W3C


RIF RDF and OWL Compatibility

W3C Editor's Draft 15 April 2008

This version:
http://www.w3.org/2005/rules/wg/draft/ED-rif-rdf-owl-20080415/
Latest editor's draft:
http://www.w3.org/2005/rules/wg/draft/rif-rdf-owl/
Previous version:
http://www.w3.org/2005/rules/wg/draft/ED-rif-rdf-owl-20080414/ (color-coded diff)
Editors:
Jos de Bruijn, Free University of Bozen/Bolzano


Abstract

Rules interchanged using the Rule Interchange Format RIF may depend on or be used in combination with RDF data and RDF Schema or OWL data models (ontologies). This document, developed by the Rule Interchange Format (RIF) Working Group, specifies the interoperation between RIF and the data and ontology languages RDF, RDFS, and OWL.

Status of this Document

May Be Superseded

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document is being published as one of a set of 3 documents:

  1. RIF Basic Logic Dialect
  2. RIF Framework for Logic Dialects
  3. RIF RDF and OWL Compatibility (this document)

Note for Working Group

This draft is ready for Working Group Review. A publication decision is scheduled for 15 April.

Please Comment By 2008-04-15

The Rule Interchange Format (RIF) Working Group seeks public feedback on these Working Drafts. Please send your comments to public-rif-comments@w3.org (public archive). If possible, please offer specific changes to the text that would address your concern. You may also wish to check the Wiki Version of this document for internal-review comments and changes being drafted which may address your concerns.

No Endorsement

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

Patents

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.


Contents

1 Overview of RDF and OWL Compatibility

The Rule Interchange Format (RIF), specifically the Basic Logic Dialect (BLD) (RIF-BLD), is a format for interchanging logical rules over the Web. Rules that are exchanged using RIF may refer to external data sources and may be based on data models that are represented using a language different from RIF. The Resource Description Framework RDF (RDF-Concepts) is a Web-based language for the representation and exchange of data; RDF Schema (RDFS) (RDF-Schema) and the OWL Web Ontology Language (OWL-Reference) are Web-based languages for representing and exchanging ontologies (i.e., data models). This document specifies how combinations of RIF BLD Rulesets and RDF data and RDFS and OWL ontologies must be interpreted; i.e., it specifies how RIF interoperates with RDF/OWL.

The RIF working group plans to develop further dialects besides BLD, most notably a dialect based on Production Rules (RIF-PRD); these dialects are not necessarily extensions of BLD. Future versions of this document may address compatibility of these dialects with RDF and OWL as well. In the remainder of this document, RIF is understood to refer to RIF BLD (RIF-BLD).

RDF data and RDFS and OWL ontologies are represented using RDF graphs. Several syntaxes have been proposed for the exchange of RDF graphs, the normative syntax being RDF/XML (RDF-Syntax). RIF does not provide a format for exchanging RDF graphs, since this would be a duplication. Instead, it is assumed that RDF graphs are exchanged using RDF/XML, or any other syntax that can be used for representing or exchanging RDF graphs.

A typical scenario for the use of RIF with RDF/OWL is the exchange of rules that either use RDF data or an RDFS or OWL ontology: an interchange partner A has a rules language that is RDF/OWL-aware, i.e., it supports the use of RDF data, it uses an RDFS or OWL ontology, or it extends RDF(S)/OWL. A sends its rules using RIF, possibly with references to the appropriate RDF graph(s), to partner B. B receives the rules and retrieves the referenced RDF graph(s) (published as, e.g., RDF/XML (RDF-SYNTAX)). The rules are translated to the internal rules language of B and are processed, together with the RDF graphs, using the RDF/OWL-aware rule engine of B. The use case Vocabulary Mapping for Data Integration (RIF-UCR) is an example of the interchange of RIF rules that use RDF data and RDFS ontologies.

A specialization of this scenario is the publication of RIF rules that refer to RDF graphs: publication is a special kind of interchange. A rule publisher A publishes its rules on the Web. There may be several consumers that retrieve the RIF rules and RDF graphs from the Web, translate the RIF rules to their own rules languages, and process them together with the RDF graphs in their own rules engine. The use case Publishing Rules for Interlinked Metadata (RIF-UCR) illustrates the publication scenario.

Another specialization of the exchange scenario is the interchange of rule extensions to OWL (RIF-UCR). The intention of the rule publisher in this scenario is to extend an OWL ontology with rules: interchange partner A has a rules language that extends OWL. A splits its ontology+rules description into a separate OWL ontology and a RIF ruleset, publishes the OWL ontology, and sends (or publishes) the RIF ruleset, which includes a reference to the OWL ontology. A consumer of the rules retrieves the OWL ontology and translates the ontology and ruleset into a combined ontology+rules description in its own rule extension of OWL.


A RIF ruleset that refers to RDF graphs and/or RDFS/OWL ontologies, or any use of a RIF ruleset with RDF graphs, is viewed as a combination of a ruleset and a number of graphs and ontologies. This document specifies how, in such a combination, the ruleset and the graphs and ontologies interoperate in a technical sense, i.e., the conditions under which the combination is satisfiable (i.e., consistent), as well as the entailments (i.e., logical consequences) of the combination. The interaction between RIF and RDF/OWL is realized by connecting the model theory of RIF (specified in (RIF-BLD)) with the model theories of RDF (specified in (RDF-Semantics)) and OWL (specified in (OWL-Semantics)), respectively.

Throughout this document the following conventions are used when writing RIF and RDF statements in examples and definitions.


The RDF semantics specification (RDF-Semantics) defines four notions of entailment for RDF graphs. At this stage it has not been decided which of these notions are of interest in RIF. The OWL semantics specification (OWL-Semantics) defines two notions of entailment for OWL ontologies, namely OWL Lite/DL and OWL Full; both notions are of interest in RIF. This document specifies the interaction between RIF and RDF/OWL for all six notions.

Editor's Note: Currently, this document only defines how combinations of RIF rulesets and RDF/OWL should be interpreted; it does not suggest how references to RDF graphs and OWL ontologies are specified in RIF, nor does it specify which of the RDF/OWL entailment regimes (simple, RDF, RDFS, D, OWL DL, OWL Full) should be used. Finally, there are two notions of interoperation with OWL DL: one that does not consider annotation properties, and one that does; it is an open issue whether this distinction should be reflected in the syntax. Possible ways to refer to RDF graphs and RDFS/OWL ontologies include annotations in RIF rulesets and extensions of the syntax of RIF. Note that no agreement has yet been reached on this issue, and that especially the issue of the specification of entailment regimes is controversial (see http://lists.w3.org/Archives/Public/public-rif-wg/2007Jul/0030.html and the ensuing thread). See the Annotations page for a proposal for extending RIF with annotations.

The Appendix: Embeddings (Informative) describes how reasoning with combinations of RIF rules with RDF and a subset of OWL DL can be reduced to reasoning with RIF rulesets, which can be seen as a guide to describing how a RIF processor could be turned into an RDF/OWL-aware RIF processor. This reduction can be seen as a guide for interchange partners that do not have RDF-aware rule systems, but want to be able to process RIF rules that refer to RDF graphs. In terms of the aforementioned scenario: if the interchange partner B does not have an RDF/OWL-aware rule system, but B can process RIF rules, then the appendix explains how B's rule system could be used for processing RIF-RDF.

2 RDF Compatibility

This section specifies how a RIF ruleset interacts with a set of RDF graphs in a RIF-RDF combination. In other words, how rules can "access" data in the RDF graphs and how additional conclusions that may be drawn from the RIF rules are reflected in the RDF graphs.

There is a correspondence between constant symbols in RIF rulesets and names in RDF graphs. The following table explains the correspondences of symbols.

Table 1. Correspondence between RDF and RIF symbols.
RDF Symbol Example RIF Symbol Example
Absolute IRI <http://www.w3.org/2007/rif> Absolute IRI "http://www.w3.org/2007/rif"^^rif:iri
Plain literal without language tag "literal string" String in the symbol space xsd:string "literal string"^^xsd:string
Plain literal with language tag "literal string"@en String plus language tag in the symbol space rif:text "literal string@en"^^rif:text
Literal with datatype "1"^^xsd:integer Symbol in symbol space "1"^^xsd:integer

There is, furthermore, a correspondence between statements in RDF graphs and certain kinds of formulas in RIF. Namely, there is a correspondence between RDF triples of the form s p o and RIF frame formulas of the form s'[p' -> o'], where s', p', and o' are RIF symbols corresponding to the RDF symbols s, p, and o, respectively. This means that whenever a triple s p o is satisfied, the corresponding RIF frame formula s'[p' -> o'] is satisfied, and vice versa.

Consider, for example, a combination of an RDF graph that contains the triples

ex:john ex:brotherOf ex:jack . 
ex:jack ex:parentOf ex:mary . 

saying that ex:john is a brother of ex:jack and ex:jack is a parent of ex:mary, and a RIF ruleset that contains the rule

Forall ?x, ?y, ?z (?x[ex:uncleOf -> ?z] :- 
     And(?x[ex:brotherOf -> ?y] ?y[ex:parentOf -> ?z]))

which says that whenever some x is a brother of some y and y is a parent of some z, then x is an uncle of z. From this combination the RIF frame formula :john[:uncleOf -> :mary], as well as the RDF triple :john :uncleOf :mary, can be derived.

