- From: <Patrick.Stickler@nokia.com>
- Date: Thu, 23 Aug 2001 10:48:12 +0300
- To: sean@mysterylights.com
- Cc: www-rdf-interest@w3.org, uri@w3.org, www-rdf-comments@w3.org
> -----Original Message----- > From: ext Sean B. Palmer [mailto:sean@mysterylights.com] > Sent: 22 August, 2001 16:37 > To: Stickler Patrick (NRC/Tampere) > Cc: www-rdf-interest@w3.org; uri@w3.org > Subject: Re: QName URI Scheme Re-Visited, Revised, and Revealing > > > > Hmmmm.... do we need then to use DAML equivalence > > constructs for basic RDF?! E.g. > > > > <qn:{http://description.org/schema/}Creator> > > daml:equivalentTo > > <qn:{http://description.org/schema/}@Creator> . > > No, because those QNames are in RDF, and RDF processors handle them by > concatenating them and forming a URI. The XML Namespace specification > defines a difference between the QNames, but it doesn't tell > you how to > handle them. The RDF specification comes in and tells people > what to do > with the QNames. It tells one to concatenate the QNames togther minus > partition information, to form a URI. Uhhh... so even if the NS spec says X and Y are different, RDF can do whatever it likes with them, including saying X = Y even if the creator(s) of the data in which X and Y occur say that X != Y?! Sorry, I don't read the NS spec that way, and I seriously doubt very many folks in the XML community at large read it that way -- i.e. that QName distinctions can be discarded by RDF and RDF still remain conformant to the NS spec. And what about QName partition distinctions, as pointed out by Sean, where in fact not only do two different QNames constitute different lexical identity (and hence can represent different semantics) but also a QName used for an element name constitutes a different lexical identity from the same QName used for a global attribute name! So there are now two distinctions defined by the NS spec that RDF discards. That can't be good. > There's no problem with > that, except > there's no way to identify a QName as a first class object > without some > extra properties, If potentially four lexically distinct QNames (i.e. two QNames which collide on direct concatenation used both for elements and global attributes) are merged by RDF into one single URI derived by the present RDF function, then how can you possibly make statements about them to differentiate them -- since their only identity referencable in a statement in RDF space is ambiguous to start with? Or are we back to using sub-graphs to define their actual identity rather than self-contained, stand-alone URIs, which is the IMO proper way to define resource identity in RDF? > or a new URI scheme. I agree there. A QName URI scheme would have to capture the complete identity of all QNames maintaining the full scope of lexical distinction present in the serialized XML. I.e. something like my revised qn URI scheme, or anything similar that does the job. But then we get that nasty problem of element and global attribute QNames having identitical semantics according to RDF's condensed serialization syntax -- in which case we *have* to explicitly define the equivalences between the different QName URI's (as above). > So there's no > particular need to map > every QName in XML RDF to your URI scheme - to do so would be > silly - but > it doesn't stop people using it. Nothing stops people from custom programming any functionality whatsoever into their own systems, no. But this is an issue of global standardization, not what can be done locally in a single isolated system. > In other words, you've accomplished all that you want to > accomplish simply > by inventing this URI scheme. No, that's not so. If I can't rely on *every* RDF engine used by every SW agent to interpret my data exacly as I have defined it, then the SW has no data integrity. Again, its about global consistency of data. > If you force people to adopt it > *instead* of > the concatenation mechanism in XML RDF, then you'll form an > utter mess, I agree (and have stated several times) that the QName URI solution is far less attractive than other solutions because it is not backwards compatible. However, since people are already thinking in terms of QNames when defining their Web based ontologies, I hardly expect that RDF officially mapping those QNames to an equivalent URI representation is going to cause much uproar (in fact, I expect it will be very positively recieved as it serves to clarify what is now a murky relationship between QNames and RDF resource URIs). Folks using XML *think* in terms of QNames insofar as their