W3C home > Mailing lists > Public > www-archive@w3.org > March 2004

Re: Graphs: intension and extension

From: Patrick Stickler <patrick.stickler@nokia.com>
Date: Mon, 15 Mar 2004 11:33:39 +0200
Message-Id: <D738A04F-7663-11D8-A56E-000A95EAFCEA@nokia.com>
Cc: chris@bizer.de, phayes@ihmc.us, www-archive@w3.org
To: "ext Jeremy Carroll" <jjc@hpl.hp.com>


Oops. Missed this. I had always been thinking 'intensionally'...  ;-)

Patrick



On Mar 14, 2004, at 22:42, ext Jeremy Carroll wrote:

>
> Thinking about other aspects of the paper, I reckon a key choice is 
> whether we
> think of a graph intentionally (like an rdfs:Class) or extensionally 
> like a
> set.
>
> The Carroll/Stickler paper went for extension, it might however be 
> better to
> go for intension.
>
> e.g.
> if there is an RDF/XML document at http://example.org/x we can talk 
> about
> <rdfx:Graph rdf:about="http://example.org/x">
>
> we can annotate it with things like dc:creator
>
> we can compare it to other graphs with
>    <rdfx:equivalentGraph> (graph isomorphism)
> and
>   <rdfx:subGraphOf> (understood as being isomorphic to a subgraph of)
>
> A blank node that names a graph is then just the usual existential ...
>
> The Carroll/Stickler paper also allows a blank node to be shared 
> between two
> graphs ... this is seeming less than useful.
>
> Here is a test case in Chris TriG notation
>
> <a> ( _:a vc:name "Jeremy" )
> <b> (_:b vc:name "Chris" )
> <c> ( <eg> dc:creator _:a )
> <d> ( <eg> dc:creator _:b )
>
> The problem is that <c> and <d> are equivalent, but if we accept all 
> four
> graphs they are saying different things. So if we accept some of the 
> graphs
> we need some mechanism for keeping track of which bnodes are which; 
> which as
> far as I can tell breaks more then we would want. I am currently 
> inclined
> just to prohibit bnodes to be shared between graphs, except in the 
> case where
> the bnode names a graph and occurs (in a triple) in exactly one graph; 
> in
> this case it is basically required to remember the graph named by the 
> bnode
> ...
>
> The point of the example I guess is that the bnode _:a is playing a 
> role in
> linking two triples; the main reason for having those two triples in 
> two
> graphs is that some people might accept one of them without the other, 
> which
> then prevents the blank node from playing that role.
>
> I guess a potential use for blank nodes shared between graphs is that 
> I can
> copy one of your graphs and then make additional statements with one 
> of your
> blank nodes in an additional graph of mine - I think we could just ban 
> that -
> it is certainly easier and the 80/20 rule seems to suggest we should.
>
> Jeremy
>
>
>
>

--

Patrick Stickler
Nokia, Finland
patrick.stickler@nokia.com
Received on Monday, 15 March 2004 04:41:35 UTC

This archive was generated by hypermail 2.4.0 : Friday, 17 January 2020 22:32:25 UTC