[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: FW: [ANN]: XQuery: A Guided Tour
Dare Obasanjo wrote: >As for XQuery being able to support other XML schema languages in future, this is nice but I'm curious as to what exactly this means in practical terms. > It seems the XQuery does not use (declarations for) - facets - key/keyref - nillable - details of user-defined derivation hierarchies (i.e., it is interested in supertypes or subtypes only as an unordered list, not as an ordered list or anything: like substitution groups, only the name is important) and creates its own type hierarchy for lists and unions which is different from WXS. (An improvement, I presume it could be folded into WXS.) Given all that, XQuery looks pretty good in this department. I could easily imagine (unambiguous) RELAX NG (with WXS datatypes and a minimal PSVI interface) fitting in. It might be pretty difficult to have an XQuery implementation that supports pluggable schema languages, of course; and the constant revalidation probably rules out a schema language that may want to access the whole document (Schematron, WXS key/integrity constraints, etc.) The main problem I see for XQuery is that XQueryX completely [expletive deleted]. For those who have missed it, XQuery defines a nice whitespace-tokenized language, but it also has another syntax that looks like XML but is not XML: different namespace rules, <![CDATA[ is a function that returns a CDATA section, <!-- a comment is a function returning a comment-->, no encoding considerations. A completely retrograde step, from what I can make out, and one I hope that needlessly clouds what otherwise is looking pretty good now. XQueryX could be trimmed without loss of power: scraps to the dogs under the table. >The working group already has seen the problems caused when one group builds a "type system" (and I use the term loosely) which they believe can work for some future language only for their assumptions to come out wrong. I originally was of the mind that this was a laudable goal of XQuery and still think it is a laudable goal to have it in the data model but wonder how feasible it is to actually create an XQuery implementation with what is basically a "pluggable" type system. A validation language with pluggable datatypes (which are basically custom checking on string values) like RELAX-NG has is fairly straightforward enough but a creating a pluggable type system where you have to deal with issues like type promotion & type substitutability is a bit harder. > Sure, but the only (public) full-featured alternative datatype system so far is J. Tennison's, which ultimately relies on WXS: it performs translations. So the choices need not be between WXS and non-WXS, but WXS and a more extensible form of WXS with which you can define your own types. Jenni's datatyping system really fits at the same level as derivation-by-list and derivation-by-union: type promotion and substitutability of primitives occurs based on primitive types' properties and facets, which Jenni's datatypes fit on top of. Cheers Rick
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