Tuesday, December 13, 2005

Modeling Talk
Last week I was invited along to a talk given by Bob at SAP. I enjoy seeing what Bob's working on when I'm not discussing OWL with him. He's a clever guy, and understands modeling quite well. I just wish I'd written about it sooner, as I won't be so clear anymore.

Probably the most important thing I got out of his talk was an overview of category theory. Andrae and Simon have both spoken about it, and I've come to understand that it's relevant, but as yet I haven't learnt anything about it. Bob gave the 30 second overview for computer scientists, which I found quite enlightening.

I finally got my copy of Types and Programming Languages (otherwise known as TAPL), and have been looking forward to reading it. But when ordering this book I discovered that Benjamin Pierce has also written a much smaller book called Basic Category Theory for Computer Scientists. I had considered getting this book (at only 117 pages, it looks like a relatively quick read), and Bob's talk has now convinced me. The only problem is that I'll have to put the order off until we move to the States... whenever that happens.

Speaking of books, I also picked up a copy of MDA Distilled, Principles of Model-Driven Architecture. Some of the work I did with SAP came out of this book (virtually guaranteed when you work with one of the authors), and it talks about the kind of dynamic modeling that I've been talking about investigating with OWL. I haven't been through all of it before now, so I thought it would be worthwhile reading it in detail.

Coincidentally, I was explaining some of my ideas to someone at work today (Indy), and referred to this book to describe some of the background. I had some idea that Herzum Software worked with MDA (which is why I thought they might be interested in this work), but I had never thought of it as a formal association. Indy quickly made it very clear that Herzum Software specifically put themselves out there as an MDA company. That makes perfect sense as it aligns with what I already knew, but being in my own little corner of the world has kept me isolated from the advertising of it. Anyway, it's nice to know that the direction I'm moving in is paralleled by the work of my new employer.

RDFS Entailment
I've also been in an email discussion about entailment on RDFS. It seems that the following statements:

  <camera:min> <rdfs:range> <xsd:float>
<_node301> <camera:min> '15.0'^^<xsd:float>
will lead to an entailment of:
  <xsd:float> <rdfs:subClassOf> <rdfs:Resource>
'15.0'^^<xsd:float> <rdf:type> <xsd:float>
'15.0'^^<xsd:float> <rdf:type> <rdfs:Resource>
It seems that I didn't cover all the possible rules which could lead to a literal in the subject position. It's quite annoying, as these are completely valid entailments, according to RDF semantics. Making special cases to avoid particular results seems like a hack.

In a similar way, it seems wrong to not allow entailments about blank nodes. I should re-visit the decision there. I think I need to re-read the semantics document to see if I can get further enlightenment. At the least, I know that I can't entail a statement with a blank node as the predicate. Like the problem with literals, the semantics document appears to justify this sort of statement, but the RDF syntax doesn't allow for it. I know this is a particular bugbear for Andrae.

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