Note that blank nodes cannot be referenced directly from RIF rules, since blank nodes are local to a specific RDF graph. Variables in RIF rules do, however, range over objects denoted by blank nodes. So, it is possible to "access" an object denoted by a blank node from a RIF rule using a variable in a rule.

Typed literals in RDF may be ill-typed, which means that the literal string is not part of the lexical space of the datatype under consideration. Examples of such ill-typed literals are "abc"^^xsd:integer, "2"^^xsd:boolean, and "<non-valid-XML"^^rdf:XMLLiteral. Ill-typed literals are not expected to be used very often. However, as the RDF recommendation (RDF-Concepts) allows creating RDF graphs with ill-typed literals, their occurrence cannot be completely ruled out.

Rules that include ill-typed symbols are not legal RIF rules, so there are no RIF symbols that correspond to ill-typed literals. As with blank nodes, variables do range over objects denoted by such literals. The following example illustrates the interaction between RDF and RIF in the face of ill-typed literals and blank nodes.

Consider a combination of an RDF graph that contains the triple

_:x ex:hasName "a"^^xsd:integer . 

saying that there is some blank node that has a name, which is an ill-typed literal, and a RIF ruleset that contains the rules

Forall ?x, ?y ( ?x[rdf:type -> ex:nameBearer] :- ?x[ex:hasName -> ?y] )
Forall ?x, ?y ( "http://a"^^rif:iri["http://p"^^rif:iri -> ?y] :- ?x[ex:hasName -> ?y] )

which say that whenever there is some x that has some name y, then x is of type ex:nameBearer and http://a has a property http://p with value y.

From this combination the following RIF condition formula can be derived:

Exists ?z ( And( ?z[rdf:type -> ex:nameBearer]  "http://a"^^rif:iri["http://p"^^rif:iri -> ?z] ))

as can the following RDF triples:

_:y rdf:type ex:nameBearer .
<http://a> <http://p> "a"^^xsd:integer . 

However, "http://a"^^rif:iri["http://p"^^rif:iri -> "a"^^xsd:integer] cannot be derived, because it is not a well-formed RIF formula, due to the fact that "a" is not an integer; it is not in the lexical space of the datatype xsd:integer.


The remainder of this section formally defines combinations of RIF rules with RDF graphs and the semantics of such combinations. A combination consists of a RIF ruleset and a set of RDF graphs. The semantics of combinations is defined in terms of combined models, which are pairs of RIF and RDF interpretations. The interaction between the two interpretations is defined through a number of conditions. Entailment is defined as model inclusion, as usual.

2.1 Syntax of RIF-RDF Combinations

This section first reviews the definitions of RDF vocabularies and RDF graphs, after which definitions related to datatypes and ill-typed literals are reviewed. Finally, RIF-RDF combinations are formally defined.

2.1.1 RDF Vocabularies and Graphs

An RDF vocabulary V consists of the following sets of names:

The syntax of the names in these sets is defined in RDF Concepts and Abstract Syntax (RDF-Concepts). Besides these names, there is an infinite set of blank nodes, which is disjoint from the sets of literals and IRIs.

DEFINITION: Given an RDF vocabulary V, a generalized RDF triple of V is a statement of the form s p o, where s, p and o are names in V or blank nodes.

DEFINITION: Given an RDF vocabulary V, a generalized RDF graph is a set of generalized RDF triples of V.

(See the (End note on generalized RDF graphs))

2.1.2 Datatypes and Typed Literals

Even though RDF allows the use of arbitrary datatype IRIs in typed literals, not all such datatype IRIs are recognized in the semantics. In fact, simple entailment does not recognize any datatype and RDF and RDFS entailment recognize only the datatype rdf:XMLLiteral. Furthermore, RDF allows expressing typed literals for which the literal string is not in the lexical space of the datatype; such literals are called ill-typed literals. RIF, in contrast, does not allow ill-typed literals in the syntax. To facilitate discussing datatypes, and specifically datatypes supported in specific contexts (required for D-entailment), the notion of datatype maps (RDF-Semantics) is used.

A datatype map is a partial mapping from IRIs to datatypes.

RDFS, specifically D-entailment, allows the use of arbitrary datatype maps, as long as the rdf:XMLLiteral datatype is included in the map. RIF BLD additionally requires the following datatypes to be included: xsd:string, xsd:decimal, xsd:time, xsd:date, xsd:dateTime, and rif:text; these datatypes are the RIF-required datatypes. A conforming datatype map is a datatype map that recognizes at least the RIF-required datatypes.


DEFINITION: A datatype map D is a conforming datatype map if it satisfies the following conditions:

  1. No RIF-supported symbol space that is not a datatype (these are rif:local and rif:iri in RIF BLD) is in the domain of D.
  2. The IRIs identifying all RIF-required datatypes are in the domain of D.
  3. D maps IRIs identifying XML schema datatypes to the respective data types (XML-SCHEMA2), rdf:XMLLiteral to the rdf:XMLLiteral datatype (RDF-Concepts), and rif:text to the rif:text primitive datatype (RIF-BLD).

The notions of well- and ill-typed literals loosely correspond to the notions of legal and illegal symbols in RIF:

DEFINITION: Given a conforming datatype map D, a typed literal (s, d) is a well-typed literal if

  1. d is in the domain of D and s is in the lexical space of D(d),
  2. d is the IRI of a symbol space supported by RIF BLD and s is in the lexical space of the symbol space, or
  3. d is not in the domain of D and does not identify a symbol space supported by RIF.

Otherwise (s, d) is an ill-typed literal.

2.1.3 RIF-RDF Combinations

A RIF-RDF combination consists of a RIF ruleset and zero or more RDF graphs. Formally:

DEFINITION: A RIF-RDF combination is a pair < R,S>, where R is a RIF ruleset and S is a set of generalized RDF graphs of a vocabulary V.

When clear from the context, RIF-RDF combinations are referred to simply as combinations.

2.2 Semantics of RIF-RDF Combinations

The semantics of RIF rulesets and RDF graphs are defined in terms of model theories. The semantics of RIF-RDF combinations is defined through a combination of the RIF and RDF model theories, using a notion of common models. These models are then used to define satisfiability and entailment in the usual way. Combined entailment extends both entailment in RIF and entailment in RDF.

The RDF Semantics document (RDF-Semantics) defines four normative kinds of interpretations, as well as corresponding notions of satisfiability and entailment:

Those four types of interpretations are reflected in the definitions of satisfaction and entailment in this section.

2.2.1 Interpretations

This section defines the notion of common-rif-rdf-interpretation, which is an interpretation of a RIF-RDF combination. This common-rif-rdf-interpretation is the basis for the definitions of satisfaction and entailment in the following sections.

The correspondence between RIF semantic structures (interpretations) and RDF interpretations is defined through a number of conditions that ensure the correspondence in the interpretation of names (i.e., IRIs and literals) and formulas, i.e., the correspondence between RDF triples of the form s p o and RIF frames of the form s'[p' -> o'], where s', p', and o' are RIF symbols corresponding to the RDF symbols s, p, and o, respectively (cf. Table 1).

2.2.1.1 RDF and RIF Interpretations

The notions of RDF interpretation and RIF semantic structure (interpretation) are briefly reviewed below.

As defined in (RDF-Semantics), a simple interpretation of a vocabulary V is a tuple I=< IR, IP, IEXT, IS, IL, LV >, where

Rdf-, rdfs-, and D-interpretations are simple interpretations that satisfy certain conditions:

As defined in (RIF-BLD), a semantic structure is a tuple of the form I = <TV, DTS, D, Dind, Dfunc, IC, IV, IF, Iframe, ISF, Isub, Iisa, I=, Iexternal, Itruth>. The specification of RIF-RDF compatibility is only concerned with DTS, D, IC, IV, Iframe, Isub, Iisa, and Itruth. The other mappings that are parts of a semantic structure are not used in the definition of combinations.

Recall that Const is the set of constant symbols and Var is the set of variable symbols in RIF.

2.2.1.2 Common RIF-RDF Interpretations

DEFINITION: A common-rif-rdf-interpretation is a pair (I, I), where I = <TV, DTS, D, Dind, Dfunc, IC, IV, IF, Iframe, ISF, Isub, Iisa, I=, Iexternal, Itruth> is a RIF semantic structure and I=<IR, IP, IEXT, IS, IL, LV> is an RDF interpretation of a vocabulary V, such that the following conditions hold:

  1. (IR union IP) = Dind;
  2. IP is a superset of the set of all k in Dind such that there exist some a, b in Dind and Itruth(Iframe(k)(a,b))=t;
  3. LV is a superset of (Dind intersection (union of the value spaces of all datatypes in DTS));
  4. IEXT(k) = the set of all pairs (a, b), with a, b, and k in Dind, such that Itruth(Iframe(k)(a,b))=t;
  5. IS(i) = IC("i"^^rif:iri) for every absolute IRI i in VU;
  6. IL((s, d)) = IC("s"^^d) for every well-typed literal (s, d) in VTL;
  7. IEXT(IS(rdf:type)) is equal to the set of all pairs <a,b> in Dind × Dind such that Itruth(Iisa(< a,b >))=t; and
  8. IEXT(IS(rdfs:subClassOf)) is a superset of the set of all pairs <a,b> in Dind × Dind such that Itruth(Isub(< a,b >))=t.


