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Re: summary of reification semantics issues (material for discussion).

From: Brian McBride <bwm@hplb.hpl.hp.com>
Date: Sun, 16 Mar 2003 20:13:53 +0000
Message-Id: <5.1.0.14.0.20030316185510.02a62058@localhost>
To: Tim Berners-Lee <timbl@w3.org>
Cc: pat hayes <phayes@ai.uwf.edu>, Graham Klyne <GK@NineByNine.org>, Frank Manola <fmanola@mitre.org>, RDF Core <w3c-rdfcore-wg@w3.org>

Tim,

Reviewing the content of this thread, I believe you make the following points:

   A it is not clear to you why the WG chose the current reification proposal
   B cwm uses reification in a manner that is not consistent with the 
current definition
   C the current reification proposal is a barrier to adoption of RDF and 
it might be better dropped.

Concerning A, the WG were considerably influenced by Ron Daniel's assertion 
that the intent of the original WG was to provide a mechanism to support 
provenance.  The use case that was highly influential is as follows:

Consider the following 2 graphs:

Graph G1:

_:s rdf:type rdf:Statement .
_:s rdf:subject subj .
_:s rdf:predicate pred .
_:s rdf:object object .
_:s foo:saidBy fred .
_:s foo:saidIn doc1 .

Graph G2:

_:s rdf:type rdf:Statement .
_:s rdf:subject subj .
_:s rdf:predicate pred .
_:s rdf:object object .
_:s foo:saidBy john .
_:s foo:saidIn doc2 .

Merge the two graphs and then determine who said what, where.  If the _:s 
nodes in each graph denote a statement (as opposed to a stating), it is 
identified by its subject predicate and object properties which would allow 
the two _:s nodes in each graph to be merged.

The WG concluded that if reified statements denoted triples, rather than 
occurrences of triples, the scenario above would lead to many modelling 
errors and further confusion.

I hope this example goes some way to persuading you that the WG is not 
entirely off its trolley in making the proposal that it has.

Concerning B, you note the current proposal is unsuitable for the ways it 
has been used in cwm.  That may be so, and therein may lie a clue that the 
representation of rules was not what it was designed for.

The WG was aware of issues such as the "{ }" mechanism in cwm, the desire 
to represent graphs within  graphs and the notion of contexts.  It decided 
that this area was beyond the scope of its current charter and has recorded 
an issue for consideration by a future WG:

   http://www.w3.org/2000/03/rdf-tracking/#rdfms-contexts

I fear this is not a trivial area into which to venture.

You note that if the current vocabulary were defined as you would prefer, 
then new vocabulary can be defined to represent the other meaning.  That 
argument is two edged.  Can you not define new vocabulary to represent the 
concepts you use in cwm just as well?  I was wondering whether you also had 
a solution for b-nodes as the objects of the reification triples.

As for C, dropping reification all together.  Reification does cause 
confusion and the WG did consider this option, but we do know that people 
use the current reification machinery.  The note on the RDF schema for P3P 
for example uses it (though I doubt anyone uses the note) and in the jena 
project we know that people use it because not only do we get support 
calls, but folks asked for us to ensure we kept the Jena 1 optimisations 
supported in Jena 2.

The WG compromised and decided try to marginalise reification to "just 
another bit of vocabulary" as far as it can.  It is not part of the 
concepts document and is mentioned in a low key way in schema.  It has to 
be acknowledged that its special treatment in the syntax means that it is 
singled out to some extent.  But then, various interesting alternative 
approaches to RDF syntax are gaining traction.

Hopefully, careful explanation in the primer will minimise further confusion.

Brian
Received on Sunday, 16 March 2003 15:13:40 EST

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