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XML Linking WG comments on XML Schema Last Call specifications

From: Eve L. Maler <Eve.Maler@east.sun.com>
Date: Tue, 30 May 2000 13:27:46 -0400
Message-Id: <>
To: www-xml-schema-comments@w3.org
Cc: w3c-xml-linking-wg@w3.org
[I really apologize for the delay in sending these.  I thought I had sent 
them last week, and noticed just now that I hadn't.]

XML Linking Working Group
Comments on the XML Schema last-call specifications
26 May 2000

XML Linking Working Group is a potential user of the XML Schema 
specifications because its XLink vocabulary has constraints that are useful 
to express in a schema.  Therefore, we have concentrated our review of the 
XML Schema specifications on whether they have the ability to express 
XLink's current needs and anticipated future needs.

The Last Call version of XLink appears to be quite expressible in XML 
Schema.  For example, Jonathan Marsh has written a schema that does a good 
job of covering the constraints in the XLink spec. [1]  Since each 
XLink-significant attribute has a fixed name, the name can simply be 
declared as the attribute name in the schema, for example, <xlink-prefix>:href.

However, our group has been asked to allow XLink users to design their own 
names for attributes that have XLink-specified meaning so that, for 
example, the XHTML <img> element can have an attribute called src that 
serves the purpose of an XLink href attribute.  We hope to meet this 
request in the future development of XLink, and expect that XML Schema can 
help.  For example, we could define a simple type called "XLink-href", and 
simply require that an attribute of this type be present when referencing 
needs to be done in XLink, no matter what name the attribute actually has.

While it is an easy matter to define such simple types, we found that there 
is no way for an XML Schema to package the desired attributes of the 
desired types; for example, the fact that a simple linking element needs 
optional "XLink-href", "XLink-role", etc. attributes would have to be 
expressed in prose, not in a schema.  Thus, a normative schema module for 
this imagined future XLink spec would consist only of some simple-type 
definitions; users of the module, once they decided on their desired 
attribute names, would have to do the work of writing a schema that adhered 
to the additional constraints.

The Linking group thus requests that the Schema group consider the addition 
of a feature that would allow the specification of "any attribute of the 
xxx type," or something similar, in an attribute-list declaration.  One 
experiment using the <anyAttribute> element has been produced along these 
lines [2]; another approach could be to create attribute equivalence classes.

A large majority of the Linking group felt that this new feature should be 
a Version 2 XML Schema work product; one person felt instead that it should 
be added to Version 1, on the grounds that it would take too long if we 
depended on the completion of an XML Schema Version 2 effort before 
publishing a normative XLink schema using this feature (given that XLink 
Version 1 is ahead of the development curve of XML Schema Version 1).

Since XLink might be called an "enabling vocabulary" rather than a complete 
application on its own, we believe that this new functionality is important 
for letting XLink users design a higher-level markup language on their own 
terms.  With this feature available, we expect that other enabling 
vocabularies focusing on types rather than markup names might arise.  Given 
the promise of intelligent processing inherent in the XML Schema typing 
system, we feel that this would strengthen XML Schema's usefulness.

Thank you for your consideration.

	Eve Maler
	the XML Linking Working Group


Eve Maler                                    +1 781 442 3190
Sun Microsystems XML Technology Center    elm @ east.sun.com
Received on Tuesday, 30 May 2000 13:27:26 UTC

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