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Equality Re: Blank Nodes Re: Toward easier RDF: a proposal

From: William Waites <wwaites@tardis.ed.ac.uk>
Date: Thu, 29 Nov 2018 13:19:30 +0000
Cc: semantic-web@w3.org, Henry Story <henry.story@bblfish.net>
Message-Id: <CF8CBC2F-16F7-43C1-84BA-8455F45399F2@tardis.ed.ac.uk>
To: David Booth <david@dbooth.org>
> Do joe and monica have the *same* address? If we know that those attributes form
> a composite key, then the answer is yes.  Otherwise, the answer is no.

I don’t think that blank nodes are actually at issue here. Consider:

    A a ComplexNumber; real 1; imag 4.
    B a ComplexNumber; real 1; imag 4.

This is much simpler and better defined than street addresses, doesn’t use blank
nodes, and yet still we would want to conclude that A = B. They are equal in that
their meanings are the same but they are not the same in that they have different
names.

Now what about:

   C a Length; value 10; unit cm.
   D a Length; value 3.94; unit in.

Those also have the same meaning but now the graph labels are different so you
can’t say “graph isomorphism” or (possibly better) “bisimulation”.

This is kind of the complement to Hugh's Colliding Kardashians. Here you can
synthetically construct alternative URIs for C and D from the properties and get
something distinct. It still doesn’t help the problem of deciding if they are equal
or if they are the same.

I was in America recently and had brought some small art prints from Europe. The
picture was 25cm x 25cm. I stood by helplessly as a slightly eccentric picture framer
measured them with an inch ruler and muttered to himself about complicated and 
unusual fractions of an inch. He did a good job in the end, but it would have been
easier using the right units. Being easier is a relevant difference so maybe C and D
aren’t the same after all, despite being equal.

Maybe sameness depends on the query context at least as much as the data 
context. Things can be the same — relevantly similar — for one purpose and
different for another. For answering queries, you need to pick which notions of
equality you want to use. And at query time you have to close the world because
you can only make answers from the information that you have.

It seems to me that the appropriate notion of equality can be different according
to the type of the entity. This must simply be defined by the person who defines
the type. It would be nice to provide a sensible default equality for when we don’t
have a more specialised one.

What do you think?

-w
Received on Thursday, 29 November 2018 13:19:58 UTC

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