Condition 1 ensures that the combination of resources and properties corresponds exactly to the RIF domain; note that if I is an rdf-, rdfs-, or D-interpretation, IP is a subset of IR, and thus IR=D. Condition 2 ensures that the set of RDF properties at least includes all elements that are used as properties in frames in the RIF domain. Condition 3 ensures that all concrete values in D are included in LV. Condition 4 ensures that RDF triples are interpreted in the same way as frame formulas. Condition 5 ensures that IRIs are interpreted in the same way. Condition 6 ensures that typed literals are interpreted in the same way. Note that no correspondences are defined for the mapping of names in RDF that are not symbols of RIF, e.g., ill-typed literals and RDF URI references that are not absolute IRIs. Condition 7 ensures that typing in RDF and typing in RIF correspond, i.e., a rdf:type b is true iff a # b is true. Finally, condition 8 ensures that whenever a RIF subclass statement holds, the corresponding RDF subclass statement holds as well, i.e., a rdfs:subClassOf b is true if a ## b is true.

One consequence of conditions 5 and 6 is that IRIs of the form http://iri and typed literals of the form "http://iri"^^rif:iri that occur in an RDF graph are treated the same in RIF-RDF combinations, even if the RIF Ruleset is empty. For example, consider the combination of an empty ruleset and an RDF graph that contains the triple

<http://a> <http://p> "http://b"^^rif:iri .
 

This combination allows the derivation of, among other things, the following triple:

<http://a> <http://p> <http://b> .

as well as the following frame formula:

"http://a"^^rif:iri ["http://p"^^rif:iri -> "http://b"^^rif:iri]

2.2.2 Satisfaction and Models

The notion of satisfiability refers to the conditions under which a common-rif-rdf-interpretation (I, I) is a model of a combination < R, S>. The notion of satisfiability is defined for all four entailment regimes of RDF (simple, RDF, RDFS, and D). The definitions are all analogous. Intuitively, a common-rif-rdf-interpretation (I, I) satisfies a combination < R, S> if I is a model of R and I satisfies S. Formally:

DEFINITION: A common-rif-rdf-interpretation (I, I) satisfies a RIF-RDF combination C=< R, S > if I is a model of R and I satisfies every RDF graph S in S; in this case (I, I) is called a simple model, or model, of C, and C is satisfiable. (I, I) satisfies a generalized RDF graph S if I satisfies S. (I, I) satisfies a closed RIF-BLD condition φ if TValI((φ)=t.


Notice that not every combination is satisfiable. In fact, not every RIF ruleset has a model. For example, the ruleset consisting of the rule

Forall ("1"^^xsd:integer="2"^^xsd:integer)

does not have a model, since the symbols "1"^^xsd:integer and "2"^^xsd:integer are mapped to the (distinct) numbers 1 and 2, respectively, in every semantic structure.

Rdf-, rdfs-, and D-satisfiability are defined through additional restrictions on I:

DEFINITION: A model (I, I) of a combination C is an rdf-model of C if I is an rdf-interpretation; in this case C is rdf-satisfiable.

DEFINITION: A model (I, I) of a combination C is an rdfs-model of C if I is an rdfs-interpretation; in this case C is rdfs-satisfiable.

DEFINITION: Given a conforming datatype map D, a model (I, I) of a combination C is a D-model of C if I is a D-interpretation; in this case C is D-satisfiable.

2.2.3 Entailment

Using the notions of models defined above, entailment is defined in the usual way, i.e., through inclusion of sets of models.

DEFINITION: Given a conforming datatype map D, a RIF-RDF combination C D-entails a generalized RDF graph S if every D-model of C satisfies S. Likewise, C D-entails a closed RIF-BLD condition φ if every D-model of C satisfies φ.


The other notions of entailment are defined analogously:

DEFINITION: A combination C simple-entails S (resp., φ) if every simple model of C satisfies S (resp., φ).

DEFINITION: A combination C rdf-entails S (resp., φ) if every rdf-model of C satisfies S (resp., φ).

DEFINITION: A combination C rdfs-entails S (resp., φ) if every rdfs-model of C satisfies S (resp., φ).

3 OWL Compatibility

The syntax for exchanging OWL ontologies is based on RDF graphs. Therefore, RIF-OWL-combinations are combinations of RIF rulesets and sets of RDF graphs, analogous to RIF-RDF combinations. This section specifies how RIF rulesets and OWL ontologies interoperate in such combinations.

OWL (OWL-Reference) specifies three increasingly expressive species, namely Lite, DL, and Full. OWL Lite is a syntactic subset of OWL DL, but the semantics is the same (OWL-Semantics). Since every OWL Lite ontology is an OWL DL ontology, the Lite species is not considered separately in this document.

Syntactically speaking, OWL DL is a subset of OWL Full, but the semantics of the DL and Full species are different (OWL-Semantics). While OWL DL has an abstract syntax with a direct model-theoretic semantics, the semantics of OWL Full is an extension of the semantics of RDFS, and is defined on the RDF syntax of OWL. Consequently, the OWL Full semantics does not extend the OWL DL semantics; however, all derivations sanctioned by the OWL DL semantics are sanctioned by the OWL Full semantics.

Finally, the OWL DL RDF syntax, which is based on the OWL abstract syntax, does not extend the RDF syntax, but rather restricts it: every OWL DL ontology is an RDF graph, but not every RDF graph is an OWL DL ontology. OWL Full and RDF have the same syntax: every RDF graph is an OWL Full ontology and vice versa. This syntactical difference is reflected in the definition of RIF-OWL compatibility: combinations of RIF with OWL DL are based on the OWL abstract syntax, whereas combinations with OWL Full are based on the RDF syntax.


Since the OWL Full syntax is the same as the RDF syntax and the OWL Full semantics is an extension of the RDF semantics, the definition of RIF-OWL Full compatibility is a straightforward extension of RIF-RDF compatibility. Defining RIF-OWL DL compatibility in the same way would entail losing certain semantic properties of OWL DL. One of the main reasons for this is the difference in the way classes and properties are interpreted in OWL Full and OWL DL. In the Full species, classes and properties are both interpreted as objects in the domain of interpretation, which are then associated with subsets of and binary relations over the domain of interpretation using rdf:type and the extension function IEXT, as in RDF. In the DL species, classes and properties are directly interpreted as subsets of and binary relations over the domain. The latter is a key property of Description Logic semantics that enables the use of Description Logic reasoning techniques for processing OWL DL descriptions. Defining RIF-OWL DL compatibility as an extension of RIF-RDF compatibility would define a correspondence between OWL DL statements and RIF frame formulas. Since RIF frame formulas are interpreted using an extension function, the same way as in RDF, defining the correspondence between them and OWL DL statements would change the semantics of OWL statements, even if the RIF ruleset is empty. Consider, for example, an OWL DL ontology with a class membership statement

a rdf:type C .

This statement says that the set denoted by C contains at least one element that is denoted by a. The corresponding RIF frame formula is

a[rdf:type -> C]

The terms a, rdf:type, and C are all interpreted as elements in the individual domain, and the pair of elements denoted by a and C is in the extension of the element denoted by rdf:type.

This semantic discrepancy has practical implications in terms of entailments. Consider, for example, an OWL DL ontology with two class membership statements

a rdf:type C .
D rdf:type owl:Class .

and a RIF ruleset

Forall ?x ?y ?x=?y 

which says that every element is the same as every other element (note that such statements can also be written in OWL using owl:Thing and owl:hasValue). From the naïve combination of the two one can derive C=D, and indeed

a rdf:type D .

This derivation is not sanctioned by the OWL DL semantics, because even if every element is the same as every other element, the class D might be interpreted as the empty set.


A RIF-OWL combination that is faithful to the OWL DL semantics requires interpreting classes and properties as sets and binary relations, respectively, suggesting that correspondence could be defined with unary and binary predicates. It is, however, also desirable that there be uniform syntax for the RIF component of both OWL DL and RDF/OWL Full combinations, because one may not know at time of writing the rules which type of inference will be used. Consider, for example, an RDF graph S with the following statement

a rdf:type C .

and a RIF ruleset with the rule

Forall ?x ?x[rdf:type -> D] :- ?x[rdf:type -> C]

The combination of the two, according to the specification of RDF Compatibility, allows deriving

a rdf:type D .

Now, the RDF graph S is also an OWL DL ontology. Therefore, one would expect the triple to be derived by RIF-OWL DL combinations as well.


To ensure that the RIF-OWL DL combination is faithful to the OWL DL semantics and to enable using the same, or similar, rules with both OWL DL and RDF/OWL Full, the interpretation of frame formulas s[p -> o] in the RIF-OWL DL combinations is slightly different from their interpretation in RIF BLD and syntactical restrictions are imposed on the use of variables, function terms, and frame formulas.


Note that the abstract syntax form of OWL DL allows so-called punning (this is not allowed in the RDF syntax), i.e., the same IRI may be used in an individual position, a property position, and a class position; the interpretation of the IRI depends on its context. Since combinations of RIF and OWL DL are based on the abstract syntax of OWL DL, punning may also be used in these combinations. This paves the way towards combination with OWL 2, which is envisioned to allow punning in all its syntaxes.


