL 0.6A Semantic Web Rule Language Combining
OWL and RuleML

• Ian Horrocks, Peter F. Patel-Schneider, Harold
  Boley, Said Tabet, Benjamin Grosof, Mike Dean
• presented by Axel Polleres
• http//www.daml.org/rules/proposal/

2
Overview

• SWRL Overview
• Abstract Syntax
• Model-Theoretic Semantics
• Not covered here XML concrete Syntax/RDF
  concrete syntax
• Usage Suggestions, why do we need rules?
• Built-Ins
• Decidability, why not?

3
SWRL Overview

• Combines OWL DL RuleML (basically Horn Rules)
• Straightforward combination
• ? Decidability is lost!
• Document defines Semantics and different Syntax
  proposals
• Examples for rules and usage in the document

4
Abstract Syntax

• Extends OWL DL abstract syntax by a further
  axiom
• axiom rule
• where
• rule 'Implies(' URIreference
  annotation antecedent consequent ')'
• antecedent 'Antecedent(' atom ')'
• consequent 'Consequent(' atom ')'
• atom description '(' i-object ')'
• dataRange '(' d-object i-object ')'
• individualvaluedPropertyID '(' i-object
  i-object ')'
• datavaluedPropertyID '(' i-object d-object
  ')'
• sameAs '(' i-object i-object ')'
• differentFrom '(' i-object i-object ')'
• builtIn '(' builtIn builtinID d-object
  ')'

Hmmm, typo?
Only syntactic sugar, can be expressed by OWL
5
Abstract Syntax contd

• i-object i-variable individualID
• d-object d-variable dataLiteral
• i-variable 'I-variable(' URIreference ')'
• d-variable 'D-variable(' URIreference ')'
• Basically, you can model Horn rules where atoms
  can be
• C(x), P(x,y), sameAs(x,y), differentFrom(x,y),
  or builtIn(r,x,...) where C is an OWL description
  or data range, P is an OWL property, r is a
  built-in relation, x and y are either variables,
  OWL individuals or OWL data values, as
  appropriate.
• Safety (cf. Ullman) is required, i.e. vars in the
  head have to be bound in the body.
• no magic we now use a more human readable
  syntax.

6
Semantics

• As usual rules are satisfied by an interpretation
  if every variable binding satisfying the
  antecedent also satisfies the consequent.
• Semantics of OWL unchanged, an interpretation
• Satisfies an OWL ontology if all axioms (incl.
  rules) are satisfied.
• No magic, no sophisticated semantics
• BUT decidability of either DL or Datalog alone
• is lost ?

7
Where has all the decidability gone?

• Problem existential quantifiers
• recursive rules
• For instance (in DL syntax)
• C1(X) - C2(X).
• C1 ? ?R.C2

C(a).
by skolemization creates C(f(a))
C(f(f(a)) etc.
Corresponds to a rule (?Y.R(X,Y)) - C2(X)
which amounts to ? X.?Y.R(X,Y)) v not C2(X)
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Where has all the decidability gone? Contd

• The simple example from the last slide might
  still be solved with a blocking strategy in the
  evaluation algorithm, but
• in general, interaction of existential
  quantifiers and recursive rules causes troubles,
  by introducing function symbols
• It is not yet completely clear to me which SWRL
  fragment is fine

9
State of discussion

• Which F-Logic fragment corresponds how to which
  SWRL fragment?
• (copied from deri-wsmo-discussion, thread
• FvHgt how weak must subsets of SWRL and
• FvHgt pragmatic FLogic be before they start to
  diverge?
• MKgt My guess is that they coincide on their pure
  Horn subsets and that
• MKgt this is the maximal such subset.
• FvHgt Thanks again for confirming this. I still
  think the rules must also be
• FvHgt non-recursive for the two to coincide (from
  another point in your
• FvHgt email, I think we agree on this).
• RVgt Personally, I don't understand why rules
  have to be non-recursive. This
• RVgt is definitely not the case for the DLP
  fragment.
• MKgt I don't understand either. Why does
  recursion fall outside of the intersection of
  SWRL and F-logic?
• MKgt But in SWRL you can have recursion, no?

10
Conclusion Outlook

• Simple combination of DL and rules loses
  decidability.
• Related works
• CARIN (Levy, Rousset, 1996)
• Recent paper by Eiter et al. (KR 2004)
• Description Logic Programs by Horrocks et al.
  (WWW 2003)
• These works partly try to combine LP and DLs
  differently or impose syntactical restrictions
  for DL or for the rules.

11
Use for WSMO

• F-Logic is FOL, thus undecidable well, ok,
  semi-decidable.
• F-Logic as in Flora is decidable but uses
  non-classical semantics, namely well-founded
  semantics for negation.
• Use cases shall show which expressivity we need.
• As long as we only use Horn-rules in F-Logic and
  dont use function symbols, we are fine, since we
  dont have existential introduction as in OWL, or
  no?
• comments please! follow the discussions on the
  list What are the next steps?