+

Rules

Mariano Rodriguez-Muro, Free University of Bozen-Bolzano

+Disclaimer

License This work is licensed under a

Creative Commons Attribution-Share Alike 3.0 License (http://creativecommons.org/licenses/by-sa/3.0/)

+Reading material

Chapter 7. Semantic Web Programming Chapter 6. Foundations of Semantic Web.

+What are rules

Means of representing knowledge An ontology language Can be seen as conditions statements:

if []then []

if x is a Man and x hasChild y Father(x)

+Why rules

Easy to understand RDFS (and sometimes OWL) is not enough:

If a person x is the brother of somebody y, and there is z such that z is the uncle of x, then z is the uncle of y too.

for all x,y,z, if hasBrother(x,y) & hasUncle(x,z) then hasUncle(y,z)

+SWRL

+SWRL

How can datalog and OWL be combined? SWRL – Semantic Web Rule Language [swirl]

• Proposal for a rule based Semantic Web Language (W3C member submission)http://www.w3.org/Submission/SWRL/

• Idea: datalog rules referring to RDF classes/propertiesSymbols in rules can be OWL identifiers or new symbols

• Various further features and syntactic forms

• Support in inference engines (very wide spread support)

7

+SWRL Syntaxes

Two syntaxes:

1. Abstract syntax based on “Extended Backus-Naur Form notation1. Directly translatable to a Human readable syntax

(SPARQL lie)

2. XML Concrete Syntax1. Parsable by existing OWL/RDF/XML parsers2. Suitable as a exchange language for the semantic

web

+Abstract Syntax

+Abstract Syntax ExampleFor the rule:

The corresponding abstract syntax:

+Example 1A very common use for rules is to move property values from one individual to a related individual

Express the fact that the style of an art object is the same as the style of the creator.

Artist(?x) & artistStyle(?x,?y) & Style(?y) & creator(?z,?x) ⇒ style/period(?z,?y)

Implies(Antecedent(Artist(I-variable(x)) artistStyle(I-variable(x) I-variable(y)) Style(I-variable(y)) creator(I-variable(z) I-variable(x)))

Consequent(style/period(I-variable(z) I-variable(y))))

+RDFS as rules

Provide a translation of all RDFS axiom as rules subClassOf subPropertyOf domain range

+Beyond RDFS

Transitivity of a property P Reflexivity of a property P Inverse of a property P Equivalence of properties/classes Sufficient conditions for class membership

+Built ins

One of the main motivations for SWRL rules Provide means to manipulate data

Comparisons Mathematical transformations List operators Modifiers for strings, dates and times Boolean and URI checks URI construction

+Built-ins exampleshasAddress(?creditCardMachine, ?ccAddress) ^hasAddress(?customer, ?custAddress) ^hasState(?ccAddress, ?ccState) ^hasState(?custAddress, ?custState) ^swrlb:equal(?custState, ?ccState) ^hasBirthday(?customer, ?bDate) ^swrlb:subtractYearMonthDurations(?diff, ?today, ?bDate) ^swrlb:greaterThanOrEqual(?diff, "P18Y0M")→ LegalCigaretteBuyer(?customer)

+Built-ins examples

foaf:Person(?person) ^foaf:gender(?person, "female") ^foaf:name(?person, ?name)→ swrlb:stringConcat(?s, "Dear Ms. ", ?name, ":") ^ hasFormalGreeting(?person, ?s)

+Empty antecedents/consequent

Empty antecedent indicate the consequent is ALWAYS true

Empty consequents indicate the antecedent is a contradiction

Allow to define FACTs and CONSTRAINTS

State class/property disjointness

State a FK-like constraint State number-restriction

constraints

+XML Concrete Syntax

• Based on OWL XML Presentation Syntax and RuleML• The SWRL XML syntax uses the OWL XML Ontology root

element and some of its subelements:• VersionInfo• PriorVersion• BackwardCompatibleWith• IncompatibleWith• Imports• Annotation• Class• EnumeratedClass

• SubClassOf• EquivalentClasses• DisjointClasses• DatatypeProperty• ObjectProperty• SubPropertyOf• EquivalentProperties• Individual• SameIndividual• DifferentIndividuals

+XML Elements (1)

• VAR

• IMP

+XML Element (2)

• Rlab• Body

• Head

+XML Elements (cont)

• classAtom

+XML Elements (cont)

• datarangeAtom

+XML Elements (cont)

• individualPropertyAxiom

+XML Elements (cont)

• datavaluedPropertyAxiom

+XML Elements (cont)

• sameIndividualAtom

+XML Elements (cont)

• differentIndividualsAtom

+XML Elements (cont)

• builtInAtom

Built in atoms include: Comparison, Mathematical transformations, List operators, Modifiers for strings, dates and times, Boolean and URI checks, URI constructions

+When RDFS/Rules

Expressivity for RDFS/OWL is enough

Use is knowledge sharing Application requires high

performance/uses specialized reasoner

Application requires standard behavior

Expressivity only captures by rules

Use is application behavior Application requires

complex reasoning Application relies on a

particular inference engine only

RDFS/OWL Rules

+Conclusion

Rules are a VERY expressive (powerful) ontology language Rules can capture most inferences provided by RDFS (part

of OWL too) Rules shouldn’t be abused. Performance of rule based

reasoning is suboptimal w.r.t. RDFS/OWL specialized reasoning.

Rules can be a powerful mean to capture application behiavor.

Note, although SWRL is the defacto rule language for the semantic web, there are new standards, i.e. RIF, that might take over in the future (further information in FSW Chapter 6)