Editor's Note: The semantics of RIF-OWL DL combinations is similar in spirit to the Semantic Web Rule Language proposal. However, a reference to SWRL from the above text does not seem appropriate.

3.1 Syntax of RIF-OWL Combinations

Since RDF graphs and OWL Full ontologies cannot be distinguished, the syntax of RIF-OWL Full combinations is the same as the syntax of RIF-RDF combinations.

The syntax of OWL ontologies in RIF-OWL DL combinations is specified by the abstract syntax of OWL DL. Certain restrictions are imposed on the syntax of the RIF rules in combinations with OWL DL. Specifically, the only terms allowed in class and property positions in frame formulas are constant symbols.

DEFINITION: A RIF-BLD condition φ is a RIF DL-condition if for every frame formula a[b -> c] in φ it holds that b is a constant and if b = rdf:type, then c is a constant.

DEFINITION: A RIF-BLD ruleset R is a DL-Ruleset if for every frame formula a[b -> c] in every rule of R it holds that b is a constant and if b = rdf:type, then c is a constant.

DEFINITION: A RIF-OWL-DL-combination is a pair < R,O>, where R is a DL-Ruleset and O is a set of OWL DL ontologies in abstract syntax form of a vocabulary V.

When clear from the context, RIF-OWL-DL-combinations are referred to simply as combinations.

3.1.1 Safeness Restrictions

In the literature, several restrictions on the use of variables in combinations of rules and Description Logics have been identified (Motik05, Rosati06) for the purpose of decidable reasoning. These restrictions are specified for RIF-OWL-DL combinations.

Given a set of OWL DL ontologies in abstract syntax form O, a variable ?x in a RIF rule Q then :- if is DL-safe if it occurs in an atomic formula in if that is not of the form s[P -> o] or s[rdf:type -> A], where P or A, respectively, occurs in one of the ontologies in O. A RIF rule Q then :- if is DL-safe, given O if every variable that occurs in then :- if is DL-safe. A RIF rule Q then :- if is weakly DL-safe, given O if every variable that occurs in then is DL-safe and every variable in if that is not DL-safe occurs only in atomic formulas in if that are of the form s[P -> o] or s[rdf:type -> A], where P or A, respectively, occurs in one of the ontologies in O.

Editor's Note: It is not strictly necessary to disallow disjunctions in the definition, but it would make the definition a lot more complex. It would require defining the disjunctive normal form of a condition formula and defining safeness with respect to each disjunct. Given that the safeness restriction is meant for implementation purposes, and that converting rules to disjunctive normal form is extremely expensive, it is probably a reasonable restriction to disallow disjunction.

DEFINITION: A RIF-OWL-DL-combination < R,O> is DL-safe if every rule in R is DL-safe, given O. A RIF-OWL-DL-combination < R,O> is weakly DL-safe if every rule in R is weakly DL-safe, given O.

Editor's Note: Do we want additional safeness restrictions to ensure that variables do not cross the abstract-concrete domain boundary?

3.2 Semantics of RIF-OWL Combinations

The semantics of RIF-OWL Full combinations is a straightforward extension of the Semantics of RIF-RDF Combinations.

The semantics of RIF-OWL-DL-combinations cannot straightforwardly extend the semantics of RIF RDF combinations, because OWL DL does not extend the RDF semantics. In order to keep the syntax of the rules uniform between RIF-OWL-Full- and RIF-OWL-DL-combinations, the semantics of RIF frame formulas is slightly altered in RIF-OWL-DL-combinations.

3.2.1 OWL Full

A D-interpretation I is an OWL Full interpretation if it interprets the OWL vocabulary and it satisfies the conditions in the sections 5.2 and 5.3 in (OWL Semantics).

The semantics of RIF-OWL Full combinations is a straightforward extension of the semantics of RIF-RDF combinations. It is based on the same notion of common-interpretations, but defines additional notions of satisfiability and entailment.

DEFINITION: Given a conforming datatype map D, a common-rif-rdf-interpretation (I, I) is an OWL-Full-model of a RIF-RDF combination C=< R, S > if I is a model of R, I is an OWL Full interpretation, and I satisfies every RDF graph S in S; in this case C is OWL-Full-satisfiable.

DEFINITION: Given a conforming datatype map D, a RIF-RDF combination C OWL-Full-entails a generalized RDF graph S if every OWL-Full-model of C satisfies S. Likewise, C OWL-Full-entails a closed RIF-BLD condition φ if every OWL-Full-model of C satisfies φ.

3.2.2 OWL DL

The semantics of RIF-OWL-DL-combinations is similar in spirit to the semantics of RIF-RDF combinations. Analogous to a common-rif-rdf-interpretation, there is the notion of common-rif-dl-interpretations, which are pairs of RIF and OWL DL interpretations, and which define a number of conditions that relate these interpretations to each other. In contrast to RIF-RDF combinations, the conditions below define a correspondence between the interpretation of OWL DL classes and properties and RIF unary and binary predicates.


3.2.2.1 Modified Semantics for RIF Frame Formulas

The modification of the semantics of RIF frame formulas is achieved by modifying the mapping function for frame formulas (Iframe), and leaving the RIF BLD semantics (RIF-BLD) otherwise unchanged.

Namely, frame formulas of the form s[rdf:type -> o] are interpreted as membership of s in the set denoted by o and frame formulas of the form s[p -> o], where p is not rdf:type, as membership of the pair (s, o) in the binary relation denoted by p.


DEFINITION: A RIF DL-semantic structure is a tuple I = <TV, DTS, D, Dind, Dfunc, IC, IV, IF, Iframe', ISF, Isub, Iisa, I=, Iexternal, Itruth>, where Iframe' is a mapping from Dind to total functions of the form SetOfFiniteFrame'Bags(D × D) → D, such that for each pair (a, b) in SetOfFiniteFrame'Bags(D × D) holds that if aIC(rdf:type), then b in Dind; all other elements of the structure are defined as in RIF semantic structures.

We define I(o[a1->v1 ... ak->vk]) = Iframe(I(o))({<I(a1),I(v1)>, ..., <I(an),I(vn)>}). The truth valuation function TValI is then defined as in RIF BLD.

DEFINITION: A RIF DL-semantic structure I is a model of a DL-Ruleset R if TValI(R)=t.

3.2.2.2 Semantics of RIF-OWL DL Combinations

As defined in (OWL-Semantics), an abstract OWL interpretation with respect to a datatype map D, with vocabulary V is a tuple I=< R, EC, ER, L, S, LV >, where

The OWL semantics imposes a number of further restrictions on the mapping functions as well as on the set of resources R, to achieve a separation of the interpretation of class, datatype, ontology property, datatype property, annotation property, and ontology property identifiers.

DEFINITION: Given a conforming datatype map D, a common-rif-dl-interpretation is a pair (I, I), where I = <TV, DTS, D, IC, IV, IF, Iframe', ISF, Isub, Iisa, I=, ITruth> is a RIF DL-semantic structure and I=<R, EC, ER, L, S, LV> is an abstract OWL interpretation with respect to D of a vocabulary V, such that the following conditions hold

  1. (O union LV)=Dind;
  2. EC(u) = set of all objects k in O such that Itruth(Iframe'(IC(rdf:type))(k,IC("u"^^rif:iri))) = t, for every class identifier u in V;
  3. ER(u) = set of all pairs (k, l) in O × (O union LV) such that Itruth(Iframe'(IC("u"^^rif:iri))( k, l ))) = t (true), for every data valued and individual valued property identifier u in V;
  4. L((s, d)) = IC("s"^^d) for every well-typed literal (s, d) in V;
  5. S(i) = IC("i"^^rif:iri) for every individual identifier i in V.

Condition 1 ensures that the relevant parts of the domains of interpretation are the same. Condition 2 ensures that the interpretation (extension) of an OWL DL class u corresponds to the interpretation of frames of the form ?x[rdf:type -> "u"^^rif:iri]. Condition 3 ensures that the interpretation (extension) of an OWL DL object or datatype property u corresponds to to the interpretation of frames of the form ?x["u"^^rif:iri -> ?y]. Condition 4 ensures that typed literals of the form (s, d) in OWL DL are interpreted in the same way as constants of the form "s"^^d in RIF. Finally, condition 5 ensures that individual identifiers in the OWL ontologies and the RIF rulesets are interpreted in the same way.


Using the definition of common-rif-dl-interpretation, satisfaction, models, and entailment are defined in the usual way:

DEFINITION: Given a conforming datatype map D, a common-rif-dl-interpretation (I, I) is an OWL-DL-model of a RIF-OWL-DL-combination C=< R, O > if I is a model of R and I satisfies every OWL DL ontology in abstract syntax form O in O; in this case C is OWL-DL-satisfiable. (I, I) is an OWL-DL-model of an OWL DL ontology in abstract syntax form O if I satisfies O. (I, I) is an OWL-DL-model of a closed RIF-BLD condition φ if Itruth(φ)=t.

DEFINITION: Given a conforming datatype map D, a RIF-OWL-DL-combination C OWL-DL-entails an OWL DL ontology in abstract syntax form O if every OWL-DL-model of C is an OWL-DL-model of O. Likewise, C OWL-DL-entails a closed RIF DL-condition φ if every OWL-DL-model of C is an OWL-DL-model of φ.


