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RE: [ALL] [HTML] reification in rdf/a

From: Peter Mika <pmika@cs.vu.nl>
Date: Mon, 12 Dec 2005 13:50:15 +0100
To: "'Mike Linksvayer'" <ml@creativecommons.org>, "'Dan Brickley'" <danbri@w3.org>
Cc: "'Ben Adida'" <ben@mit.edu>, "'SWBPD list'" <public-swbp-wg@w3.org>, <public-rdf-in-xhtml-tf@w3.org>
Message-ID: <003c01c5ff1a$9b033b40$fb1e2582@fspc055>

Hi All,

The question is not so much the definition of owl:sameAs, but the definition
of rdf:Statement. The place to look seems to be the RDF Semantics document,
which explicitly mentions the use case described by Dan:

"Although a graph is defined as a set of triples, several such tokens with
the same triple structure might occur in different documents. Thus, it would
be meaningful to claim that the blank node in the second graph above does
not refer to the triple in the first graph, but to some other triple with
the same structure. This particular interpretation of reification was chosen
on the basis of use cases where properties such as dates of composition or
provenance information have been applied to the reified triple, which are
meaningful only when thought of as referring to a particular instance or
token of a triple.

-- from Section 3.1 at http://www.w3.org/TR/rdf-mt/#Reif

According to the intended (but not formalized) meaning of rdf:Statement
described in the RDF MT document two statements are not necesarily
owl:sameAs even if all their parts (subject, predicate, object) are
owl:sameAs.

Namely, if that would be true, then

_:xxx rdf:type rdf:Statement .
_:xxx rdf:subject <ex:subject> .
_:xxx rdf:predicate <ex:predicate> .
_:xxx rdf:object <ex:object> .

_:yyy rdf:type rdf:Statement .
_:yyy rdf:subject <ex:subject> .
_:yyy rdf:predicate <ex:predicate> .
_:yyy rdf:object <ex:object> .

_:xxx <ex:property> <ex:foo> .

would entail

_:yyy <ex:property> <ex:foo> .

This is explicitly disclaimed by the RDF MT document.

Aside: I don't think we should get rid of the RDF reification vocab despite
all misunderstandings and abuses. SPARQL introduces the notion of named
graphs, how would we exchange such information in RDF without reification?