data models, vocabularies, and ontologies are concerned. In fact, it's tough to get folks to think in terms even of (namespace)name rather than ns:name! People are already using QNames as universal identifiers. The fact that there is *not* an official, standard URI representation for QNames is what is surprising... I don't see a QName URI scheme as a hard sell, nor do I see it causing a mess, rather the opposite, of cleaning up the present mess in RDF QName to URI mapping. > because the Semantic Web deals with URIs, and not just > QNames. Firstly, I'm not suggesting that folks to use "QNames" for all resources. In fact, one would expect that QName URIs would be used only for abstract concepts such as properties, classes, etc. Noone is prevened from using other URI schemes for other types of resources, and in fact, we *need* to be able to use other URI schemes to do alot of the neat stuff that will be possible on the SW, and to ground the abstract concepts and relationships into the "reality" of the web. Secondly, RDF itself is "URI Scheme blind". It does not itself care what URI scheme you use. All that matters is that your universal identifiers (which coincidentally happen to be URIs) are unique, and consistently identify the same resource. Whether some *other* application outside the scope of RDF-space can do something with those identifiers, such as interpret them as URIs and perform some task with them, is irrelevant insofar as the RDF model is concerned. > Using the > concatenation mechanism is an excellent and quick way to form > those URIs > out of QNames. Quick? Maybe. Excellent? No! It was a very clever hack that works with HTTP URLs using HTML fragment syntax, but as with most hacks it does not generalize or scale sufficiently to meet a broader range of needs -- namely those of the SW in all its forms. It also does not maintain lexical distinctions defined by the NS spec, and for that reason alone, its validity is suspect. It has IMO been sufficiently demonstrated that the current method used by RDF for deriving URIs is unsafe, discriminates against arbitrary URI schemes, and does not maintain data integrity. I don't intend to go over all that again. > ... and now that you've > come up with > representing an XML QName (i.e. with all the details > preserved as in the > XML namespace specification) as a URI, that's fine. But it's true utility and value to RDF would be as the official URI representation within triples -- making QNames a first class identifier for abstract resources. If it's not standardized and mandated for all RDF applications, it's not a solution to the present problem(s). > Now, there are only two errors with your URI scheme. The > first is that the > characters "{" and "}" are disallowed in URIs per section 2.4.3 of RFC > 2396. This can be easily gotten round by using "(" and ")" > instead, e.g.:- > > qn:(http://www.w3.org/1999/xhtml)title > > ... > > Because the "#" character is also disallowed by section 2.4.3 > of RFC 2396. > Perhaps you can just %HH encode it? > > qn:(http://infomesh.net/2001/08/example%23)myElement > Thanks for pointing these out. I just copied James Clark's syntax (which was not specifically intended for use as URI and missed those encoding problems. I was focusing more on the idea of a single, standardized URI scheme for QNames, not specifically what that URI scheme would be, etc. Parentheses and escaping should do just fine (though the latter makes the URI scheme harder for humans to process -- but that's a problem of any URI scheme that includes embedded URI refs). Fortunately, if RDF also adopts support for QNames as URI attribute values, then even in RDF Schemas folks won't have to "touch" the QName URIs directly too often, but can simply define prefixes and use them in their minimal, abbreviated forms. The only place where one would see the full qn URIs would be just in the actual triples store. > qn = 'qn:' '(' absoluteURIexc [ '%23' fragment ] ')' ( > elem | glob | > per ) > elem = NCName > glob = '@' NCName > per = NCName '@' NCname Thanks for the revised grammar. If such an approach is adopted by the RDF core working group, then we can roll up our sleaves and put some spit and polish on the URI scheme itself. Cheers, Patrick -- Patrick Stickler Phone: +358 3 356 0209 Senior Research Scientist Mobile: +358 50 483 9453 Software Technology Laboratory Fax: +358 7180 35409 Nokia Research Center Video: +358 3 356 0209 / 4227 Visiokatu 1, 33720 Tampere, Finland Email: patrick.stickler@nokia.com
Received on Thursday, 23 August 2001 03:48:25 UTC