Recall that in an abstract OWL interpretation I the sets O, which is used for interpreting individuals, and LV, which is used for interpreting literals (data values), are disjoint and that EC maps class identifiers to subsets of O and datatype identifiers to subsets of LV. The disjointness entails that data values cannot be members of a class and individuals cannot be members of a datatype.

In RIF, variable quantification ranges over Dind. So, the same variable may be assigned to an abstract individual or a concrete data value. Additionally, RIF constants (e.g., IRIs) denoting individuals can be written in place of a data value, such as the value of a data-valued property or in datatype membership statements; similarly for constants denoting data values. Such statements cannot be satisfied in any common-rif-dl-interpretation, due to the constraints on the EC and ER functions. The following example illustrates several such statements.

Consider the datatype xsd:string and a RIF-OWL DL combination consisting of the set containing only the OWL DL ontology

ex:myiri rdf:type ex:A .

and a RIF ruleset containing the following fact

ex:myiri[rdf:type -> xsd:string]

This combination is not OWL-DL-satisfiable, because ex:myiri is an individual identifier and S maps individual identifiers to elements in O, which is disjoint from the elements in the datatype xsd:string.

Consider a RIF-OWL DL combination consisting of the set containing only the OWL DL ontology

ex:hasChild rdf:type owl:ObjectProperty .

and a RIF ruleset containing the following fact

ex:myiri[ex:hasChild -> "John"^^xsd:string]

This combination is not OWL-DL-satisfiable, because ex:hasChild is an object property, and values of object properties may not be concrete data values.

Consider a RIF-OWL DL combination consisting of the OWL DL ontology

ex:A rdfs:subClassOf ex:B

and a RIF ruleset containing the following rule

Forall ?x ?x[rdf:type -> ex:A]

This combination is not OWL-DL-satisfiable, because the rule requires every element, including every concrete data value, to be a member of the class ex:A. However, the mapping EC in any abstract OWL interpretation requires every member of ex:A to be an element of O, and concrete data values may not be members of O.

3.2.2.3 Annotation properties

Note that the above definition of RIF-OWL DL compatibility does not consider ontology and annotation properties, in contrast to the definition of compatibility of RIF with OWL Full, where there is no clear distinction between annotation and ontology properties and other kinds of properties. Therefore, it is not possible to "access" or use the values of these properties in the RIF rules. This limitation is overcome in the following definition. It is envisioned that the user will choose whether annotation and ontology properties are to be considered. It is currently expected that OWL 2 will not define a semantics for annotation and ontology properties; therefore, the below definition cannot be extended to the case of OWL 2.

DEFINITION: Given a conforming datatype map D, a common-rif-dl-interpretation (I, I) is a common-DL-annotation-interpretation if the following condition holds

  6. ER(u) = set of all pairs (k, l) in O × O such that Itruth(Iframe'(IC("u"^^rif:iri))( k, l) ) = t (true), for every IRI u in V.


Condition 6, which strengthens condition 3, ensures that the interpretation of all properties (also annotation and ontology properties) in the OWL DL ontologies corresponds with their interpretation in the RIF rules.

DEFINITION: Given a conforming datatype map D, a common-DL-annotation-interpretation (I, I) is an OWL-DL-annotation-model of a RIF-OWL-DL-combination C=< R, O > if I is a DL-model of R and I satisfies every OWL DL ontology in abstract syntax form O in O; in this case C is OWL-DL-annotation-satisfiable.

DEFINITION: Given a conforming datatype map D, a RIF-RDF combination C OWL-DL-annotation-entails an OWL DL ontology in abstract syntax form O if every OWL-DL-annotation-model of C is an OWL-DL-model of O. Likewise, C OWL-DL-annotation-entails a closed RIF-BLD condition φ if every OWL-DL-annotation-model of C is an OWL-DL-model of φ.

The difference between the two kinds of OWL DL entailment can be illustrated using an example. Consider the following OWL DL ontology in abstract syntax form

Ontology (ex:myOntology
  Annotation(dc:title "Example ontology"))

which defines an ontology with a single annotation (title). Consider also a ruleset consisting of the following rule:

Forall ?x, ?y ( ?x[ex:hasTitle -> ?y] :- ?x[dc:title -> ?y])

which says that whenever something has a dc:title, it has the same ex:hasTitle.

The combination of the ontology and the ruleset OWL-DL-annotation-entails the RIF condition formula ex:myOntology[ex:hasTitle -> "Example ontology"^^xsd:string]; the combination does not OWL-DL-entail the formula.

4 References

4.1 Normative References

[OWL-Semantics]
OWL Web Ontology Language Semantics and Abstract Syntax, P. F. Patel-Schneider, P. Hayes, I. Horrocks, Editors, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-owl-semantics-20040210/. Latest version available at http://www.w3.org/TR/owl-semantics/.
[RDF-Concepts]
Resource Description Framework (RDF): Concepts and Abstract Syntax, G. Klyne, J. Carroll (Editors), W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-concepts-20040210/. Latest version available at http://www.w3.org/TR/rdf-concepts/.
[RDF-Semantics]
RDF Semantics, P. Hayes, Editor, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-mt-20040210/. Latest version available at http://www.w3.org/TR/rdf-mt/.
[RIF-BLD]
RIF Basic Logic Dialect Harold Boley, Michael Kifer, eds. W3C Editor's Draft, 15 April 2008, http://www.w3.org/2005/rules/wg/draft/ED-rif-bld-20080415/. Latest version available at http://www.w3.org/2005/rules/wg/draft/rif-bld/.
[XML-Schema2]
XML Schema Part 2: Datatypes, W3C Recommendation, 2 May 2001. This version is http://www.w3.org/TR/2001/REC-xmlschema-2-20010502/. Latest version available at http://www.w3.org/TR/xmlschema-2/.

4.2 Informational References

[CURIE]
CURIE Syntax 1.0, M. Birbeck and S. McCarron, W3C Working Draft, 26 November 2007, http://www.w3.org/TR/2007/WD-curie-20071126/. Latest version available at http://www.w3.org/TR/curie/.
[DLP]
Description Logic Programs: Combining Logic Programs with Description Logics, B. Grosof, R. Volz, I. Horrocks, S. Decker. In Proc. of the 12th International World Wide Web Conference (WWW 2003), 2003.
[Motik05]
Query Answering for OWL-DL with rules, B. Motik, U. Sattler, R. Studer, Journal of Web Semantics 3(1): 41-60, 2005.
[RDF-Schema]
RDF Vocabulary Description Language 1.0: RDF Schema, D. Brickley, R.V. Guha, Editors, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-schema-20040210/. Latest version available at http://www.w3.org/TR/rdf-schema/.
[RDF-Syntax]
RDF/XML Syntax Specification (Revised), D. Beckett, Editor, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-rdf-syntax-grammar-20040210/. Latest version available at http://www.w3.org/TR/rdf-syntax-grammar/.
[RFC-3066]
RFC 3066 - Tags for the Identification of Languages, H. Alvestrand, IETF, January 2001. This document is http://www.isi.edu/in-notes/rfc3066.txt.
[RIF-PRD]
RIF Production Rule dialect, C. de Sainte Marie, Editor, Editor's Draft. Latest version available at http://www.w3.org/2005/rules/wg/wiki/PRdialect.
[RIF-UCR]
RIF Use Cases and Requirements, A. Ginsberg, D. Hirtle, F. McCabe, P.-L. Patranjan, Editors, W3C Working Draft, 10 July 2006, http://www.w3.org/TR/2006/WD-rif-ucr-20060710/. Latest version available at http://www.w3.org/TR/rif-ucr/.
[OWL-Reference]
OWL Web Ontology Language Reference, M. Dean, G. Schreiber, Editors, W3C Recommendation, 10 February 2004, http://www.w3.org/TR/2004/REC-owl-ref-20040210/. Latest version available at http://www.w3.org/TR/owl-ref/.
[Rosati06]
DL+log: Tight Integration of Description Logics and Disjunctive Datalog, R. Rosati, Proceedings of the 10th International Conference on Principles of Knowledge Representation and Reasoning, pp. 68-78, 2005.
[Turtle]
Turtle - Terse RDF Triple Language, D. Beckett and T. Berners-Lee, W3C Team Submission, 14 January 2008, http://www.w3.org/TeamSubmission/2008/SUBM-turtle-20080114/. Latest version available at http://www.w3.org/TeamSubmission/turtle/.

5 Appendix: Embeddings (Informative)

RIF-RDF combinations can be embedded into RIF Rulesets in a fairly straightforward way, thereby demonstrating how a RIF-compliant translator without native support for RDF can process RIF-RDF combinations.

RIF-OWL combinations cannot be embedded in RIF, in the general case. However, there is a subset of RIF-OWL DL combinations that can be embedded.


Throughout this section the function tr is defined, which maps symbols, triples, RDF graphs, and OWL DL ontologies in abstract syntax form to RIF symbols, statements, and rulesets.

5.1 Embedding RIF-RDF Combinations

5.1.1 Embedding Symbols

Given a combination C=< R,S>, the function tr maps RDF symbols of a vocabulary V and a set of blank nodes B to RIF symbols, as defined in following table.

