Network Working GroupP. Stickler
Expires: October 12, 2002April 13, 2002

The 'val:' URI Scheme for Denoting and Describing Datatype Values

Status of this Memo

This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026.

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Copyright (C) The Internet Society (2002). All Rights Reserved.


This document describes the 'val:' Uniform Resource Identifier (URI) scheme for denoting and describing datatype values which are represented by a pairing of the datatype and a lexical form.


Table of Contents


1. Overview

XML Schema [5] defines a simple datatype as consisting of a value space (a set of values), a lexical space (a set of lexical forms, or strings), and an N:1 mapping from members of the lexical space to members of the value space. Thus, in the context of a given datatype, a lexical form (member of the lexical space) denotes one and only one value (member of the value space). Thus the pairing of a datatype and a lexical form unambiguously identifies a specific value of that datatype.

The 'val:' URI scheme is intended to provide a simple but consistent means by which a lexical form can be paired with a datatype URI allowing one to denote the datatype value represented by that pairing.



These examples are provided for illustrative purposes only and do not necessarily constitute actual URIs. See the BNF definition below for an explicit definition of 'val:' URI syntax.

'val:' URIs are particularly useful for applications such as RDF [4] where one may wish to make statements about such value resources. E.g. to assign language specific labels to members of an enumeration:

   <rdf:Description rdf:about="val:(">
      <rdfs:label xml:lang="en">Finnish</rdfs:label>
      <rdfs:label xml:lang="fi">Suomi</rdfs:label>
      <rdfs:label xml:lang="se">Finska</rdfs:label>

There is the additional benefit in RDF whereby if canonical lexical forms are used, every datatype value represented by a 'val:' URI in an RDF graph will share a common graph node, which both provides a considerable amount of graph compression compared to other literal datatyping schemes as well as facilitates queries or inference based on equality of datatype values.


2. BNF for the 'val:' URI Scheme

This is a BNF-like description of the 'val:' Uniform Resource Identifier syntax, using the conventions of RFC 822[2], except that "|" is used to designate alternatives, and brackets [] are used around optional or repeated elements. Briefly, literals are quoted with "", optional elements are enclosed in [brackets], and elements may be preceded with <n>* to designate n or more repetitions of the following element; n defaults to 0.

This BNF description adopts sub-definitions defined in RFC 2396 "Uniform Resource Identifiers (URI): Generic Syntax" [1] which are not repeated here.

val-URI       = "val:" "(" datatypeURI ")" lexicalForm
datatypeURI   = absoluteURI [ "%23" fragment ]
lexicalForm   = 1*uric

absoluteURI   = <as defined by RFC 2396>
fragment      = <as defined by RFC 2396>
uric          = <as defined by RFC 2396>

Any occurrences of the characters '(', ')', or '%' in either the absoluteURI or fragment segments must be URI escaped. E.g. the datatype URI "foo:abc(xyz)#def%23bar" with value "zzz" would be encoded as "val:(foo:abc%28xyx%29%23def%2523bar)zzz".

Note that the URI scheme prefix "val:" is considered to be a valid URI denoting this URI scheme, though it is not itself a valid URI according to this URI scheme.


3. Security Considerations

This document raises no known security issues.



[1] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifiers (URI): Generic Syntax", RFC 2396, August 1998.
[3] International Organization for Standardization, "ISO/IEC 11578:1996 Information technology -- Open Systems Interconnection -- Remote Procedure Call", August 2001.
[4] Lassila, O. and R. Swick, "Resource Description Framework (RDF) Model and Syntax Specification", February 1999.
[5] "XML Schema ...".


Author's Address

  Patrick Stickler
  Nokia Research Center
  Visiokatu 1
  Tampere 33720


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