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www-tag: Potential new issue: PSVI considered harmful



Anyone (Jeni:-) who thought I was "strident" in my criticism of the over
dependence of XPath2 on W3C Schema might be entertained by the thread
which starts at:

http://lists.w3.org/Archives/Public/www-tag/2002Jun/0085.html

A few highlights...



Tim Bray:

  4. Work on XQuery and other things that require a Type-Augmented Infoset 
  must not depend on schema processing, and should not have normative 
  linkages to any schema language specifications.

Simon St.Laurent:

  I'd say this item is a crucial requirement if the W3C wants to avoid a 
  serious fork in XML development.  Efforts to impose the PSVI as part of the 
  XML core are not very welcome in a lot of places.


James Clark:

   I see several different problems with the PSVI.
   
   (a) It makes documents less self-contained.
   
   (b) Applications that depend on a PSVI now require a very complex, 
   heavy-weight schema validation process, rather than a relatively simple 
   parsing process.
   
   (c) Applications that depends on a PSVI must agree not only on the choice 
   of schema language but also on the choice of mechanism to locate the 
   schema. As has been pointed out, xsi:schemaLocation is just hint; there is 
   no single way that is mandated for an application to locate a schema. XML 
   Schema does not specify a single way to get from a URI specifying a 
   document to a PSVI; it only specifies the way to get to a PSVI from a URI 
   specifying a document together with a mapping from namespace URIs to schema 
   locations.
   
   (d) The PSVI is not XML; this is the most insidious problem. With something 
   like default values, you can perform a normalization process and produce a 
   self-contained document where defaults are explicit. The declaration of 
   default values defines a mapping from an XML infoset to another instance of 
   an XML infoset. It is not necessary to add complexity to applications to 
   deal with default values. However, the W3C XML Schema PSVI is not like 
   this; a PSVI is not an XML infoset. You cannot perform the PSVI infoset 
   augmentation as a separate XML to XML transformation. All applications 
   dealing with the PSVI are dealing with a different, more complex structure 
   than applications that deal with pure XML. Applications communicating with 
   the PSVI become much more tightly coupled: when applications communicate 
   using the XML infoset, they do not have to share an address space, because 
   there is a standard serialization of an XML infoset as XML, but this does 
   not apply with the PSVI.  I believe this is a catastrophic architectural 
   mistake in XML Schema, and it needn't have been like this: schema infoset 
   augmentation could and should be defined as an XML to XML transformation 
   process.
   
   

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