Subject: Re: [xsl] XSLT/XPath 2.0 (was "Identifying two tags...") From: Jeni Tennison <jeni@xxxxxxxxxxxxxxxx> Date: Fri, 10 May 2002 11:48:06 +0100 |
Hi Dan, > Thanks for the tip! I'll most certainly do that (although I fear > that "I'd like to be spared of 80% of the spec" is not exactly what > the WG would like to hear...). Yeah well, they might not want to hear it, but if enough people say it enough times perhaps they might start having to. And I do think that they're genuinely interested in learning what users think, especially about *which* 80% they don't like. I posted my 80% list to XML-Dev yesterday (because it came up there; I've previously made most of those comments both here and on the proper comments lists as well) -- have a look at http://lists.xml.org/archives/xml-dev/200205/msg00548.html > What you're saying is really food for though: if you of all people > are having a hard time "getting your head around" the intricacies of > the type system in XSLT 2, what are my (and other, more average > Joes' out there) chances? And even if I did take 6 months off for > quality time with XQuery semantics and friends, what good would it > do me (besides the obvious bragging rights)? I have the feeling that much of what makes XPath/XSLT 2.0 seem complicated is simply that it hasn't been described well. If you're had your head buried in the details for months, then it's often hard to write it down without skipping over the assumptions that you've come to take for granted, or spending too long on the exceptional, difficult cases. Again, we can help here by asking the dumb questions, like "what's the difference between 'treat' and 'assert'?" > Heck, I can't even quite grasp what that PSVI thingy is supposed to > be (it's one of those "almost, but in reality: no" things). Since > all the other stuff seems to be built on that, I'll admit that my > chances look quite slim. The PSVI is the stuff that you get out of validating an instance document with an XML Schema. You know that if you validate an instance against a DTD, it's different from what you get if you look at the instance without the DTD? With the DTD, you get additional defaulted attributes, plus you get information about the types of attributes, like "this is an ID attribute". In some data models, you also get the element and attribute declarations, so you can refer to them later on. What parts of that set of information an application actually needs to know depends on the application. XPath 1.0, for example, doesn't care about element or attribute declarations, or about most of the data types aside from ID attributes, but it does keep the defaulted attributes. The PSVI is exactly the same thing, except it's what you get after schema validation. It's the same instance as you get normally, with extra defaulted/fixed values, and indications of the types of elements and attributes. You get element and attribute declarations, just like from a DTD. Plus, because you have user-defined types in XML Schema and they're all part of a hierarchy, you get the type hierarchy as well. There are a few other things, but that's the important stuff. Like XPath 1.0, XPath 2.0 picks and chooses from this set of information to build up its data model. It keeps hold of information about the types of particular elements and attributes, and some information about the type hierarchy (like how types are derived from each other). I found that looking at the PSVI output of XSV (http://www.ltg.ed.ac.uk/~ht/xsv-status.html), which is described at http://www.w3.org/2001/05/serialized-infoset-schema.html, helped to visualise what it looked like; just remember that it wouldn't normally be passed around as an XML structure! Cheers, Jeni --- Jeni Tennison http://www.jenitennison.com/ XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list
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