http://norman.walsh.name/2010/04/09/xmldefault 13 13 2010-04-09T11:27:38-04:00 Norman Walsh 2010 Norman Walsh A look at the intersection of the XML model PI, the XML stylesheet PI, and XProc. What is the “default XML processing model?” That question has been open for a long time, since the very beginning of XML really. There are a lot of different opinions, some of them captured in the W3C’s Technical Architecture Group discussion of the issue “xmlFunctions-34”. (Disclaimer: I contributed to that issue while I was a member of the TAG.) It's in the charter of the XML Processing Model Working Group, which I chair, to provide an answer to this question. I don't think it has an answer. I don't subscribe to the notion that XML documents have one and only one intrinsic meaning. I think the best we can do is describe one (or a few) possible models and give them labels. That will allow the authors of other specifications, and applications, to say “we do TYPEX processing on XML documents”, where “TYPEX” is one of the labels. That'll give us a shorthand for talking about some common processing models. That may not seem very satisfying. Maybe it isn't. The point of this essay isn't to make or defend that position. When the WG produces it's first public working draft of a document that attempts to answer the “default XML processing model” question, I'll let you know. The right answer isn't what I think it is, it's what community consensus drives us to. No, the point of this essay is something else, something a little more complicated than I think we would reasonably expect to put in that document (though I could be wrong). The XML community has had an Associating Style Sheets with XML documents specification for a long time. It will soon have an Associating Schemas with XML documents specification. (That link is to an early editor's draft, there's nothing official yet, but it's coming soon.) What are the two most common things that many (not all!) users want to do with XML documents? Validate them and transform them. Well, if the document tells you how to validate it and how to style it, then isn't one possible answer to the default processing question simple: validate like I say and style like I say? If it is, shouldn't we be able to express that processing using an XProc pipeline? Of course we should. And we can: default.xpl. I'm happy and relieved to find that we can express that processing in XProc. I'm a little, but only a little, surprised to see how complex that pipeline is. Weighing in at 320+ lines it works for a few narrow cases. I still need to integrate support for RELAX NG compact syntax schemas and NVDL processing, at least. I may also want to support a few more stylesheet options, I'm not sure.