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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: RE: Namespace use cases
 On Thu, 9 Jul 2009 17:59:56 -0700, Jim Tivy wrote: > I would love to hear of some success or horror stories of multi-namespaces > in authoring content. Anyone out there doing this in DITA, DocBook, open > office... > > What ever happened to the compound document? Web services are an obvious use case. It's true that SOAP is execrable, but the 'simplest subset' continues to drive it: there's an "envelope" namespace, which indicates 'this is a message conforming to certain expectations' (that breaks down when you add in the headers and the ws-* interoperability mess; perhaps that's a namespaces counterexample?), and the content is in some other namespace. That namespace defines its own rules. You can't create a "carrier" namespace without the ability to say something about the content carried; HTML5 is a rather brilliant example of how limited such content would be if Powers That Be had to approve every message dialect. Several XML dialects are designed for inclusion in other dialects--SVG, MathML are obvious ones. XPath is an interesting case: it's not an XML dialect, but does need to be able to navigate arbitrary dialects. Compare the rigidity and complexity of CSS selectors for a counterexample. If XML namespaces weren't such a horrible muddle, there would probably be more such dialects (and less resistance from, for instance, HTML proponents, to adopting the technique). "Microformats" are an example where namespaces would be valuable, but since microformats are typically HTML-based, instead the designers are typically forced to contort a limited set of elements and attributes into unnatural shapes to carry the message. Sure, you and I can agree that "<abbr class="key 42">moltuae</abbr> is an encoding for the meaning of life, the universe, and everything, but it's ... somewhat non-obvious, shall we say. Providing a means of extensibility would enable, in HTML 5, microformats (which are probably not intended for display in the browser, but which are nonetheless valuable to their intended users). On a side note ... someone, I think it was Jim Tivy, suggested in an earlier message that attributes ought to be in the default namespace. I would suggest that that's not a good idea. If you think of attributes as having a "namespace" scoped by their containing element, things are a lot more clear. The "no-namespace" or "global" namespace (xmlns="") is where elements live if they're not assigned to a namespace. It should be obvious that if you have multiple dialects from different authorities competing for names in that namespace, you're going to lose, probably quickly. The same is not true of attributes; attributes in "no namespace" are scoped by their parent element--a single authority. <p class="important">Blah</p> current state, html as default namespace <html:p class="important>Blah</p> current state, xmlns:html=html namespace <html:p html:class="important">Blah</p> proposed change. take attributes out of their parent's namespace and into the default namespace, and *every* *attribute* of every element in a non-default namespace now has to be prefixed, even though it's governed by the same authority that defined the default namespace. The scoping of attributes to their container elements is often confusing to people new to XML, but it is, in my opinion, the right decision (given the verbosity that would otherwise be introduced). As they stand, namespaces in XML are awkward and clumsy (needlessly so, as others have already pointed out, using for example the Java package style technique for establishing a namespace authority, which is fully as general as the URIs-that-aren't-really-URIs defined by the namespaces spec), but not broken ... until you encounter prefixes in content, at which point ... it's broken. Is it broken because of namespaces, or because of namespace prefixes in content? Well, given the choice of (uselessly verbose and special-character-studded) URIs as the namespace identifier, it's hard to demand that they be replicated; lots of folks are willing to complain about the replication of the prefix "aliases" anyway. Clark notation, after all, would solve the problem ... but hasn't been widely adopted, and I think for exactly that reason. Amy! -- Amelia A. Lewis amyzing {at} talsever.com "How does one hate a country, or love one? ... What is love of one's country; is it hate of one's uncountry? Then it's not a good thing. Is it simply self-love? That's a good thing, but one mustn't make a virtue of it, or a profession." -- Therem Harth rem ir Estraven 
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