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Re: Layers

From: Thomas Baker <tom@tombaker.org>
Date: Wed, 2 May 2012 08:23:34 -0400
To: Pat Hayes <phayes@ihmc.us>
Cc: Dan Brickley <danbri@danbri.org>, Sandro Hawke <sandro@w3.org>, public-rdf-wg <public-rdf-wg@w3.org>
Message-ID: <20120502122334.GA50682@julius>
On Tue, May 01, 2012 at 11:12:48PM -0500, Pat Hayes wrote:
> > The Layers metaphor fits very nicely with the notion of Levels in the FRBR use
> > case as described at [1]:
> > 
> >    This proposal views descriptions of WEMI entities as bundles of statements
> >    made at different levels of abstraction, from the most concrete Item level
> >    to the most abstract Work level.
> 
> Hmmmm. Sorry to be a bit of a pooper, but this worries me. When Sandro +
> Danbri came up with the "levels" terminology, I hated it because it suggested
> that a "vertical" dimension between layers was meaningful and even important,
> which it isn't. It has no semantic meaning at all: the 'layers' have no
> intrinsic ordering, and in fact are best thought of as constituting a set
> rather than a sequence or list. 

Yes, I agree!  I was assuming that any "order" between levels would be
determined by rules specific to the interpretation of (in this case) FRBR --
rules orthogonal to the graphs (or layers) themselves.

>                                  But Sandro calmed me down by saying, no, its
> just that they are transparent, like sheets of celluloid. But my worry has
> now reared its head again, because WEMI "levels" really do have a vertical
> order, and one that is semantically significant (level of abstraction).

As I see it (which is how the question was framed when we discussed it on 11
April [1]) the question is: where does the semantics of graphs (or layers) end,
and the semantics of rules overlaid on those semantics -- I think someone
called this "magic," as a placeholder -- begin?

[1] http://www.w3.org/2011/rdf-wg/meeting/2012-04-11

> If we endorse this kind of usage, I am afraid that others will start using the
> "depth" of a "layer" to mean time sequence, where deeper means older; and
> others to mean importance, where deeper means less trusted; and others to
> mean all kinds of other things. Which just re-creates the kind of confused
> ad-hocery that we have now. 

I completely agree.

> Its OK to use layers to handle levels, supported by a suitable ontology
> maybe, but what I DONT want us to even hint at doing is to encourage people
> to use some kind of ordering of layers based on an implementation accident or
> something meaningless like lexical ordering of the "graph names" to encode
> anything meaningful. 

Absolutely - not even a hint.  But by analogy, has there been a problem with
implementers reading significance into the order of triples in an RDF dataset?

Tom

-- 
Tom Baker <tom@tombaker.org>
Received on Wednesday, 2 May 2012 12:24:10 GMT

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