rdf:text: A Datatype for Internationalized Text

W3C Working Draft 21 April 2009

This version:

http://www.w3.org/TR/2009/WD-rdf-text-20090421/

Latest version:

http://www.w3.org/TR/rdf-text/

Previous version:

http://www.w3.org/TR/2008/WD-rdf-text-20081202/

Authors:

Jie Bao, Rensselaer Polytechnic Institute, Troy, New York, USA

Axel Polleres, DERI Galway at the National University of Ireland, Galway, Ireland

Boris Motik, Oxford University, Oxford, UK

This document is also available in these non-normative formats: PDF version.

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Copyright © 2009 W3C^® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

Abstract

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May Be Superseded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/.

Summary of Changes

Last Call

The Working Group believes it has completed its design work for the technologies specified this document, so this is a "Last Call" draft. The design is not expected to change significantly, going forward, and now is the key time for external review, before the implementation phase.

Please Comment By 12 May 2009

The OWL Working Group and the Rule Interchange Format (RIF) Working Group seek public feedback on this Working Draft. Please send your comments to public-owl-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 and see if the relevant text has already been updated.

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.

Many RDF [RDF] applications need a mechanism for representing text in various different languages, retrieving the text written in a specific language, and other kinds of language-specific processing. To facilitate this, RDF provides plain literals with a language tag, which form the basis for processing text in different languages in RDF. Apart from such literals, however, RDF also provides for plain literals without a language tag and typed literals. RDF thus provides three distinct types of literals each of which is treated in a separate way, which increases complexity for specifications based on RDF such as RIF and OWL. Furthermore, RDF does not provide a name for the set of all plain literals, which, for example, prevents one from stating that the range of some OWL property must be a plain literal with a language tag.

To address these deficiencies, this specification defines a datatype called rdf:text. This datatype provides a name for the set of all data values assigned to plain literals, which is why the datatype uses the rdf: prefix. Furthermore, typed rdf:text literals are semantically equivalent to plain literals, which allows specifications built on top of RDF to consider only typed literals. Since the rdf:text datatype just provides additional forms for writing plain literals, its addition does not change the semantics of RDF. Furthermore, when exchanging RDF graphs between RDF tools, typed rdf:text literals must be replaced with plain literals, thus maximizing interoperability between RDF tools that support rdf:text and those that do not.

RDF tools may use other mechanisms for representing text in different languages, such as using the xml:lang attribute on the data values of the rdf:XMLLiteral datatype. The rdf:text datatype does not provide a replacement for such mechanisms.

A character is an atomic unit of text. Each character has a Universal Character Set (UCS) code point [ISO/IEC 10646] (or, equivalently, a Unicode code point [UNICODE]) that MUST match the Char production from XML [XML] thus ensuring compatibility with XML Schema Datatypes, version 1.1 [XML Schema Datatypes]. Code points are sometimes represented in this document as U+ followed by a four-digit hexadecimal value of the code point.

A string is a finite sequence of zero or more characters. The length of a string is the number of characters in it. Strings are written in this specification by enclosing them in double quotes. Two strings are identical if and only if they contain exactly the same characters in exactly the same sequence.

A language tag is a string matching the langtag production from BCP 47 [BCP 47]. Furthermore, note that this definition corresponds to the well-formed rather than the valid class of conformance in BCP 47. A language tag MAY contain subtags that are not registered in the IANA Language Subtag Registry, although an rdf:text implementation MAY also choose to reject such invalid language tags.

This specification uses Uniform Resource Identifiers (URIs) for naming datatypes and their components, which are defined in RFC 3986 [RFC 3986]. For readability, URIs prefixes are often abbreviated by a short prefix name according to the convention of RDF [RDF]. The following prefix names are used throughout this document:

• the prefix name xs: stands for http://www.w3.org/2001/XMLSchema#
• the prefix name rdf: stands for http://www.w3.org/1999/02/22-rdf-syntax-ns#
• the prefix name fn: stands for http://www.w3.org/2005/xpath-functions#
• the prefix name rtfn: stands for http://www.w3.org/2009/rdf-text-functions#

Datatypes are defined in this document along the lines of XML Schema Datatypes [XML Schema Datatypes]. Each datatype is identified by a URI and is described by the following components:

• The value space is a set determining the set of values of the datatype. Elements of the value space are called data values.
• The lexical space is a set of strings that can be used to refer to data values. Each member of the lexical space is called a lexical form, and it is mapped to a particular data value.
• The facet space is a set of pairs of the form ⟨ F v ⟩, where F is a URI called a constraining facet, and v is an arbitrary data value called a constraining value. Each such pair is mapped to a subset of the value space of the datatype.

