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LC-151: typed references - XML Schema Last Call Issue Response

From: Frank Olken <olken@lbl.gov>
Date: Fri, 13 Oct 2000 12:20:45 -0700
Message-ID: <39E7608D.EB7C5222@lbl.gov>
To: Jane Hunter <jane@dstc.edu.au>
CC: mpeg7-ddl <" mpeg7-ddl"@darmstadt.gmd.de>, "'www-xml-schema-comments@w3.org'\"" <www-xml-schema-comments@w3.org>, "w3c-xml-schema-ig@w3.org" <w3c-xml-schema-ig@w3.org>, X3D Contributors <x3d-contributors@web3d.org>
                                        October 13, 2000

Dear Ms. Hunter,


The XML Schema Working Group has spent the last several months
working through the comments received from the public on the last-call
draft of the XML Schema specification.  We thank you for the comments
you made on our specification during our last-call comment period, and
want to make sure you know that all comments received during the
last-call comment period have been recorded in our last-call issues
list (http://www.w3.org/2000/05/12-xmlschema-lcissues).

        I am writing on behalf of the XML Schema WG 
concerning your last call comments concerning the issue of array 
specifications in XML Schema.  This issue is known to the XML Schema 
WG as LC-151. typed-refs: How do I restrict IDREFs to particular 
(element) types?

LC-151. typed-refs: How do I restrict IDREFs to particular (element)
types?
----------------------------------------------------------------------------

Issue Class: A Locus: structures Cluster: 20 keys Status: unassigned
Assigned to: Frank Olken Originator: Jane Hunter (MPEG-7)

Description
------------

How does a schema author use key constraints to specify that a value 
(which otherwise behaves like an SGML or XML ID) is restricted to
pointing 
at one (or more) particular element type(s)?

Interactions and Input
----------------------

Input from Jane Hunter:

4. Typed References
MPEG-7 requires 'typed references' or the ability to constrain IDs 
and IDREFs to particular elements: 

        <element name="SummaryDS">
                <complexType>.....</complexType>
        </element>

        <attribute name="SummaryDSRef" type="IDREF"
refType="SummaryDS"/>


Resolution
----------

There is some confusion in the Schema WG as to exactly what sort
of constraint you are seeking.  We did consider both cases.

Case 1:  constrain IDREFs to particular named elements
Case 2:  constrain IDREFs to types of elements (of any name).


Case 1 is addressed by the various identity constraints provided by
the current draft of Schema Language.  These effectively permit one
to constrain certain IDs to point to certain named elements.  This
appears to be what is effectively being sought in the example supplied.
See section 4.3.8 XML Representation of Identity-constraint Definition 
Schema Components of the XML Schema Part 1: Structures,
W3C Working Draft 22 September 2000 at the 
URL: http://www.w3.org/TR/xmlschema-1/
Note that this version of the constraint is similar to traditional
key, foreign key constraints in relational database management systems.
Note that these constraints are only operative within
a single XML document.


Case 2 is the analog of reference declarations in programming languages,
in which a pointer is constrained to point to a certain type of 
variable, not particular variables.  Case 2 is not presently supported
in XML Schema Language Version 1.0.  It was excluded because:

        1) limited demand for it
        2) additional complexity to schema language
        3) the expectation that XSLT WG efforts to add type predicateds
to  
        XPATH and related XML Query WG efforts would eventually afford
        a syntax to facilitate incorporating such constraints into the
        existing identity constraints.

Summary
-------

        Case 1 - Constrain References to named elements - supported in 
                Version 1.0 of XML Schema Language

        Case 2 - Constrain References to certain complextypes - not now,
                possibly later



Is this response adequate ?
------------------------------

The XML Schema Working Group wants to know your opinion
of our response to your last call comments.  This information
will be included with the package submitted to the W3C
Executive Director as part of the recommendation to take
the XML Schema Language to Candidate Recommendation.
We would appreciate your response as soon as possible.

Please indicate whether you wanted Case 1 or Case 2, or both.

For each case you are concerned with
please choose from one of the following responses, adding 
whatever details, explanation you wish:

1)  "GOOD ENOUGH"  - You are satisfied with the Schema WG response
to your comments on XML Schema Language.  The response meets 
your requirements.  The matter may be considered resolved.

2) "STOP THE PRESSES"  - You are not happy with the response
to your comments on XML Schema Language.  Either the response
is unclear or inadequate.  The issue is of sufficient importance
and urgency that you want it called to the attention of the 
W3C Executive Director and you ask that the XML Schema Language 
delayed in advancing to Candidate Recommendation until the 
issue is resolved. 

3)  "LATER - VERSION 1.1"  - You are not happy with the response,
but are prepared to defer reconsideration until XML Schema Lang.
Version 1.1 is drafted.  It is anticipated (hoped) that Version 1.1
will be completed by mid-2001.  Version 1.1 is intended primarily
to fix small issues needed by other W3C Working Groups to proceed 
with their work (especially XML Query Language).  You request that
your comments be reconsidered when drafting the Version 1.1 
requirements document.

4) "LATER - VERSION 2.0"  - You are not happy with the response,
but are prepared to defer consideration until XML Schema Language
Version 2.0 is drafted.  It is anticipated that Version 2.0 would
not be completed until late 2001 or early 2002.  Version 2.0 may
include major revisions, e.g., multiple inheritance, etc.
You request that your comments be reconsidered when drafting the 
Version 2.0 requirements document.

5) "NO LONGER CARE"  - You are not happy with the response, but
no longer care to pursue the matter, because ....



                  Frank Olken
                  XML Schema Language Working Group

  Lawrence Berkeley National Laboratory   (510) 486-5891 (voice)
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  E-mail:  olken@lbl.gov
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Received on Friday, 13 October 2000 15:20:37 GMT

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