Best,

Peter


> -----Original Message-----
> From: public-swbp-wg-request@w3.org [mailto:public-swbp-wg-request@w3.org]
> On Behalf Of Mike Linksvayer
> Sent: Saturday, December 10, 2005 12:13 AM
> To: Dan Brickley
> Cc: Ben Adida; SWBPD list; public-rdf-in-xhtml-tf@w3.org
> Subject: Re: [ALL] [HTML] reification in rdf/a
> 
> 
> Hi, delurking for a second, as this is important for CC and I've always
> been personally a bit confused by the notion that reification is
> problematic (apart from the usual RDF/XML syntax issues).
> 
> The inference below that b was licensed on the same date as a seems
> correct.  owl:sameAs says a and b are the same thing.  Of course any
> assertion about one is true of the other, though it is silly to say
> "other", as we've said they are identical.
> 
> I'm not entirely certain upon re-reading
> http://www.w3.org/TR/owl-ref/#sameAs-def whether "same" means ==
> or .equals() (i.e., identity or equality), which could make a
> difference.
> 
> But in any case, any ambiguity there is would arise from inappropriate
> use of owl:sameAs, not reification.
> 
> <http://a.example.org> dc:title "foo" .
> <http://b.example.org> dc:title "Foo" .
> <http://b.example.org> owl:sameAs <http://a.example.org> .
> 
> presumably allows one to infer
> 
> <http://a.example.org> dc:title "Foo" .
> 
> No reification involved.  Looks analogous to the example below that does
> involve reification.  (I don't see a problem in either case, but if
> there is a problem, it is in both cases.)
> 
> This is important to CC because we'd like people to be able to make
> assertions about license assertions (e.g., who, effective date,
> warranty), in addition to straightforward assertions about works and
> licenses.  The latter are too imprecise when a work is potentially
> offered under multiple licenses by multiple rights holders.
> 
> 
> On Fri, 2005-12-09 at 17:05 -0500, Dan Brickley wrote:
> > * Ben Adida <ben@mit.edu> [2005-12-09 13:03-0500]
> > >
> > > Dan,
> > >
> > > Thanks for this point, and apologies for my delayed response.
> > >
> > > Is it the sense of the Working Group that reification is, itself, not
> > > a Best Practice for RDF?
> > >
> > > I ask because your concern seems to be a generic concern about
> > > reification, not one about RDF/A's implementation of it specifically.
> >
> >
> > Yup. It's a general point about the RDF'99 reification vocab
> > (rdf:Statement but more specifically the associated properties
> > rdf:predicate rdf:subject and rdf:object). The same objection applies
> > to any and all RDF notifications, eg. GRDDL.
> >
> > I'm interested to find out if others on the WG share my concern, and
> > hope that any recharted WG could offer some written down advice in this
> > area.
> >
> > Dan
> > > So far, we've tried to build as much of RDF as possible into RDF/A,
> > > without prejudice as to which feature we implement, other than the
> > > inherent limitations of trying to express RDF in XHTML with as much
> > > simplicity as possible. To forgo reification for the reason below
> > > would be a slight departure from our approach to date: it would mean
> > > that we think the concept of reification is, itself, not a best
> > > practice and thus not a practice we want to encourage with a compact
> > > RDF/A reification notation.
> > >
> > > Thus, I'm wondering if this belief -- that reification is generally
> > > bad news -- is widespread within the working group.
> > >
> > > -Ben
> > >
> > > On Nov 4, 2005, at 12:53 PM, Dan Brickley wrote:
> > >
> > > >
> > > >
> > > >I have a concern about RDF/A encouraging usage of the old RDF
> > > >Reification vocab, eg. as in
> > > >http://www.w3.org/2001/sw/BestPractices/HTML/2005-rdfa-
> > > >primer#id0x0382be98
> > > >
> > > ><link about="" rel="cc:license"
> > > >     href="http://creativecommons.org/licenses/by-nc-nd/2.5/">
> > > >  <meta property="dc:date" content="2005-10-18" />
> > > ></link>
> > > >
> > > >yields:
> > > >
> > > >[ rdf:subject <>;
> > > > rdf:predicate cc:license ;
> > > > rdf:object <http://creativecommons.org/licenses/by-nc-nd/2.5/> ]
> > > >dc:date "2005-10-18" .
> > > >
> > > >My concern is that this vocab (rdf:subject/predicate/object/etc)
> > > >appears to be a quoting mechanism, but isn't. So it interacts with
> > > >other RDF statements in a way that is likely to cause confusion.
> > > >
> > > >Imagine in the data above that <> is <http://a.example.org/> (to
> > > >avoid relative URIs confusing things)...
> > > >
> > > >What inferences go through if we believe that (excuse the notation,
> > > >not sure if this is turtle)
> > > >
> > > ><http://a.example.org/> owl:sameAs <http://b.example.org/> .
> > > ><http://creativecommons.org/licenses/by-nc-nd/2.5/> owl:sameAs
> > > ><http://creativecommons.org:80/licenses/by-nc-nd/2.5/> .
> > > >
> > > >I think this gets us.....
> > > >
> > > >[ rdf:subject <http://b.example.org>;
> > > > rdf:predicate cc:license ;
> > > > rdf:object <http://creativecommons.org:80/licenses/by-nc-nd/2.5/> ]
> > > >dc:date "2005-10-18" .
> > > >
> > > >While in the current example, the "dc:date = 2005-10-18" claim
> > > >might be true, I don't think it follows that all such attempts at
> > > >reification will make sense given the interaction with OWL-powered
> > > >identity reasoning.
> > > >
> > > >For example,
> > > >
> > > >
> > > ><link about="http://z.example.com/" rel="cc:license"
> > > >     href="http://creativecommons.org/licenses/by-nc-nd/2.5/">
> > > >  <meta property="xyz:reviewerNotes" content="Dan reviewed this." />
> > > ></link>
> > > >...if there are a number of other things that are owl:sameAs http://
> > > >z.example.com/  .... for which the reification statements might be
> > > >inappropriate, depending on the semantics of the meta property.
> > > >
> > > >Hope these hurried notes make sense,
> > > >
> > > >Dan
> > > >
> > > >
> >
> --
>   Mike Linksvayer
>   http://creativecommons.org/about/people#21
> 
Received on Monday, 12 December 2005 12:53:46 GMT

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