SWRL 0.5 (aka Rules for OWL)

• SWRL adds a rule construct to OWL (DL).
  • Antecedent implies consequent
  • Both antecedent and consequent are bits of OWL applied to variables and individuals or data values.
  • parent(?a,?b) ∧ brother(?b,?c) → uncle(?a,?c)
  • bodyPart(?a,?b) ∧ disease(?b,?c) → disease(?a,?c)
  • child(?a,?b) ∧ Student(?b) → dependent(?a,?b)
• SWRL is a simple extension to OWL (DL).
  • SWRL maintains the general semantic flavor of OWL DL.
    • Rules are material implications.
    • Simple extension of OWL DL model theory.
  • No non-monotonic features are involved.
  • Model theory for SWRL rules is a simple extension to the DL model theory for OWL.

Why add Rules to OWL?

• OWL can do a lot.
  • Define classes:
    • Student ≤ Person ∩ (∃ ssn xsd:int)
  • Define properties:
    • ssn ≤ Person × xsd:int
  • Provide information about individuals:
    • john ∈ Student ∩ (ssn : "565"^^xsd:int)
• OWL can't do everything.
  • In particular, OWL is very weak in reasoning with properties,
    • No property composition constructor,
    • Can't define uncle (as the composition of parent and brother);
  • Limitations help to ensure that reasoning is decidable for OWL DL and OWL Lite.
• Rules can provide much of what OWL can't do.
  • Defining properties, inheritance, ....
  • E.g., the brother of your parent is your uncle.
  • E.g., diseases of a body part are diseases of the body.

Syntax (abstract syntax version)

rule ::= 'Implies(' { annotation } antecedent consequent ')'

antecedent ::= 'Antecedent(' { atom } ')'
consequent ::= 'Consequent(' { atom } ')'

atom ::= description '(' i-object ')'
atom ::= individualvaluedPropertyID '(' i-object i-object ')'
atom ::= datavaluedPropertyID '(' i-object d-object ')'
atom ::= sameAs '(' i-object i-object ')'
atom ::= differentFrom '(' i-object i-object ')'

i-object ::= 'I-variable(' URIreference ')' | individualID
d-object ::= 'D-variable(' URIreference ')' | dataLiteral

All variables in a rule must occur (at least) in the antecedent.

Examples

Transferring style from an artist to his/her paintings
(from Guus Schreiber).

• Artist(?x) ∧ Style(?y) ∧ artistStyle(?x, ?y) ∧ creator(?x, ?z)
→ style/period(?z, ?y)

Inheriting trauma from body parts
(often needed in medical ontologies).

• Location(?x) ∧ Trauma(?y) ∧ isLocationOf(?x, ?y) ∧ isPartOf(?x,?z)
→ isLocationOf(?z, ?y)

Computability and Implementability

• OWL DL plus rules (of almost any sort) is undecidable.
  • SWRL is undecidable.
• SWRL plus rules can be implemented. (Really!)
  • See presentations later