Wednesday, August 30, 2006


I'm finally getting off my rear end and writing some code to do OWL in Mulgara. (I hear the gasps of disbelief from the peanut gallery).

So I needed to start with some OWL axioms. You know, owl:Class is an owl:Class, it's a rdfs:subClass of owl:Class, etc. So I went to a handy little file I have named owl.rdf. Now I distinctly recall that the file was not originally named owl.rdf but I wasn't sure why I'd renamed it. All the same, I've had it for some time, and it looks official, so I decided to work with it.

The easiest way to convert it into Krule axioms was to load this file into Mulgara, and then have an iTQL client build the RDF for me. I had a one line bug (why is it always one line?) which took me a while to track down to a typo in a constructor where I effectively said:
  parameter = this.parameter;
But finally my output was as expected.

At this stage I noticed something odd in the axioms. Out of curiosity I went back to the owl.rdf file and discovered this:
  <Class rdf:ID="Thing">
<unionOf rdf:parseType="Collection">
<Class rdf:about="#Nothing"/>
<complementOf rdf:resource="#Nothing"/>

<Class rdf:ID="Nothing">
<complementOf rdf:resource="#Thing"/>
So on one hand it says that owl:Thing is the union of owl:Nothing and everything, and on the other hand, owl:Thing is the complement of owl:Nothing. I can see a rational behind both points of view, but they can't both be true at one. owl:Thing either includes owl:Nothing or it doesn't. (Or am I missing something really big here?)

So where did this file come from? Unfortunately, I picked it up with wget. If I'd used Safari, then it would store the URL in metadata which I could view with mdls. Apple metadata might be a nice patch to work on for wget. Fortunately, when I went to the W3C to search for the file, I suddenly remembered where it came from.

RDF uses Uniform Resource Identifiers (URI) to identify resources. Note that these are Identifiers and not the more familiar Uniform Resource Locators (URL). URLs are a subset of URIs, but their purpose is for location rather than purely identification. A URI may look like a URL, but it may not be specifying the location of anything at all. This is often confusing for first-time users of RDF, who expect that the identifiers they see should be taking them somewhere useful.

Now just because RDF says that you don't need something at the address of a given URI, that doesn't mean you can't put something there. In fact, many people do, including the W3C. For this reason, one day I decided to put the URI for the OWL domain into wget and see if I got anything. The result was this RDF file called owl. Once I remembered that, I also remembered changing the name to the more useable owl.rdf.

So here is this file sitting at the W3C, hiding in plain sight. I find it amusing that it can have such a contradiction in it.

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