Re: Will the next version of XML Schema have aschema-for-schem
On Thu, 2007-10-18 at 11:29 +0100, Anthony B. Coates (XML-Dev) wrote: > Rick, is that historically correct? Was anyone in the XML Schema Working > Group ever arguing *for* content models that depend on attribute values or > similar? I wasn't there, but W3C XML Schema (to me) has always looked > like a schema language which takes a lot of inspiration the way data is > structured in object-oriented languages (and to a lesser extend, > relational databases). Yes. Me. See http://lists.w3.org/Archives/Public/www-xml-schema-comments/1999AprJun/0061.html 8 and half years ago, and I still don't know if it on their radar. One of the troubles then was that there was no implemented schema language that used grammars that did it, then, and the WG was not theory-friendly but implementation-friendly: the WG's method was to consolidate the ideas from the various trial balloons that members had implemented, if you pardon my euphemistic usage of "consolidate". Schematron didn't count, and many of the experts on the WG did have a lot of treasured database and OO experience to contribute. This is why XSD is used for everything except actual validation nowadays. It has lots of extra machinery for bundling up constraints into higher-level components and naming the kinds of things that parameter entities can do, but basically no attention was paid to being more powerful than DTDs at the grammar level: this is why people feel it has disappointing bang per buck. (I am not referring to the issue of what class of TCS grammar is used, btw.) My little 1999 paper "Validate This: Content Models on Different Targets" has the basic idea: you should be able to include any type of node in the DOM in the content model. RELAX NG ended up adopting a simplified version where they allowed elements, attributes or particular token values to be particles. I doubt if anyone on the XSD WG bothered to read or understand the paper (1999 was not a good year for my prose) and they certainly weren't listening to Murata-san by that stage. (An interesting idea in it is that you can use the same system to validate IDREF, since the presence of an IDREF requires an ID elsewhere.) I extended the idea in the paper "Axis Models & Path Models: Extending DTDs with XPaths" at http://xml.ascc.net/en/utf-8/validaxis.html has the discussion proposal. In fact, I suggested that the particles in a grammar could in fact be any XPath, not just an element particle or an attribute particle: "path models". That paper also has "Axis models" which use grammars, but along any axis, not just following-sibling axis. So a simplified schema on the child axis for HTML might be (html, (head, (meta|object|script)) | (body, ((p, (b|i))| (ul, li, (b|i))| (ol, li, (b|i))) For a database dump kind of schema, where there are no positional dependencies, the axis model can give the entire schema in a single production, along the child axis. (For anyone interested, I also developed a third approach, using the document-order axis, using a simplified grammar, and allowing partial ordering and missing sections, as the "Hook" schema language, a toy language that was the smallest schema language until Eric's Examplotron. http://xml.ascc.net/hook/) I also argued that we needed to support arbitrary syntaxes for simple datatypes: how to map notations to standard types was the key. This is because the job of markup is to annotate data in the form that the user is familar with, not always just for machine-to-machine transfer in inhuman formats. This is the approach that ISO DTLL has taken. (I remember making the point at XML 200? at the town-hall meeting on Schemas where Schematron was able to specify many more of the document constraints than others but was not declared the winner despite the objective evidence, hrmmmph.) My earliest suggestion was "Notation Schemas" http://lists.w3.org/Archives/Public/www-xml-schema-comments/1999AprJun/0030.html now archived at http://xml.coverpages.org/jelliffe-notation19990513.html > That is to say, I didn't think it was a case of "they did it this way > because they were forced to", I always thought it was more "they did it > this way because they chose to", which is a different kettle of fish. No, the simpleContent and complexContent elements were quite late additions made as hacks, IIRC. Cheers Rick Jelliffe
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