W3C has acknowledged receipt of a Member Submission from the National Research Council of Canada, Network Inference, and Stanford University for SWRL: A Semantic Web Rule Language Combining OWL and RuleML. The submission has been made in association with the Joint US/EU ad hoc Agent Markup Language Committee.

The SWRL submission package contains three components in addition to the principal prose document: (1) an RDF Schema partially describing the RDF Concrete Syntax of SWRL; (2) an OWL ontology partially describing the RDF Concrete Syntax of SWRL; (3) an XML Schema for the SWRL XML Concrete Syntax.

The document "contains a proposal for a Semantic Web Rule Language (SWRL) based on a combination of the OWL DL and OWL Lite sublanguages of the OWL Web Ontology Language with the Unary/Binary Datalog RuleML sublanguages of the Rule Markup Language. SWRL includes a high-level abstract syntax for Horn-like rules in both the OWL DL and OWL Lite sublanguages of OWL. A model-theoretic semantics is given to provide the formal meaning for OWL ontologies including rules written in this abstract syntax. An XML syntax based on RuleML and the OWL XML Presentation Syntax as well as an RDF concrete syntax based on the OWL RDF/XML exchange syntax are also given, along with several examples."

Statements concerning copyrights, trademarks, service marks, and patents related to the submission have been made by W3C members and non-members, including the National Research Council of Canada, Network Inference, Stanford University, Lucent Technologies, Macgregor, Inc., Massachusetts Institute of Technology, and BBN Technologies; each of these organizations has granted to the W3C "a perpetual, nonexclusive, royalty-free, world-wide right and license under any of its copyrights in this contribution to copy, publish and distribute the contribution under the W3C document licenses."

Bibliographic Information

SWRL: A Semantic Web Rule Language Combining OWL and RuleML. By Ian Horrocks (Network Inference), Peter F. Patel-Schneider (Bell Labs Research, Lucent Technologies), Harold Boley (National Research Council of Canada), Said Tabet (Macgregor, Inc.), Benjamin Grosof (Sloan School of Management, MIT), and Mike Dean (BBN Technologies). W3C Member Submission. 21-May-2004. Version URL: http://www.w3.org/Submission/2004/SUBM-SWRL-20040521/. Latest version URL: http://www.w3.org/Submission/SWRL/.

SWRL Overview

SWRL provides a high-level abstract syntax "that extends the OWL abstract syntax described in the OWL Semantics and Abstract Syntax document. An extension of the OWL model-theoretic semantics is also given to provide a formal meaning for OWL ontologies including rules written in this abstract syntax.

The proposed rules are of the form of an implication between an antecedent (body) and consequent (head). The intended meaning can be read as: whenever the conditions specified in the antecedent hold, then the conditions specified in the consequent must also hold.

Both the antecedent (body) and consequent (head) consist of zero or more atoms. An empty antecedent is treated as trivially true (i.e., satisfied by every interpretation), so the consequent must also be satisfied by every interpretation; an empty consequent is treated as trivially false (i.e., not satisfied by any interpretation), so the antecedent must also not be satisfied by any interpretation. Multiple atoms are treated as a conjunction. Note that rules with conjunctive consequents could easily be transformed (via the Lloyd-Topor transformations) into multiple rules each with an atomic consequent.

Atoms in these rules can be of the form C(x), P(x,y), sameAs(x,y) or differentFrom(x,y), where C is an OWL description, P is an OWL property, and x,y are either variables, OWL individuals or OWL data values. It is easy to see that OWL DL becomes undecidable when extended in this way as rules can be used to simulate role value maps...

An XML syntax is also given for these rules based on RuleML and the OWL XML presentation syntax. Furthermore, an RDF concrete syntax based on the OWL RDF/XML exchange syntax is presented. The rule syntaxes are illustrated with several running examples. Finally, we give usage suggestions and cautions..." [from the SWRL Submission 'Introduction']

W3C Team Comment on the SWRL Submission

"Rule languages have a long history, dating from Prolog in the early 1970s, through a variety of academic and industrial developments, including recent Semantic Web rule languages from W3C Team members, Metalog and N3.

Rules offer users the ability to express certain logical relationships in a form suitable for machine processing. They are declarations like 'if P is true, then Q must also be true,' and for some applications they are easy for people to understand and efficient for machines to use in computation.

As a rule language for the Semantic Web, SWRL uses URIs to identify things, making it essentially compatible with RDF and OWL. In RDF, an organization can express that a particular person in an employee and is also granted access to all internal documents. In SWRL, one can express the rule that all employees are granted access to internal documents. Given this rule and the fact that someone is an employee, a SWRL reasoner can conclude that the person is granted access.

SWRL is unique in being an extension of OWL DL, so that users of OWL DL can add rules to their ontologies and maintain clear semantics. Some rule systems offer meta-processing (rules about rules), and with the addition of OWL comes the possibility for new confusion in rules about OWL axioms and OWL axioms about rules; the design of SWRL 0.6 carefully steers clear of these potentially-confusing areas...

The development of languages like SWRL, with high expressive power, also raises the question of the computational complexity of the implementation: as the submission authors correctly note, there will be the need to select suitable subsets of the language that can be implemented in efficient ways, balancing expressive power against execution speed and termination of the computation.

We encourage further development of SWRL or similar languages to address reasoning on the Web, understanding that graceful interoperation within the RDF model and with the XQuery functions and operators may well be the key to the success of components of the Semantic Web..." [excerpted from the comments of Massimo Marchiori and Sandro Hawke]