RE: Towards a TAG consideration of CURIEs

To add another element to the mix, the IPTC has defined, and will be
using, QCodes (Qualified Codes) for the forthcoming News Architecture
and all G2 standards based upon it: NewsML-G2, NITF-G2, SportsML-G2,
EventsML-G2, etc.

Some of you will recall my presentation to the W3C AC last year about 
this subject.  We were then still considering the use of CURIEs but 
have since concluded that what we need are QNames without the 
restriction on the leading character of the right-hand side.  So we 
created QCodes, which don't have this limitation.

Henry pointed out at the Edinburgh AC meeting that if we used simple 
concatenation to give people access to information about terms in 
our taxonomies, we could end up with illegal fragment IDs.  So:

If:
   <subject qcode="iptc:123456"/>
and if:
   iptc -> http://www.iptc.org/NewsCodes#
and if we used simple concatenation, we'd get:
   iptc -> http://www.iptc.org/NewsCodes#123456

There is, of course, the other option:
   iptc -> http://www.iptc.org/NewsCodes/
then if we used simple concatenation, we'd get:
   iptc -> http://www.iptc.org/NewsCodes/123456

We've decided to side-step this by specifying that the concatenation
rules are taxonomy-specific and are up to the provider of each 
taxonomy.

For us the bottom line is, as I said in Edinburgh, that we require
tuples without some of the constraints that QNames took from XML.  
The construction of a URI pointing to useful info (and usable for 
RDF) we see as icing on the cake.

We think that some Semantic Web specs and tools may gag on QCodes 
but this is where theory meets the real World.

Misha Wolf
News Standards Manager, Reuters
http://www.iptc.org/ | http://www.iptc.org/NAR/


-----Original Message-----
From: www-tag-request@w3.org [mailto:www-tag-request@w3.org] On Behalf
Of Henry S. Thompson
Sent: 30 March 2007 12:06
To: www-tag@w3.org
Subject: Towards a TAG consideration of CURIEs


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I took an action at the last TAG telcon (minutes forthcoming) to try
to draft a statement of where the current CURIE draft [1] (actually
quotes below are from a Member-only editors' draft [1a], which
contains some minor changes to the syntax) raises architectural
issues.

*Executive Summary*

Do the expected benefits of CURIEs outweigh the potential costs in
introducing a _third_ syntax for identifiers into the languages of the
Web?

*Background*

XML Namespaces introduced the notion of expanded names, that is, names
in the form of a pair of namespace name (possibly empty) and local
name.  It further introduced an abbreviation mechanism, involving
prefixes and namespace declarations.  The word 'QName' has come to be
used for both the syntactic form such abbreviations take (i.e.
  (NCName ':')? NCName
) and the two-part name such abbreviations stand for.  As such, QNames
are clearly distinct from URIs.  Their use as identifiers, however,
immediately raises the question of their relationship with URIs.

The TAG considered the use of QNames as shorthand for URIs in issue
rdfmsQnameUriMapping-6 [2], and issued a finding on the the related
subject of using QNames as names for things other than XML elements
and attributes, called "Using Qualified Names (QNames) as Identifiers
in XML Content" [3].

In that finding, we find

  "We observe also that there is an overlap in the lexical space of
   QNames and URIs.

  "Specifications that use QNames to represent {URI, local-name}
   pairs SHOULD NOT allow both forms in attribute values or element
   content where they would be indistinguishable."

and also

   "Where there is a compelling reason to use QNames instead of URIs
    for identification, it is imperative that specifications provide a
    mapping between QNames and URIs, if such a mapping is possible."

The Architecture of the World Wide Web summarises these points in its
section on QNames [4].

*CURIEs*

Unlike QNames, CURIEs are explicitly intended as abbreviations for
URIs.  None-the-less they use an extension of the syntax of QNames,
namely
  NCName ':' [pretty unconstrained string]

*Architectural issues*

If the CURIE WD is eventually adopted, we will have three related
forms of identification:

 1) URIs themselves;
 2) CURIEs as abbreviations of (absolute) URIs;
 3) QNames as abbreviations for expanded names, which in _some_
    circumstances are mapped by convention or explicit algorithm to
    URIs.

In [3] and [4] the potential confusions of overlapping syntax and
function arising from the use of QNames as identifiers and even as
abbreviations for URIs is accepted as a fact with good historical and
pragmatic motivations.

The fundamental architectural question raised by the CURIE
specification is then whether the expected benefits outweigh the
potential costs in introducing a _third_ syntax for identifiers into
the languages of the Web.

A subsidiary question depends on exactly what the intended scope of
application of this specification is -- if it is as widely-targetted
as it appears to be, would it not be better to consider an addendum to
the relevant RFCs, e.g. 3986 and 3987 [5] [6]?

And finally, the question of how CURIE would integrate with the typing
and typed-data manipulation facilities provided by W3C XML Schema and
XPath 2.0/XSLT 2.0/XQuery also needs careful consideration.

ht

[1] http://www.w3.org/TR/2007/WD-curie-20070307/
[1a] http://www.w3.org/MarkUp/Group/2007/ED-curie-20070322/
[2] http://www.w3.org/2001/tag/issues.html?type=1#rdfmsQnameUriMapping-6
[3] http://www.w3.org/2001/tag/doc/qnameids.html
[4] http://www.w3.org/TR/webarch/#xml-qnames
[5] http://www.ietf.org/rfc/rfc3986.txt
[6] http://www.ietf.org/rfc/rfc3987.txt
- -- 
 Henry S. Thompson, HCRC Language Technology Group, University of
Edinburgh
                     Half-time member of W3C Team
    2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
            Fax: (44) 131 650-4587, e-mail: ht@inf.ed.ac.uk
                   URL: http://www.ltg.ed.ac.uk/~ht/
[mail really from me _always_ has this .sig -- mail without it is forged
spam]
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Received on Friday, 30 March 2007 11:36:13 UTC