A plain literal is a string with an optional language tag [RDF]. A plain literal without a language tag is interpreted in an RDF interpretation by itself. A plain literal with a language tag is written as "abc"@langTag, and it is interpreted in an RDF interpretation as a pair ⟨ "abc" , "langTag" ⟩.

A typed literal consists of a string and a datatype URI [RDF], it is written as "abc"^^datatypeURI, and it is interpreted in an RDF interpretation as the data value that the datatype identified by datatypeURI assigns to the lexical form "abc".

The italicized keywords MUST, MUST NOT, SHOULD, SHOULD NOT, and MAY specify certain aspects of the normative behavior of tools implementing this specification, and are interpreted as specified in RFC 2119 [RFC 2119].

In RDF implementations based on the entailment rules from Section 7 of the RDF Semantics [RDF Semantics], this equivalence can be achieved by means of the entailment rules shown in Table 3. These are analogous to rules xsd 1a and xsd 1b of the RDF Semantics [RDF Semantics] that establish semantic equivalence between typed xs:string literals and plain literals without a language tag. No rule is necessary to establish the correspondence between typed rdf:text literals and typed xs:string literals, as this is achieved indirectly via xsd 1a, xsd 1b, and the rules shown in Table 3.

Despite the semantic equivalence between typed rdf:text literals and plain literals, the presence of typed rdf:text literals in an RDF graph might cause interoperability problems between RDF tools, as not all RDF tools will support rdf:text. Therefore, before exchanging an RDF graph with other RDF tools, an RDF tool that suports rdf:text MUST replace in the graph each typed rdf:text literal with the corresponding plain literal. The notion of graph exchange includes, but is not limited to, the process of serializing an RDF graph using any (normative or nonnormative) RDF syntax.

This section defines functions that construct and operate on rdf:text data values. The terminology used and the way in which these functions are described are in accordance with the XQuery 1.0 and XPath 2.0 Functions and Operators [XPathFunc]. The error codes used in this section are given in Appendix G of the XPath 2.0 specification [XPath20] and Appendix C of XQuery and XPath function specification [XPathFunc].

rtfn:text-from-string( $arg1 as xs:string ) as rdf:text
rtfn:text-from-string( $arg1 as xs:string, $arg2 as xs:string) as rdf:text

Summary: returns the data value ⟨ $arg1, lowercase($arg2) ⟩ if $arg2 is present, and returns the data value $arg1 otherwise. Both arguments must be of type xs:string or one of its subtypes, and $arg2 — if present — must be a (nonempty) language tag; otherwise, this function raises type error err:FORG0006. Note that, since the lexical forms of rdf:text require language tags to be in lowercase, this function converts $arg2 to lowercase.

 rtfn:string-from-text( $arg as rdf:text) as xs:string

Summary: returns the string part s from the argument $arg, which must be an rdf:text data value of the form ⟨ s, l ⟩ or of the form s. If $arg is not of type rdf:text, this function raises type error err:FORG0006.

 rtfn:lang-from-text( $arg as rdf:text ) as xs:lang

Summary: returns the language tag l if $arg is an rdf:text data value of the form ⟨ s, l ⟩, and returns the empty string if $arg is an rdf:text data value of the form s. If $arg is not of type rdf:text, this function raises type error err:FORG0006.

The notion of collations used in this section is taken from Section 7.3.1 of XPath and XQuery function specification [XPathFunc].

 rtfn:compare( $comparand1  as rdf:text?, $comparand2 as rdf:text? ) as xs:integer?

 rtfn:compare( $comparand1  as rdf:text?, $comparand2 as rdf:text?, $collation as xs:string )  as xs:integer?

Summary: if either $comparand1 or $comparand2 is not of type rdf:text, of if $collation is specified but is not of type xs:string, this function raises type error err:FORG0006. Otherwise, the function returns the empty sequence if one of the arguments is empty, if one of $comparand1 and $comparand2 has a language tag and the other one does not, or if the language parts of $comparand1 and $comparand2 are unequal; otherwise, this function returns -1, 0, or 1 depending on whether the value of the string-part of $comparand1 (or $comparand1 itself, respectively, if it has no language tag) is respectively less than, equal to, or greater than the value of the string-part of $comparand2 (or $comparand2 itself, respectively, if it has no language tag). The collation used by the invocation of this function is determined according to the rules in Section 7.3.1 of the XPath and XQuery functions specification [XPathFunc].

The first version of this function backs up the XQuery operators "eq", "ne", "gt", "lt", "le", and "ge" on rdf:text values.

