Subject: [xsl] www-tag: Potential new issue: PSVI considered harmful From: David Carlisle <davidc@xxxxxxxxx> Date: Mon, 17 Jun 2002 11:12:51 +0100 |
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. _____________________________________________________________________ This message has been checked for all known viruses by Star Internet delivered through the MessageLabs Virus Scanning Service. For further information visit http://www.star.net.uk/stats.asp or alternatively call Star Internet for details on the Virus Scanning Service. XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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