Mapping RDF symbols to RIF.
RDF Symbol RIF Symbol Mapping
IRI i in VU Constant with symbol space rif:iri tr(i) = "i"^^rif:iri
Blank node x in B Variable symbols ?x tr(x) = ?x
Plain literal without a language tag xxx in VPL Constant with the datatype xsd:string tr("xxx") = "xxx"^^xsd:string
Plain literal with a language tag (xxx,lang) in VPL Constant with the datatype rif:text tr("xxx"@lang) = "xxx@lang"^^rif:text
Well-typed literal (s,u) in VTL Constant with the symbol space u tr("s"^^u) = "s"^^u
Ill-typed literal (s,u) in VTL Constant s^^u' with symbol space rif:local that is not used in C tr("s"^^u) = "s^^u'"^^rif:local

The embedding is not defined for combinations that include RDF graphs with RDF URI references that are not absolute IRIs.

5.1.2 Embedding Triples and Graphs

The mapping function tr is extended to embed triples as RIF statements. Finally, two embedding functions, trR and trQ embed RDF graphs as RIF rulesets and conditions, respectively. The following section shows how these embeddings can be used for reasoning with combinations.

We define two mappings for RDF graphs, one (trR) in which variables are Skolemized, i.e., replaced with constant symbols, and one (trQ) in which variables are existentially quantified.

The function sk takes as an argument a formula R with variables, and returns a formula R', which is obtained from R by replacing every variable symbol ?x in R with "new-iri"^^rif:iri, where new-iri is a new globally unique IRI.

RDF Construct RIF Construct Mapping
Triple s p o . Frame formula tr(s)[tr(p) -> tr(o)] tr(s p o .) = tr(s)[tr(p) -> tr(o)]
Graph S Ruleset trR(S) trR(S) = the set of all sk(Forall tr(s p o .)) where s p o . is a triple in S
Graph S Condition (query) trQ(S) trQ(S) = Exists tr(x1), ..., tr(xn) And(tr(t1) ... tr(tm)), where x1, ..., xn are the blank nodes occurring in S and t1, ..., tm are the triples in S

5.1.3 Embedding Simple Entailment

The following theorem shows how checking simple-entailment of combinations can be reduced to checking entailment of RIF conditions by using the embeddings of RDF graphs of the previous section.

Theorem A combination C=<R,{S1,...,Sn}> simple-entails a generalized RDF graph S iff (R union trR(S1) union ... union trR(Sn)) entails trQ(S). C simple-entails a RIF condition φ iff (R union trR(S1) union ... union trR(Sn)) entails φ.

5.1.4 Built-ins required

The embeddings of RDF and RDFS entailment require a number of built-in predicate symbols to be available to appropriately deal with literals.

Editor's Note: It is not yet clear which built-in predicates will be available in RIF. Therefore, the built-ins mentioned in this section may change. Furthermore, built-ins may be axiomatized if they are not provided by the language.

Given a vocabulary V,

5.1.5 Embedding RDF Entailment

We axiomatize the semantics of the RDF vocabulary using the following RIF rules and conditions.

The compact URIs used in the RIF rules in this section and the next are short for the complete URIs with the rif:iri datatype, e.g., rdf:type is short for "http://www.w3.org/1999/02/22-rdf-syntax-ns#type"^^rif:iri

RRDF = (Forall tr(s p o .)) for every RDF axiomatic triple s p o .) union
(
   Forall ?x ?x[rdf:type -> rdf:Property] :- Exists ?y,?z (?y[?x -> ?z]),
Forall ?x ?x[rdf:type -> rdf:XMLLiteral] :- wellxml(?x),
Forall ?x "1"^^xsd:integer="2"^^xsd:integer :- And(?x[rdf:type -> rdf:XMLLiteral] illxml(?x))
)


Theorem A combination <R,{S1,...,Sn}> is rdf-satisfiable iff (RRDF union R union trR(S1) union ... union trR(Sn)) has a model.

Theorem A combination C=<R,{S1,...,Sn}> rdf-entails a generalized RDF graph T iff (RRDF union R union trR(S1) union ... union trR(Sn)) entails trQ(T). C simple-entails a RIF condition φ iff (RRDF union R union trR(S1) union ... union trR(Sn)) entails φ.

5.1.6 Embedding RDFS Entailment

We axiomatize the semantics of the RDF(S) vocabulary using the following RIF rules and conditions.

RRDFS = RRDF union

(Forall tr(s p o .)) for every RDFS axiomatic triple s p o .) union
(

     Forall ?x ?x[rdf:type -> rdfs:Resource],
Forall ?u,?v,?x,?y ?u[rdf:type -> ?y] :- And(?x[rdfs:domain -> ?y] ?u[?x -> ?v]),
Forall ?u,?v,?x,?y ?v[rdf:type -> ?y] :- And(?x[rdfs:range -> ?y] ?u[?x -> ?v]),
Forall ?x ?x[rdfs:subPropertyOf -> ?x] :- ?x[rdf:type -> rdf:Property],
Forall ?x,?y,?z ?x[rdfs:subPropertyOf -> ?z] :- And (?x[rdfs:subPropertyOf -> ?y] ?y[rdfs:subPropertyOf -> ?z]),
Forall ?x,?y,?z1,?z2 ?z1[?y -> ?z2] :- And (?x[rdfs:subPropertyOf -> ?y] ?z1[?x -> ?z2]),
Forall ?x ?x[rdfs:subClassOf -> rdfs:Resource] :- ?x[rdf:type -> rdfs:Class],
Forall ?x,?y,?z ?z[rdf:type -> ?y] :- And (?x[rdfs:subClassOf -> ?y] ?z[rdf:type -> ?x]),
Forall ?x ?x[rdfs:subClassOf -> ?x] :- ?x[rdf:type -> rdfs:Class],
Forall ?x,?y,?z ?x[rdfs:subClassOf -> ?z] :- And (?x[rdfs:subClassOf -> ?y] ?y[rdfs:subClassOf -> ?z]),
Forall ?x ?x[rdfs:subPropertyOf -> rdfs:member] :- ?x[rdf:type -> rdfs:ContainerMembershipProperty],
Forall ?x ?x[rdfs:subClassOf -> rdfs:Literal] :- ?x[rdf:type -> rdfs:Datatype],
Forall ?x ?x[rdf:type -> rdfs:Literal] :- lit(?x),
Forall ?x "1"^^xsd:integer="2"^^xsd:integer :- And(?x[rdf:type -> rdfs:Literal] illxml(?x))
)


Theorem A combination <R1,{S1,...,Sn}> is rdfs-satisfiable iff (RRDFS union R1 union trR(S1) union ... union trR(Sn)) has a model.

Theorem A combination <R,{S1,...,Sn}> rdfs-entails a generalized RDF graph T iff (RRDFS union R union trR(S1) union ... union trR(Sn)) entails trQ(T). C rdfs-entails a RIF condition φ iff (RRDFS union R union trR(S1) union ... union trR(Sn)) entails φ.

5.1.7 Embedding D-Entailment

We axiomatize the semantics of the data types using the following RIF rules and conditions.

RD = RRDFS union

(Forall u[rdf:type -> rdfs:Datatype] | for every IRI u in the domain of D) union
(Forall "s"^^u[rdf:type -> "u"^^rif:iri] | for every well-typed literal (s , u ) in VTL) union
(Forall ?x, ?y dt(?x,?y) :- And(?x[rdf:type -> ?y] ?y[rdf:type -> rdfs:Datatype]),

Forall ?x "1"^^xsd:integer="2"^^xsd:integer :- And(?x[rdf:type -> rdfs:Literal] illD(?x)))

Theorem A combination <R,{S1,...,Sn}>, where R does not contain the equality symbol, is D-satisfiable iff (RD union R union trR(S1) union ... union trR(Sn)) is satisfiable and does not entail Exists ?x And(dt(?x,u) dt(?x,u')) for any two URIs u and u' in the domain of D such that the value spaces of D(u) and D(u') are disjoint, and does not entail Exists ?x dt(s^^u,"u'"^^rif:iri) for any (s, u) in VTL and u' in the domain of D such that s is not in the lexical space of D(u').

Editor's Note: Since this condition is very complex we might consider discarding this theorem, and suggest the above set of rules (RD) as an approximation of the semantics.


Theorem A D-satisfiable combination <R,{S1,...,Sn}>, where R does not contain the equality symbol, D-entails a generalized RDF graph T iff (RD union R union trR(S1) union ... union trR(Sn)) entails trQ(T). C D-entails a RIF condition φ iff (RD union R union trR(S1) union ... union trR(Sn)) entails φ.

Editor's Note: The restriction to equality-free rulesets is necessary because, in case different datatype URIs are equal, D-interpretations impose stronger conditions on the interpretation of typed literals than RIF does.

5.2 Embedding RIF-OWL DL Combinations

It is known that expressive Description Logic languages such as OWL DL cannot be straightforwardly embedded into typical rules languages such as RIF BLD.

In this section we therefore consider a subset of OWL DL in RIF-OWL DL combinations. We define OWL DLP, which is inspired by so-called Description Logic programs (DLP), and define how reasoning with RIF-OWL DLP combinations can be reduced to reasoning with RIF.

5.2.1 Identifying OWL DLP

Our definition of OWL DLP removes disjunction and extensional quantification from consequents of implications and removes negation and equality.

We introduce OWL DLP through its abstract syntax, which is a subset of the abstract syntax of OWL DL. The semantics of OWL DLP is the same as OWL DL.

