[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: XLink a special case in the self-describing Web?
> > (1) XML Namespaces do not provide a way for a single element to > > conform to an element type in each of several schemas. Therefore, > > there is no way for a single element to be recognized as > > conforming to both the X:Foo and the Y:Bar element types. > That is correct as stated, but it reflects a misconception. > Namespaces provide nothing but a superficial mapping of GIs and > attribute names into so-called "universal names", which are simply > an ordered pair of the form {string, Name} where "string" has the > syntax of a URI. The SGML/XML semantic model is unchanged. Well, OK, but this "misconception", as you put it, is implicit in every single one of the industrial uses of XML Namespaces that I have so far encountered. Calling it a "misconception" seems to me merely a way to deflect criticism from formalists like me, while at the same time leading the public away from something that enables information interchange in a powerful and potentially nonproprietary way, and toward something that isn't as powerful and is far more likely to engender system-vendor dependencies in everybody's information. The SGML/XML semantic model may be unchanged, but it is, in fact, being replaced with something that doesn't work in the public interest, under the banner, "Down with those #$% DTDs!" We need to acknowledge that fact. It is not helpful to retreat into statements like "There is nothing semantic about XML Namespaces", when in fact most people use names in order to label things meaningfully. Most people literally can't imagine using names for any other purpose. In the Biblical creation myth, Adam's task, as assigned by Jahweh, was to name everything. I doubt that the idea of there being a possibility of names with no semantics would ever have occurred to Adam, and maybe not even to Jahweh. If I name a class of things "squirrel", the only reason I do that is that all members of that class share the characteristics of squirrels. > Note that by definition X:Foo and Y:Bar cannot represent the same > universal name, but X:Foo and Y:Foo can and do if the prefixes X: > and Y: are bound to the same string. I wasn't clear. When I said "X" I meant "whatever X means". I wasn't making a statement about syntax. Please read "X" as "the universal name-prefix for which X is the abbreviation". > > So, if my above understandings are correct, I tentatively conclude > > from this that XLink is not a namespace or a schema in the usual > > sense, because, among all of the kinds of element definitions that > > are possible, only the XLink element types are, de facto, exempt > > from the "one element, one element type name in one semantic space > > of element type names" rule. > The namespace for XLink at present is > "http://www.w3.org/1999/xlink". The current XLink draft gives > meaning to the universal attribute names > {"http://www.w3.org/1999/xlink", "type"}, > {"http://www.w3.org/1999/xlink", "href"}, and others, but does not > give meaning to any universal GIs. You are free to use a name such > as {"http://www.w3.org/1999/xlink", "simple-link"} as a GI if you > want, but XLink does not say what it means. That's what I thought. So the actual generic identifier of an XLink element is immaterial to its recognition by any processing system as an XLink element. The question, "What kind of element is this?" is not answered by the generic identifier (element type name); it is answered by attributes other than the generic identifier. (The generic identifier is itself just an attribute; it is the value of the one-and-only nameless attribute, which all elements are required to have.) > > Can anybody create sets of attributes, just as has been done with > > XLink, that will constitute a semantic space, and thus effectively > > have elements identify themselves as conforming to certain element > > types without requiring that the generic identifier be used to > > identify the element type? > > Yes, indeed. If one is sufficiently canny, one can do so in conformance > to the rules of SGML architectures as well, as I believe XLink does. Well, almost but not quite. The basic difference has to do with establishing a context for each of the pseudo-element-types that XLink describes. With architectural forms, there is always a known set of contexts within which an element of a particular type is constrained to appear, even if that set of contexts is "anywhere at all", as is (correctly, I believe) the case with XLink elements. I haven't found any formal expression of that "anywhere at all" context constraint in XLink such that the same kind of trick could be used for element types that really should only appear in particular contexts. (Actually, XLink locator elements should only appear inside extended XLinks, but I haven't found a formal machine-interpretable expression of that constraint that I could use to define and constrain architectures other than the XLink architecture, using the same software to validate instances for conformance with such context constraints. We could do at least that much with DTDs.) Validation of the syntax of namespaces, as they are actually used in documents, when they are arbitrarily mixed with other namespaces, is a basic business requirement that W3C designs have so far ignored. The W3C evidently expects all XML processing software to re-invent and re-implement all aspects of syntactic validation of every Namespace, including functionalities needed by the overwhelming majority of business namespaces. It's a recipe for Babel, and overcoming the effects of that Babel is impossibly burdensome for developers of industrial XML Schemas, when an industry must be served by more than one software vendor, and when its information interchange architectures must incorporate any number of extensions to the industrial vocabulary, and the industrial vocabulary must be able to be mixed with the vocabularies of any number of other namespaces. > > If anybody can already do this, is this a > > methodology to which XML Schemas can provide validation services, by > > checking to see whether all of the attributes have been used in > > syntactically valid ways? If so, how? > I think so, but I don't know exactly how. Neither do I, and that's the heart of my question. It would be wonderful if XLink, for example, were defined in such a way that we could use the same formalizing definitions for inheritable architectures other than the XLink architecture, in addition to XLink itself. The ISO architectural forms syntax is just one way to accomplish that goal. Instead of repeating my usual rant extolling the virtues of ISO architectural forms, let me instead (try to) be both syntax-agnostic and standardizing-body agnostic. What are the alternative formalisms that meet the commonplace business requirement that a single element be recognized as conforming to both the X:Foo and the Y:Bar element types? XLink demonstrates a way -- for exactly one architecture, namely itself. Is W3C developing a *general* solution that can be used for *all* architectures? Is anyone else in the running? Or is this a problem that W3C's vendor-members simply don't want to be solved? If so, it's already too late, because there is already an international standard that solves it, and that internationally standard solution -- architectural forms -- is already being adopted by the financial community, primarily because there is no relevant W3C Recommendation. -Steve -- Steven R. Newcomb, President, TechnoTeacher, Inc. srn@t... http://www.techno.com ftp.techno.com NEW ADDRESS effective May 1, 2000: voice: +1 972 359 8160 fax +1 972 359 0270 405 Flagler Court Allen Texas 75013-2821 USA *************************************************************************** This is xml-dev, the mailing list for XML developers. To unsubscribe, mailto:majordomo@x...&BODY=unsubscribe%20xml-dev List archives are available at http://xml.org/archives/xml-dev/ ***************************************************************************
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