The two functions may be viewed as declared XQuery functions with the following definitions:

declare function  rtfn:compare( $comparand1 as rdf:text?, $comparand2 as rdf:text? ) as xs:integer?
 {
  return
    if ( fn:compare (  rtfn:lang-from-text( $comparand1 ),   rtfn:lang-from-text( $comparand2 ) ) = 0 ) then
       fn:compare (  rtfn:string-from-text( $comparand1 ) ,  rtfn:string-from-text( $comparand2 ) )
 }

declare function  rtfn:compare( $comparand1  as rdf:text?, $comparand2 as rdf:text? $collation as xs:string ) as xs:integer?
 {
  return
    if ( fn:compare ( fn:lang-from-text( $comparand1 ),   rtfn:lang-from-text( $comparand2 ) ) = 0 ) then
       fn:compare (  rtfn:string-from-text( $comparand1 ) ,  rtfn:string-from-text( $comparand2 ), $collation)
 }

 rtfn:length($arg as rdf:text) as xs:integer

Summary: returns the number of characters in the string part s if $arg is an rdf:text data value of the form ⟨ s, l ⟩ or a string value s, respectively. If $arg is not of type rdf:text, this function raises type error err:FORG0006.

This function may be viewed as a declared XQuery function with the following definition:

declare function  rtfn:text-length($arg as rdf:text?) as xs:integer
 {
  return
     fn:string-length (  rtfn:string-from-text( $arg ) )
 }

 rtfn:matches-language-range($arg as rdf:text?, $range as xs:string) as xs:boolean

Summary: This function is only defined if $arg is a sequence of length 0 or 1 of type rdf:texts and $range is of type xs:string; if the parameters do not satisfy these typing conditions, the function raises a type error err:FORG0006. If the typing conditions are fulfilled, the function returns true in case $arg is an rdf:text data value of the form ⟨ s, l ⟩ with l a language tag that matches the extended language range $range as specified by the extended filtering algorithm for "Matching of Language Tags" [BCP-47]; otherwise, it returns false. This means that the function returns false if the argument is a string rdf:text data value. An empty input sequence is treated as a rdf:text data value consisting of the empty string, and accordingly on such input this function also returns false.

The RIF WG and the OWL WG made parallel efforts to support strings written in different languages. This specification is the outcome of a collaboration between the two groups, and it is based on the work on the rif:text datatype on the RIF side and the owl:internationalizedString datatype on the OWL side. A short description of the design process is available here.

[RFC 2119]

RFC 2119: Key words for use in RFCs to Indicate Requirement Levels. Network Working Group, S. Bradner. Internet Best Current Practice, March 1997.

[RFC 3986]

RFC 3986 - Uniform Resource Identifier (URI): Generic Syntax. T. Berners-Lee, R. Fielding, and L. Masinter, IETF, January 2005.

[RFC 4647]

RFC 4647 - Matching of Language Tags. A. Phillips and M. Davis, IETF, September 2006.

[UNICODE]

The Unicode Standard. Unicode The Unicode Consortium, Version 5.1.0, ISBN 0-321-48091-0, as updated from time to time by the publication of new versions. (See http://www.unicode.org/unicode/standard/versions for the latest version and additional information on versions of the standard and of the Unicode Character Database)."

[ISO/IEC 10646]

ISO/IEC 10646-1:2000. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 1: Architecture and Basic Multilingual Plane and ISO/IEC 10646-2:2001. Information technology — Universal Multiple-Octet Coded Character Set (UCS) — Part 2: Supplementary Planes, as, from time to time, amended, replaced by a new edition or expanded by the addition of new parts. [Geneva]: International Organization for Standardization. ISO (International Organization for Standardization).

[BCP 47]

BCP-47 - Tags for Identifying Languages. A. Phillips, M. Davis, eds., IETF, September 2006, http://www.rfc-editor.org/rfc/bcp/bcp47.txt.

[RDF]

Resource Description Framework (RDF): Concepts and Abstract Syntax. Graham Klyne, Jeremy J. Carroll, and Brian McBride, eds., W3C Recommendation 10 February 2004.

[RDF Semantics]

RDF Semantics. Patrick Hayes, ed., W3C Recommendation 2004

[XML]

Extensible Markup Language (XML) 1.0 (Fifth Edition). Tim Bray, Jean Paoli, C. M. Sperberg-McQueen, Eve Maler, and François Yergeau, eds., W3C Recommendation 26 November 2008.

[XML Schema Datatypes]

W3C XML Schema Definition Language (XSD) 1.1 Part 2: Datatypes. D. Peterson, S. Gao, A. Malhotra, C. M. Sperberg-McQueen, H. S. Thompson, eds., W3C Working Draft 30 January 2009.

[XPath20]

XML Path Language (XPath) 2.0. Anders Berglund, Scott Boag, Don Chamberlin, Mary F. Fernández, Michael Kay, Jonathan Robie, and Jérôme Siméon, eds. W3C Recommendation 23 January 2007.

[XPathFunc]

XQuery 1.0 and XPath 2.0 Functions and Operators. Ashok Malhotra, Jim Melton, and Norman Walsh, eds. W3C Recommendation 23 January 2007.