The basic syntax of ontologies and identifiers remains the same.

ontology ::= 'Ontology(' [ ontologyID ] { directive } ')'
directive ::= 'Annotation(' ontologyPropertyID ontologyID ')'
        | 'Annotation(' annotationPropertyID URIreference ')'
        | 'Annotation(' annotationPropertyID dataLiteral ')'
        | 'Annotation(' annotationPropertyID individual ')'
        | axiom
        | fact
datatypeID ::= URIreference
classID ::= URIreference
individualID ::= URIreference
ontologyID ::= URIreference
datavaluedPropertyID ::= URIreference
individualvaluedPropertyID ::= URIreference
annotationPropertyID ::= URIreference
ontologyPropertyID ::= URIreference
dataLiteral ::= typedLiteral | plainLiteral
typedLiteral ::= lexicalForm^^URIreference
plainLiteral ::= lexicalForm | lexicalForm@languageTag
lexicalForm ::= as in RDF, a unicode string in normal form C
languageTag ::= as in RDF, an XML language tag


Facts are the same as for OWL DL, except that equality and inequality (SameIndividual and DifferentIndividual), as well as individuals without an identifier are not allowed.

fact ::= individual 
individual ::= 'Individual(' individualID { annotation } 
  { 'type(' type ')' } { value } ')'
value ::= 'value(' individualvaluedPropertyID individualID ')'
       | 'value(' individualvaluedPropertyID  individual ')'
       | 'value(' datavaluedPropertyID  dataLiteral ')'
type ::= Rdescription

The main restrictions posed by OWL DLP on the OWL DL syntax are on descriptions and axioms. Specifically, we need to distinguish between descriptions which are allowed on the right-hand side (Rdescription) and those allowed on the left-hand side (Ldescription) of subclass statements.

We start with descriptions that may be allowed on both sides

dataRange ::= datatypeID | 'rdfs:Literal'
description ::= classID
           | restriction
           | 'intersectionOf(' { description } ')'
restriction ::= 'restriction(' datavaluedPropertyID dataRestrictionComponent 
  { dataRestrictionComponent } ')'
           | 'restriction(' individualvaluedPropertyID individualRestrictionComponent 
  { individualRestrictionComponent } ')'
dataRestrictionComponent ::= 'value(' dataLiteral ')'
individualRestrictionComponent ::= 'value(' individualID ')'

We then proceed with the individual sides

Ldescription ::= description
           | Lrestriction
           | 'unionOf(' { Ldescription } ')'
           | 'intersectionOf(' { Ldescription } ')'
           | 'oneOf(' { individualID } ')'


Lrestriction ::= 'restriction(' datavaluedPropertyID LdataRestrictionComponent 
  { LdataRestrictionComponent } ')'
           | 'restriction(' individualvaluedPropertyID LindividualRestrictionComponent 
  { LindividualRestrictionComponent } ')'
LdataRestrictionComponent ::= 'someValuesFrom(' dataRange ')'
           | 'value(' dataLiteral ')'
LindividualRestrictionComponent ::= 'someValuesFrom(' description ')'
           | 'value(' individualID ')'


Rdescription ::= description
           | Rrestriction
           | 'intersectionOf(' { Rdescription } ')'
Rrestriction ::= 'restriction(' datavaluedPropertyID RdataRestrictionComponent 
  { RdataRestrictionComponent } ')'
           | 'restriction(' individualvaluedPropertyID RindividualRestrictionComponent 
  { RindividualRestrictionComponent } ')'
RdataRestrictionComponent ::= 'allValuesFrom(' dataRange ')'
           | 'value(' dataLiteral ')'
RindividualRestrictionComponent ::= 'allValuesFrom(' description ')'
           | 'value(' individualID ')'

Finally, we turn to axioms. We start with class axioms.

axiom ::= 'Class(' classID  ['Deprecated'] 'complete' { annotation } { description } ')'
axiom ::= 'Class(' classID  ['Deprecated'] 'partial' { annotation } { Rdescription } ')'
axiom ::= 'DisjointClasses(' Ldescription Ldescription { Ldescription } ')'
       | 'EquivalentClasses(' description { description } ')'
       | 'SubClassOf(' Ldescription Rdescription ')'


axiom ::= 'Datatype(' datatypeID ['Deprecated']  { annotation } )'

Property axioms in OWL DLP restrict those in OWL DL by disallowing functional and inverse functional properties, because these involve equality.

 axiom ::= 'DatatypeProperty(' datavaluedPropertyID ['Deprecated'] { annotation } 
               { 'super(' datavaluedPropertyID ')'}
               { 'domain(' description ')' } { 'range(' dataRange ')' } ')'
       | 'ObjectProperty(' individualvaluedPropertyID ['Deprecated'] { annotation } 
               { 'super(' individualvaluedPropertyID ')' }
               [ 'inverseOf(' individualvaluedPropertyID ')' ] [ 'Symmetric' ] 
               [ 'Transitive' ]
               { 'domain(' description ')' } { 'range(' description ')' } ')'
       | 'AnnotationProperty(' annotationPropertyID { annotation } ')'
       | 'OntologyProperty(' ontologyPropertyID { annotation } ')'
 axiom ::= 'EquivalentProperties(' datavaluedPropertyID datavaluedPropertyID  { datavaluedPropertyID } ')'
       | 'SubPropertyOf(' datavaluedPropertyID  datavaluedPropertyID ')'
       | 'EquivalentProperties(' individualvaluedPropertyID individualvaluedPropertyID  { individualvaluedPropertyID } ')'
       | 'SubPropertyOf(' individualvaluedPropertyID  individualvaluedPropertyID ')'

5.2.2 Embedding RIF DL-rulesets into RIF BLD

Recall that the semantics of frame formulas in DL-rulesets is different from the semantics of frame formulas in RIF BLD.

Frame formulas in DL-rulesets are embedded as predicates in RIF BLD.

The mapping tr is the identity mapping on all RIF formulas, with the exception of frame formulas.

Mapping RIF DL-rulesets to RIF rulesets.
RIF Construct Mapping
Term x tr(x)=x
Atomic formula x that is not a frame formula tr(x)=x
a[b -> c], where a,c are terms and brdf:type is a constant tr(a[b -> c])=b'(a,c), where b' is a constant symbol obtained from b that does not occur in the original ruleset or the ontologies
a[rdf:type -> c], where a is a term and c is a constant tr(a[rdf:type -> c])=c'(a), where c' is a constant symbol obtained from c that does not occur in the original ruleset or the ontologies
Exists ?V1 ... ?Vn(φ) tr(Exists ?V1 ... ?Vn(φ))=Exists ?V1 ... ?Vn(tr(φ))
And(φ1 ... φn) tr(And(φ1 ... φn))=And(tr(φ1) ... tr(φn))
Or(φ1 ... φn) tr(Or(φ1 ... φn))=Or(tr(φ1) ... tr(φn))
φ1 :- φ2 tr(φ1 :- φ2)=tr(φ1) :- tr(φ2)
Forall ?V1 ... ?Vn(φ) tr(Forall ?V1 ... ?Vn(φ))=Forall ?V1 ... ?Vn(tr(φ))
Ruleset(φ1 ... φn) tr(Ruleset(φ1 ... φn))=Ruleset(tr(φ1) ... tr(φn))

5.2.3 Embedding OWL DLP into RIF BLD

The embedding of OWL DLP into RIF BLD has two stages: normalization and embedding.

5.2.3.1 Normalization

Normalization splits the OWL axioms so that the mapping of the individual axioms results in rules. Additionally, it simplifies the abstract syntax and removes annotations.

Editor's Note: Embedding OWL-DL-annotation semantics would require maintaining the annotation properties.

Normalizing OWL DLP.
Complex OWL Normalized OWL
trN(
Ontology( [ ontologyID ] 
 directive1
 ... 
 directiven )
)
trN(directive1)
...
trN(directiven)
trN(Annotation( ... ))
trN(
Individual( individualID 
  annotation1 
  ...
  annotationn 
  type1
  ...
  typem
  value1
  ...
  valuek )
)

trN(Individual( individualID type1 ))

  ...

trN(Individual( individualID typem ))

Individual( individualID value1 )
  ...
Individual( individualID valuek )
trN(
Individual( individualID 
  type(intersectionOf(
   description1
   ...
   descriptionn
  ))
)

trN(Individual( individualID type(description1) ))

  ...

trN(Individual( individualID type(descriptionn) ))

trN(
Individual( individualID type(X))
)
Individual( individualID type(X))
X is a classID or value restriction
trN(
Individual( individualID 
 type(restriction(propertyID 
 allValuesFrom(X))))
)
trN(
SubClassOf( oneOf(individualID) 
  restriction(propertyID allValuesFrom(X)))

)

trN(
Class( classID  [Deprecated] 
 complete 
  annotation1 
  ...
  annotationn 
  description1
  ...
  descriptionm )
)

trN(

EquivalentClasses(classID  
 intersectionOf(description1
  ...
  descriptionm )
)
trN(
Class( classID  [Deprecated] 
 partial
  annotation1 
  ...
  annotationn 
  description1
  ...
  descriptionm )
)

trN(

SubClassOf(classID  
 intersectionOf(description1
  ...
  descriptionm )
)
trN(
DisjointClasses(
  description1
  ...
  descriptionm )
)
trN(SubClassOf(intersectionOf(description1 description2) owl:Nothing))
 ...

trN(SubClassOf(intersectionOf(description1 descriptionm) owl:Nothing))

 ...

trN(SubClassOf(intersectionOf(descriptionm-1 descriptionm) owl:Nothing))

trN(
EquivalentClasses(
  description1
  ...
  descriptionm )
)

trN(SubClassOf(description1 description2)) trN(SubClassOf(description2 description1))

  ...

trN(SubClassOf(descriptionm-1 descriptionm)) trN(SubClassOf(descriptionm descriptionm-1))

trN(
SubClassOf(description X)
)
SubClassOf(description X)
X is a description that does not contain intersectionOf
trN(
SubClassOf(description 
  ...intersectionOf(
    description1 
    ...
    descriptionn
  )...)
)

trN(SubClassOf(description ...description1...))

