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   RE: A certain difficulty

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  • From: Mark Birbeck <Mark.Birbeck@iedigital.net>
  • To: xml-dev@xml.org
  • Date: Sat, 26 Feb 2000 14:17:47 -0000

David Megginson wrote:
> Mark Birbeck <Mark.Birbeck@iedigital.net> writes:
> > Of course we could say everyone must use "dc:Creator", but then how
> > do I find everyone who ever knitted a jumper? Well of course I go to
> > the knitting site that has used RDF to refer to subsets of other
> > data, because they have a class called "Knitter".
> Take a look at the class.  Consider the following:
>   <rdf:RDF ...>
>   <megg:Jumper rdf:about="http://www.foo.com/1">
>     <dc:creator rdf:about="http://www.people.org/a"/>
>   </megg:Jumper>
>   <megg:Book rdf:about="http://www.amazon.com/ids/00002">
>     <dc:creator rdf:about="http://www.people.org/b"/>
>   </megg:Book>
>   </rdf:RDF>
> If you want to find every knitter but not every author, then simply
> look only for dc:creator properties attached to an object of class
> megg:Jumper.  The alternative is to create an endless stream of new
> properties
>   create
>   sweaterCreator
>   bookCreator
>   carCreator
>   webPageCreator
>   cartoonCreator
>   flyingSaucerCreator
>   emailCreator
>   badSmellCreator
> etc.  Of course, you have no choice but to do this kind of thing when
> you're using flat HTML <meta> tags, but RDF was designed specifically
> to allow you to apply modern OO techniques to the problem.

I must say I prefer old-fashioned OO techniques then ;-) where I'm not
limited to only having one class. Why in your model have you decided
that 'creator' is the must use tag for anyone who creates something, but
'hook' and 'jumper' are valid objects? If your argument was consistent
then you should just have a generic object that can be a person, book or
jumper, and a 'created by' tag. And then I can say 'this jumper was
created by that book' which we may want to inhibit.

Which is the nub of my criticism of this 'one schema fits all' stuff. It
just doesn't cut it. I am suggesting that the preferred model is to say
that 'author' and 'knitter' are sub-classes of 'creator'. Then if you
want all creators you can find them, and if you want all authors you can
find them. But more than that, you can say that the 'creator' of a
'book' must be an 'author', and not a 'car' or 'elephant'.

Another problem with your example is that you're dealing with instances
of the class and not the class itself. I might not have any information
in my database about books that you have written David, but I could
still use RDFS to find out that you are an author. With the example you
gave above, I would only know that you are an author if I have a book in
my database.

What do you think?

Best regards,


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