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Re: refining closure text for rdfs-isDefinedBy-semantics

From: Frank Manola <fmanola@mitre.org>
Date: Fri, 07 Jun 2002 11:37:38 -0400
Message-ID: <3D00D342.32965C06@mitre.org>
To: Dan Brickley <danbri@w3.org>
CC: w3c-rdfcore-wg@w3.org

Dan--

Below are some difficulties I see with what you're proposing.

Dan Brickley wrote:
> 
snip
> 
> We can note that:
> 
> RDF provides a property rdfs:isDefinedBy that is a specialisation of the
> rdfs:seeAlso property. This property is used to relate an RDF vocabulary
> term (such a Class or Property) to the context within which it
> is defined.

I'd like to be clearer as to what things are included in "RDF vocabulary
term" (what else besides Classes and Properties).  Also, I suspect "the
context within which it is defined" will muddy the waters, rather than
clearing them.

> 
> For every RDF property there is exactly one correct value for the
> rdfs:isDefinedBy property.

If this is intended to be a constraint, I don't understand how you'll
enforce it.  Note also that this doesn't say there can't be more than
one rdfs:isDefinedBy property, it just says there is only one "correct
value".  So there might be multiple rdfs:isDefinedBy properties, several
of which have the "correct value" (whatever that is), and others which
have "incorrect values".

> 
> When RDF graphs are written in the RDF/XML 1.0 syntax, this value
> corresponds to the XML namespace URIref used in the serialized
> representation of the RDF properties.

But identifying the namespace doesn't, per se, identify someplace where
something is defined, unless you're assuming that the namespace URI is
also the URI of a schema.  The text you quoted from the M&S seemed to
say that this was the case, but do we say that in any of our current
specs?  I don't think so, and without that, I think it's misleading to
suggest (by using the name "rdfs:isDefinedBy") that the namespace URI is
something that defines anything (as opposed to something that simply
distinguishes a collection of names).

> 
> can we take this as a basis for progress?

I don't think so.

> 
> The main clarification is that we represent explicitly the M+S claim that
> there is just one schema for each property, and that rdfs:isDefinedBy
> can thus be used to represent (within the RDF graph) the relationship that
> holds between a property and the namespace within which it is defined.

I don't think your text represents this explicitly, since I don't think
there's anything in our current specs that says that the namespace URI
identifies a schema.  The easiest way to represent the above explicitly
would be to simply say it, e.g., say, someplace,

a.  Each property is defined by exactly one RDF schema  
b.  The [or a] URI of this schema is used as the XML namespace URIref in
the serialized representation of the RDF properties
c.  rdfs:isDefinedBy can thus be used to represent (within the RDF
graph) the relationship that holds between a property and the namespace
(and schema) within which it is defined.

There are still some problems here (like, what exactly is a "schema"? 
if a property is described by other properties in multiple places, is
that one schema or many?), but at least it's more explicit.

--Frank




-- 
Frank Manola                   The MITRE Corporation
202 Burlington Road, MS A345   Bedford, MA 01730-1420
mailto:fmanola@mitre.org       voice: 781-271-8147   FAX: 781-271-8752
Received on Friday, 7 June 2002 11:37:49 EDT

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