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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: syntax, model
 elharo@m... (Elliotte Rusty Harold) writes: >>So are you against any schema technology on principle? If not, why >>not? How exactly does the thing described by a schema differ from >>a data model? > >That's a non-sequitur. Why would you think Simon is opposed to any >schema technology on principle just because he's opposed to sharing >data models? Allow me to rephrase: > >Simon: It's a bad idea for school children to share combs. It spreads lice. > >Tyler: So are you opposed to all hair care products in principle? Thanks - I think Elliotte's rephrasing is strikingly accurate, provided we ignore that apparently public health people are re-examining how lice spreads. Looking slightly beyond that, however, I think the syntax/model issue has a lot to with how I evaluate different schema technologies. (This may also help answer Tyler's followup, "How exactly does the thing described by a schema differ from a data model?") W3C XML Schema feels to me like an effort to create a new data model description language that happens to be connected to XML syntax. The emphasis on types throughout, the notion that an XML tree could/should be annotated with additional information about what the 'real' type is, and type machinery that means adding an attribute moves a declaration from a simple to a complex type are all strong signals that data models are the focus here. While W3C XML Schema has lots of problems, I suspect this choice was the founding catastrophe for the rest of the project. RELAX NG does operate on a data model to some degree, but it doesn't feel obsessed with creating data models. RELAX NG feels to me like it is exclusively about creating patterns which can be tested against XML documents. Does a document fit this pattern, or not? Where does it fail? To me, that's much more coherent with the notion that XML documents contain structure expressed syntactically. The math is crucial, but doesn't interfere - much the way that the math behind regular expressions doesn't abstract them away from their work of matching patterns in text. Schematron, similarly, operates on the XPath data model, and does some very nifty things with the operators XPath provides, but it doesn't create new models. I wish Schematron was a bit more syntactic - it would be very nice to have Schematron schemas issue warnings and reports about things like entity usage - but I also recognize where it came from, and think it's a pretty coherent tool for analyzing structures expressed through syntax. I guess it's fair to say that to me: * The things described by W3C XML Schemas are in fact data models and only tangentially XML documents.[1] That's as good cause as any for discarding WXS and recommending that people avoid it. * The things described by RELAX NG schemas are XML documents or simulacra thereof, and the combination of that with a sane mathematical foundation and a convenient compact syntax are as good cause as any for using RELAX NG to describe XML documents. * Schematron can probably be applied to nearly any kind of data, and its similarity to unit testing had appeal at a presentation I gave on Tuesday night, but it happens to be a great way to test for certain conditions in XML documents without creating much data model overhead. So yes, some of these things create or use data models intended to be shared. The less data model involved, the less cumbersome the solution, at least in my experience. The deeper the shared data model, the stronger the poison. ----------------------------- [1] - Yes, it's possible to use only anonymous types, per Tim Ewald. While I applaud his work, it really goes against the grain of WXS. There are also features in WXS which feel like they apply more directly to XML documents in a RELAX NG or Schematron way, notably keys and maybe substitution groups, but in the WXS context they seem mostly to muddy the waters further. 
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