W3C home > Mailing lists > Public > semantic-web@w3.org > July 2012

Re: Why do we name nodes and not edges?

From: Pat Hayes <phayes@ihmc.us>
Date: Tue, 31 Jul 2012 01:04:25 -0500
Cc: Austin William Wright <aaa@bzfx.net>, Melvin Carvalho <melvincarvalho@gmail.com>, Semantic Web <semantic-web@w3.org>
Message-Id: <3FA3AA28-A6F3-4694-A7D7-434D8B4F1380@ihmc.us>
To: Steve Harris <steve.harris@garlik.com>

On Jul 27, 2012, at 3:37 AM, Steve Harris wrote:

> Yeah, but that example uses reification, which is at best frowned upon.

Look. If you want to refer to, and describe in RDF, an RDF triple, then you will be using reification whether you call it that or not. Reification, in this context, *means* describing RDF triples in RDF. And RDF reification is not "frowned upon" as far as the specs are concerned, so if this is what you want to do, go ahead and use it. It is perfectly legal, conformant, RDF. 

> 
> Increasingly my reaction to these kinds of questions is: maybe you shouldn't be using RDF.
> 
> RDF has limits of expressivity, [all IMHO] it's best for describing things in a way that the descriptions can easily be consumed by other reasonably generic processors - once you start delving off into obscure corners - e.g. something that was said by person X, believed by person Y, but not person Z, and then published by W - then you're no longer in the territory of easily. Even once you've somehow parsed that lot, doing anything useful with it - in an even vaguely generic way - is beyond complex.

Its messy, but its not hard to write stuff like this in RDF, after inventing the appropriate URI vocabulary to do it all, of course. And as for doing anything useful with it, well yes that might well be a problem, but not because its written in RDF. This stuff is just hard to do anything useful with, period. 

Pat Hayes

> 
> - Steve
> 
> On 2012-07-26, at 16:30, Austin William Wright wrote:
> 
>> At least in RDF, resources (the node of the graph) are first class citizens.. You can describe edges as resources, you just need give the resource an identifier first:
>> 
>> <triple1234>
>>     a rdf:Statement ;
>>     rdf:subject <foo> ;
>>     rdf:predicate <http://example.com/edge/123456> ;
>>     rdf:object 1 .
>> 
>> Since edges/RDF statements with the same subject, predicate, and object must be the same edge, this identifies edges. Any rdf:Statement resources with the same values for subject, predicate, object, would be different URIs for the same resource.
>> 
>> We don't see this more often because usually edges aren't resources "of significance", there's not much reason to describe specific facts. Generally, people make statements on entire graphs of RDF statements, the graph getting a URI. These don't usually get stored themselves as RDF statements for practical database reasons, but you could, as an RDF Collection of rdf:Statement resources.
>> 
>> Austin Wright.
>> 
>> On Wed, Jul 25, 2012 at 8:07 AM, Melvin Carvalho <melvincarvalho@gmail.com> wrote:
>> Sorry if this topic has been covered before, but I have a question based on the axioms of the web, in particular:
>> 
>> Axiom 0a: Universality 2    Any resource of significance should be given a URI.
>> 
>> In this case we consider the web to be a directed graph (of nodes and edges), where a node corresponds to a resource but edge does not.
>> 
>> We are encouraged to make nodes universal by giving them a URI.
>> 
>> Why dont edges get the same treatment, ie encouragment to give it a (universal) name.  Is it even practical?
>> 
>> I know there's such thing as reification but that seems to be unpopular (maybe before my time).
>> 
>> I'm just curious as to whether this seems asymmetrical, that nodes are seemigly treated in one way, and edges in another?
>> 
> 
> -- 
> Steve Harris, CTO
> Garlik, a part of Experian
> +44 7854 417 874  http://www.garlik.com/
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Received on Tuesday, 31 July 2012 06:04:59 UTC

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