A Datatype for Internationalized Text

W3C Editor's Draft 04 November 2008

This version:

http://www.w3.org/2007/OWL/draft/ED-owl2-rdf-text-20081104/

Latest editor's draft:

http://www.w3.org/2007/OWL/draft/owl2-rdf-text/

Previous version:

http://www.w3.org/2007/OWL/draft/ED-owl2-rdf-text-20080922/ (color-coded diff)

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

---

Copyright © 2008 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May Be Superseded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/.

Please Comment By 2008-11-07

The OWL Working Group seeks public feedback on these Working Drafts. 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 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.

Internationalized text — that is, text that additionally conveys information in terms of a language tag — is used in several existing W3C specifications, such as RDF, XML, OWL, and RIF. This specification defines a datatype called rdf:text in order to allow specifications such as RDF, OWL, and RIF to refer to internationalized text literals in an interoperable way. Parallel efforts have been made to support internationalized strings by several W3C working groups, including the OWL WG and the RIF WG. Collaboration between the two working groups on the choice of language constructs for internationalized strings has lead to the present specification [1][2].

Parts of this document are based on the current work on rif:text [3] (RIF WG) and owl:internationalizedString [4] (OWL WG), for more details see a summary.

A character is an atomic unit of communication. The structure of characters is not further specified in this document, other than to note that each character has a Universal Character Set (UCS) code point [ISO/IEC 10646] (or, equivalently, a Unicode code point [UNICODE]). The set of available characters is assumed to be infinite, and it is thus independent from the current version of UCS and Unicode.

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

A language tag is a string of the form as specified in BCP 47 [BCP 47].

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

• the xsd: prefix stands for the XML Schema namespace URI http://www.w3.org/2001/XMLSchema#
• the rdf: prefix stands for http://www.w3.org/1999/02/22-rdf-syntax-ns#

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 value, 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 a value. Each such pair is mapped to a subset of the value space of the datatype.

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].

The datatype identified by the URI http://www.w3.org/1999/02/22-rdf-syntax-ns#text (abbreviated rdf:text) allows for the representation of internationalized text literals. In addition to the RIF and OWL specifications, this datatype is expected to supersede RDF's plain literals with language tags, cf. [5], which is why this datatype has been added into the rdf: namespace.

Value Space. The value space of rdf:text is the set of all pairs of the form ⟨ "text" , "lang" ⟩, where "text" is a string and "lang" is either the empty string "" or a lowercase language tag.

Lexical Space. A lexical value of an rdf:text literal is a string "val" that contains at least one character @ and that satisfies the following condition:

Each such lexical value is assigned a data value ⟨ "abc", "lc-tag" ⟩, where "lc-tag" is the string "tag" converted to lowercase.

Facet Space. The facet space of the rdf:text datatype is shown in Table 1.

The xsd:string datatype is a datatype defined in XML Schema Datatypes [XML Schema Datatypes] as having the value space equal to the set of all strings. Thus, the value space of xsd:string is not a subset of the value space of rdf:text, which may cause problems for certain applications of this specification. A similar problem arises with XML Schema datatypes that are derived from xsd:string.

To overcome this difficulty, specifications that use rdf:text MAY choose to interpret the datatypes from the following list in a slightly different way. The resulting datatypes have value spaces that are isomorphic with the value spaces from XML Schema Datatypes [XML Schema Datatypes], but that are subsets of the value space of rdf:text.

• xsd:string
• xsd:normalizedString
• xsd:token
• xsd:language
• xsd:Name
• xsd:NCName
• xsd:NMTOKEN
• xsd:ID
• xsd:IDREF
• xsd:ENTITY

Value Space. For DT a datatype from the above list, the value space of DT is a set of pairs of the form ⟨ "text" , "" ⟩ where "text" is a string matching the restrictions of DT as specified in XML Schema Datatypes [XML Schema Datatypes] and "" is the empty string.

Lexical Space. For DT a datatype from the above list, the lexical space of DT is a string "text" that matches the restrictions of DT as specified in XML Schema Datatypes [XML Schema Datatypes]. Each lexical value "text" is assigned a data value ⟨ "text" , "" ⟩.

Facet Space. Each datatype DT from the above list supports the constraining facets xsd:minLength, xsd:maxLength, xsd:length, and xsd:pattern. The facet value of each pair for DT is the same as in Table 1, with the difference that the result is a subset of the value space of DT rather than of rdf:text.

