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

From: Tim Berners-Lee <timbl@w3.org>
Date: Wed, 19 Mar 2003 11:29:32 -0500
Cc: pat hayes <phayes@ai.uwf.edu>, Graham Klyne <GK@NineByNine.org>, Frank Manola <fmanola@mitre.org>, RDF Core <w3c-rdfcore-wg@w3.org>
To: Brian McBride <bwm@hplb.hpl.hp.com>
Message-Id: <F7359304-5A27-11D7-BCE4-000393914268@w3.org>

On Sunday, Mar 16, 2003, at 15:13 US/Eastern, Brian McBride wrote:

> 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.

Yes, giving
_:s rdf:type rdf:Statement .
_:s rdf:subject subj .
_:s rdf:predicate pred .
_:s rdf:object object .
_:s foo:saidBy john .
_:s foo:saidIn doc1 .
_:s foo:saidIn doc2 .

> 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 don't follow how they concluded that, as the example above suffers 
from no confusion that I can see. The triple is stated by two files.  
(Maybe I have misunderstood the way the WG uses
"statement" and "stating".  I assumed a statement means the abstract 
tripe, and a stating is
the fact that that triple occurs somewhere.)  Here "saidIn" expressed a 
stating, by
relating the document to a triple. Works fine as far as I can see -- 
and useful, to boot.

> 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.

I can't say it does. Maybe we have all our terms backward or something. 
or maybe I have missed
something obvious above. If there is a modeling problem, then can you 
derive something ridiculous?

> 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:

Indeed.  I don't expect the group to put in {} or the equivalent at 
this stage,
I was really explaining that my attempts to use reifications in the 
style of the spec didn't work, and I abandoned it - as implementation 

>   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.

Yes, indeed I can define another vocabulary.

> 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.

Apart from test cases, do we have some axioms or some evidence of what 
it is supposed to mean? Pointers?

>  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.

optimizations? Got a pointer to the details of this? The user's email?

> The WG compromised and decided try to marginalise reification to "just 
> another bit of vocabulary" as far as it can.

The trouble is, a parser is required to output it when someone puts an 
ID on a statement.
And putting an ID on a statement may seem, to the uninitiated, to be a 
reasonable thing to do.

> 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.

I would prefer to see it removed from parser conformance requirements 
to RDF M&S - or it will become much more difficult to weed out later.

> Brian
Received on Wednesday, 19 March 2003 11:42:48 UTC

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