Re: implications of xpath 2.0 for schematron, XSD
Schematron is expected to become an ISO standard in about 3 months now, barring unforeseens. I expect it will gently become part of the fabric over the year after that. I don't anticipate standards bodies will put out their own Schematron schemas particularly, more that they will point to Schematron as a good way for users to test their additional constraints. Bryan Rasmussen wrote: >I was wondering if anyone has given thought to the >implications of xpath 2.0 for schematron (possible future xpath 2.0 using >schematron), especially as it relates to the PSVI. > > I have! There is a little implementation change for xpath 2, relating to treating empty results as boolean false, but there is no reason that Schematron cannot use Xpath 2.0 operating on the PSVI. In the jargon of the standard, that is just a different "query language binding." >As lots of people have been using Schematron as an extension to other schema >languages, via the use of embedded Schematron, what would the implications in >that situation be? > The ISO DSDL framework does not have anything in a particular about Schematron embedded in XML Schemas (or in RELAX NG for that matter), ISO Schematron allows it. There is an annex F Using Schematron as a vocabulary that blesses the practise. See http://www.schematron.com/iso/P25.html#GEN38 It is just a matter for the implementation to validate the document with XML schemas before validating with Schematron. >In one way I think it would be great, in that one thing I have found Schematron >useful for is developing algorithmic checks on text content, and XPath 2.0 >extends the ability to do this greatly. > > Yes. I am not sure that XPath 2 gives that much more than EXSLT supports already, and I am not convinced that the PSVI provides anything that casting allows in expressions. But technical power is less important than familiarity: as more people get more savvy at XPath 2 over the PSVI, it will be natural for them to want to add constraints, and ISO Schematron is fine by that. >Finally, one of the things that have often worried me vis-a-vis the future of >Schematron is that I figured at some point it would just be absorbed into W3C >Schema, given that it was just too useful not to be. But the concept of W3C 2.5 >or something, with Xpath capabilities and a reliance on Xpath 2.0 seems uhm >scary. Of course that could conceivably also be a problem in the context of >xsd:unique, though obviously not as hairy. > > We tried to make ISO Schematron W3C-friendly (well, friendly to every-one) by allowing elements to be used in isolation. So if they had a demand for constraints, and wanted to add sch:assert (or, better, sch:rule or sch:pattern) as a standard extension element, there is nothing stopping them from the ISO standard side. xsd:unique and xsd:key are currently not typed, but they are streamable in XML Schemas. Many people made noises that there could have been a better system, but I haven't seen much in the way of concrete suggestions. It would be interesting to know how many implementations of XML Schemas for data-binding or application generation actually use the unique and key declarations to do something special. Otherwise, a general purpose constraint language is just as (well, a lot more) powerful. At one stage MSM sounded me out about making up a W3C Technical Note about using Schematron with XML Schemas or something (probably I would have nicked Eddie Robertsson's material) but it never eventuated. W3C schema people have always been quite encouraging about Schematron; they see it fits well as a constraint layer. Supporting Schematron would be an easy option for them, and probably would reduce distractions from their efforts. The main things working against it are, I would say, that they are so busy with XML Schemas and XQuery they don't have the capacity, or it is outside their product lines, or that XML Schemas operates streaming and would be a better fit with a steaming version of XPath, such as STX, rather than full random-access XPath (I don't think this is a problem, myself.) B.t.w., I am presenting a day seminar on Schematron in Beligum next week. Schematron will also feature in QA seminar in the same series. For more info http://www.xmlbelux.be/events/index.html Cheers Rick Jelliffe
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