In syntaxes such as the RIF presentation syntax [6], the OWL 2 functional-style syntax [7], or the TURTLE syntax [8], literals are written using the form "rep"^^datatypeURI. This specification defines a convenient representation for rdf:text and xsd:string literals. In particular, literals of the form "text@lang"^^rdf:text where "lang" is not empty can be abbreviated as "text"@lang; furthermore, literals of the form "text"^^xsd:string can be abbreviated as "text". If an implementation supports abbreviation of literals, it SHOULD abbreviate the literals eagerly whenever possible.

The abbreviated literals can be written using the following grammar. A subset of the N-triples quoting mechanism is employed in order to allow strings to contain quotes.

Text matching the abbreviatedXSDStringLiteral production SHOULD be mapped to an xsd:string literal, and text matching the abbreviatedRDFTextLiteral production SHOULD be mapped to an rdf:text literal.

This section defines constructor functions, operators, and functions on the rdf:text datatype. The terminology used and structure to describe these functions and operators is 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].

fn:text-from-string-lang( $arg1 as xsd:string, $arg2 as xsd:string) as rdf:text

Summary: returns the data value ⟨ $arg1, $arg2 ⟩ of type rdf:text. The arguments both have to be of type xsd:string or one of its subtypes; otherwise, this function raises type error err:FORG0006.

fn:text-from-string( $arg as xsd:string) as rdf:text

Summary: returns the data value ($arg, "") of type rdf:text. The argument has to be of type xsd:string or one of its subtypes; otherwise, this function raises type error err:FORG0006.

fn:string-from-text( $arg as rdf:text) as xsd:string

Summary: extracts the string part s from the argument $arg = ⟨ s, l ⟩ of type rdf:text. The argument $arg has to be of type rdf:text; otherwise, this function raises type error err:FORG0006.

fn:lang-from-text( $arg as rdf:text ) as xsd:lang

Summary: extracts the language tag l from the argument $arg = ⟨ s, l ⟩ of type rdf:text. The argument $arg has to be of type rdf:text; otherwise, 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].

op:text-equal( $comparand1 as rdf:text,  $comparand2 as rdf:text ) as xsd:boolean

declare function op:text-equal( $comparand1 as rdf:text,  $comparand2 as rdf:text ) as xsd:boolean
 {
  return
    if ( fn:compare ( fn:lang-from-text( $comparand1 ),  fn:lang-from-text( $comparand2 ) ) = 0 &&
         fn:compare ( fn:string-from-text( $comparand1 ) , fn:string-from-text( $comparand2 ) = 0 )
    then fn:true()
    else fn:false()
 }

fn:text-compare( $comparand1  as rdf:text?, $comparand2 as rdf:text? ) as xsd:integer?

fn:text-compare( $comparand1  as rdf:text?, $comparand2 as rdf:text?, $collation as xsd:string )  as xsd:integer?

Summary: returns the empty sequence if one of the arguments is empty 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 is respectively less than, equal to, or greater than the value of the string-part of $comparand2. 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].

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

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

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

fn:text-length($arg as rdf:text) as xsd:integer

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

fn:matches-language-range($input as rdf:text?, $range as xsd:string) as xsd:boolean

Summary: returns true if the language tag part of $input is a valid language tag according to BCP-47 [BCP-47], and if it matches the language-range expression supplied as $range as specified by the algorithm for "Matching of Language Tags" which is part of BCP-47 [BCP-47]; otherwise, it returns false.

An empty input sequence is treated as a rdf:text value consisting of the empty string and the empty language tag. Since the empty string is not a valid language tag according to BCP-47 [BCP-47], on such input this function returns false.

[RFC 3986]

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

[RFC 4646]

RFC 4646 - Tags for Identifying Languages. M. Phillips and A. Davis. IETF, September 2006, http://www.ietf.org/rfc/rfc4646.txt. Latest version is available as BCP-47, (details).

[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.

[UNICODE]

The Unicode Standard. The Unicode Consortium.

[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.

[XML Namespaces]

Namespaces in XML 1.0 (Second Edition). Tim Bray, Dave Hollander, Andrew Layman, and Richard Tobin, eds. W3C Recommendation 16 August 2006.

[XML Schema Datatypes]

XML Schema Part 2: Datatypes Second Edition. Paul V. Biron and Ashok Malhotra, eds. W3C Recommendation 28 October 2004.

[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.

[IRC Log July 21, 2008]

Joint meeting of OWL, RIF and I18N WGs..