...

trN(SubClassOf(description ...descriptionn...))

trN(Datatype( ... ))
trN(
DatatypeProperty( propertyID [ Deprecated ]
  annotation1 
  ...
  annotationn 
  super(superproperty1)
  ...
  super(superpropertym)
  domain(domaindescription1)
  ...
  domain(domaindescriptionj)
  range(rangedescription1)
  ...
  range(rangedescriptionk) )
)
SubPropertyOf(propertyID superproperty1)
  ... 
SubPropertyOf(propertyID superpropertym)

trN(SubClassof(restriction(propertyID someValuesFrom(owl:Thing)) domaindescription1))

  ...

trN(SubClassof(restriction(propertyID someValuesFrom(owl:Thing)) domaindescriptionj))

trN(SubClassof(owl:Thing restriction(propertyID allValuesFrom(rangedescription1)))

  ...

trN(SubClassof(owl:Thing restriction(propertyID allValuesFrom(rangedescriptionk)))

trN(
ObjectProperty( propertyID [ Deprecated ]
  annotation1 
  ...
  annotationn 
  super(superproperty1)
  ...
  super(superpropertym)
  [ inverseOf( inversePropertyID ) ] 
  [ Symmetric ] 
  [ Transitive ]
  domain(domaindescription1)
  ...
  domain(domaindescriptionl)
  range(rangedescription1)
  ...
  range(rangedescriptionk) )
)
SubPropertyOf(propertyID superproperty1)
  ... 
SubPropertyOf(propertyID superpropertym)

trN(SubClassof(restriction(propertyID someValuesFrom(owl:Thing)) domaindescription1))

  ...

trN(SubClassof(restriction(propertyID someValuesFrom(owl:Thing)) domaindescriptionl))

trN(SubClassof(owl:Thing restriction(propertyID allValuesFrom(rangedescription1)))

  ...

trN(SubClassof(owl:Thing restriction(propertyID allValuesFrom(rangedescriptionk)))

ObjectProperty( propertyID
  [ inverseOf( inversePropertyID ) ] )
ObjectProperty( propertyID [ Symmetric ] )
ObjectProperty( propertyID [ Transitive ] )
trN(
EquivalentProperties(
  property1
  ...
  propertym )
)

trN(SubPropertyOf(property1 property2)) trN(SubPropertyOf(property2 property1))

  ...

trN(SubPropertyOf(propertym-1 propertym)) trN(SubPropertyOf(propertym propertym-1))

The result of the normalization is a set of individual property value, individual typing, subclass, subproperty, and property inverse, symmetry and transitive statements.

5.2.3.2 Embedding

We now proceed with the embedding of normalized OWL DL ontologies into a RIF DL-ruleset. The embedding extends the embedding function tr. The embeddings of IRIs and literals is as defined in the Section Embedding Symbols.

Editor's Note: This embedding assumes that for a given datatype identifier D, there is unary built-in predicate isD, called the "positive guard" for D, which is always interpreted as the value space of the datatype denoted by D and there is a built-in isNotD, called the "negative guard" for D, which is always interpreted as the complement of the value space of the datatype denoted by D.

Embedding OWL DLP.
Normalized OWL RIF DL-ruleset
trO(
directive1 
 ... 
directiven
)
trO(directive1)
...
trO(directiven)
trO(
Individual( individualID type(A) )
)

tr(individualID)[rdf:type -> tr(A)]

A is a classID
trO(
Individual( individualID 
  type(restriction(propertyID value(b))) )
)

tr(individualID)[tr(propertyID) -> tr(b)]

trO(
Individual( individualID 
  value(propertyID b) )
)

tr(individualID)[tr(propertyID) -> tr(b)]

trO(
SubPropertyOf(property1 property2)
)

Forall ?x, ?y (?x[tr(property2) -> ?y] :- ?x[tr(property1) -> ?y])

trO(
ObjectProperty(propertyID)
)
trO(
ObjectProperty(property1
  inverseOf(property2) )
)

Forall ?x, ?y (?y[tr(property2) -> ?x] :- ?x[tr(property1) -> ?y])

Forall ?x, ?y (?y[tr(property1) -> ?x] :- ?x[tr(property2) -> ?y])

trO(
ObjectProperty(propertyID
  Symmetric )
)

Forall ?x, ?y (?y[tr(propertyID) -> ?x] :- ?x[tr(propertyID) -> ?y])

trO(
ObjectProperty(propertyID
  Transitive )
)

Forall ?x, ?y, ?z (?x[tr(propertyID) -> ?z] :- And( ?x[tr(propertyID) -> ?y] ?y[tr(propertyID) -> ?z]))

trO(
SubClassOf(description1 description2)
)

trO(description1,description2,?x)

trO(description1,X,?x)

Forall ?x (trO(X, ?x) :- trO(description1, ?x )

X is a classID or value restriction
trO(description1,D,?x)

Forall ?x (trO(owl:Nothing, ?x) :- And( isNotD(?x) trO(description1, ?x ) )

D is a datatypeID and isNotD is the "negative guard" for D
trO(description1,restriction(property1 allValuesFrom(...restriction(propertyn allValuesFrom(X)) ...)),?x)

Forall ?x, ?y1, ..., ?yn (trO(X, ?yn) :- And( trO(description1, ?x)?x[tr(property1) -> ?y1] ?y1[tr(property2) -> ?y2] ... ?yn-1[tr(propertyn) -> ?yn]))

X is a classID or value restriction
trO(description1,restriction(property1 allValuesFrom(...restriction(propertyn allValuesFrom(D)) ...)),?x)

Forall ?x, ?y1, ..., ?yn (trO(owl:Nothing, ?yn) :- And( trO(description1, ?x)?x[tr(property1) -> ?y1] ?y1[tr(property2) -> ?y2] ... ?yn-1[tr(propertyn) -> ?yn] isNotD(?yn)))

D is a datatypeID or value restriction
trO(A,?x)

?x[rdf:type -> tr(A)]

A is a classID
trO(D,?x)

isD(?x)

D is a datatypeID and isD is the "guard" for the datatype
trO(intersectionOf(description1 ... descriptionn, ?x)

And(trO(description1, ?x) ... trO(descriptionn, ?x))

trO(unionOf(description1 ... descriptionn, ?x)

Or(trO(description1, ?x) ... trO(descriptionn, ?x))

trO(oneOf(value1 ... valuen, ?x)

Or( ?x = trO(value1) ... ?x = trO(valuen))

trO(restriction(propertyID someValuesFrom(description)), ?x)

Exists ?y(And(?x[tr(propertyID) -> ?y] trO(description, ?y) ))

trO(restriction(propertyID value(valueID)), ?x)

?x[tr(propertyID) -> tr(valueID) ]

trO(owl:Thing, ?x)

?x = ?x

trO(owl:Nothing, ?x)

"1"^^xsd:integer="2"^^xsd:integer

5.2.4 Reasoning with RIF-OWL DLP Combinations

Theorem A RIF-OWL-DL-combination <R,{O1,...,On}>, where O1,...,On are OWL DLP ontologies, is OWL-DL-satisfiable iff tr(R union trO(trN(O1)) union ... union trO(trN(On))) has a model.

Theorem An OWL-DL-satisfiable RIF-OWL-DL-combination C=<R,{O1,...,On}>, where O1,...,On are OWL DLP ontologies, OWL-DL-entails a closed RIF condition φ iff tr(R union trO(trN(O1)) union ... union trO(trN(On))) entails φ.

6 End Notes

RDF URI References: There are certain RDF URI references that are not absolute IRIs (e.g., those containing spaces). It is possible to use such RDF URI references in RDF graphs that are combined with RIF rules. However, such URI references cannot be represented in RIF rules and their use in RDF is discouraged.

Generalized RDF graphs: Standard RDF graphs, as defined in (RDF-Concepts), do not allow the use of literals in subject and predicate positions and blank nodes in predicate positions. The RDF Core working group has listed two issues questioning the restrictions that literals may not occur in subject and blank nodes may not occur in predicate positions in triples. Anticipating lifting of these restrictions in a possible future version of RDF, we use the more liberal notion of generalized RDF graph. We note that the definitions of interpretations, models, and entailment in the RDF semantics document (RDF-Semantics) also apply to such generalized RDF graphs.

We note that every standard RDF graph is a generalized RDF graph. Therefore, our definition of combinations applies to standard RDF graphs as well.

We note also that the notion of generalized RDF graphs is more liberal than the notion of RDF graphs used by SPARQL; generalized RDF graphs additionally allow blank nodes and literals in predicate positions.