W3C home > Mailing lists > Public > public-rdf-wg@w3.org > December 2011

Re: [ALL] agenda telecon 14 Dec

From: Richard Cyganiak <richard@cyganiak.de>
Date: Wed, 14 Dec 2011 23:44:49 +0000
Cc: Guus Schreiber <guus.schreiber@vu.nl>, RDF WG <public-rdf-wg@w3.org>
Message-Id: <9124DD28-4A4C-48C7-BAF2-4D0B3E43659D@cyganiak.de>
To: Pat Hayes <phayes@ihmc.us>
On 14 Dec 2011, at 21:02, Pat Hayes wrote:
>>> But that design refers to 'named graphs', which already introduces new semantics (that is, semantics which goes beyond that defined in the RDF specs.)
>> 
>> Isn't it a fallacy to think that because a symbol is called X, its formal meaning has to be what's implied by X?
> 
> Yes, but that does not seem to have anything to do with what we are talking about. I am concerned with the actual meaning of this rather important phrase. 

We are creating an engineered artefact. The actual meaning of the terms that we use in defining this artefact is whatever we define it to be. “Named graph” is a neologism and its definition is whatever we make it.

>> “Blank node identifiers” exist in concrete syntaxes but do not occur at all in the formal semantics, much less “identify” anything.
>> 
>> The xsd:anyURI datatype is supported in RDF Semantics, and its value space is IRIs, but these IRIs don't “denote” or “identify” in the formal semantics. They simply are inert string values.
>> 
>> Same with “graph names”, they exist in the abstract syntax, but that doesn't mean they have to occur in the formal semantics. They're ways of grouping RDF triples together, that's all.
> 
> Well, that is a meaning which the semantics should address. Presumably they have some function: if that functionality produces any entailments that would not hold without it, then the semantics should address and clarify these. 

I don't think that it should produce any additional entailments and hence see no need for the model theory to address it.

>> Research on adding formal semantics for named graphs
> 
> Named graphs already have a precise semantics. Both the phrase and the full semantics of it were published in the original paper I co-authored with Jeremy and others.

Has this semantics ever been implemented? What systems rely on it to function or interoperate?

> If you mean something else by "named graph", please use a different terminology (and so should the specs). 

I'm using it in the sense of Prud'hommeaux and Seaborne, Eds.: SPARQL Query Language for RDF. W3C Recommendation, 15 January 2008.

> I understood that people want to be able to use an IRI to 'label' a graph (to be the fourth field in a quad store) while at the same time considering that IRI to denote something else, such as a person. OK, but this means that the relationship between the IRI and the graph it labels cannot be that of naming.

Why do you say that it cannot be? Of course it can be.

An IRI can denote one thing – in the precise technical sense of “denote” as used in RDF Semantics (and, as of RDF 1.1 ED, in RDF Concepts).

The same IRI can be the graph name of another thing – in the precise technical sense of “graph name” as used in SPARQL 1.0, SPARQL 1.1, and RDF 1.1 Concepts.

Reasonable people might argue that this would be confusing or a bad idea, but saying that it is somehow impossible is plainly false.

> Under these conditions, with the RDF semantics unchanged, the fourth field in a quad store, or the IRI associated with a graph in a SPARQL RDF Dataset, is *not* the name of the graph, and these items are *not* named graphs. 

The IRI is not the *denotation* of the graph, in the technical sense of “denotation” used in RDF Semantics.

Something is a named graph if and only if it meets the definition of the term “named graph”. How else could it be?

> However, Concepts currently says they are named graphs.

Yes. And that makes anything that meets that definition a “named graph in the RDF Concepts sense”.

> OK, you can take the position: to hell with what 'named graph' used to mean, Concepts is currently *defining* it to be simply a <IRI, graph> pair; end of story.

That's what SPARQL said in 2008.

My position is: why change what works?

> Which is a coherent position, as long as you stop there. But in fact, everyone wants more: they want to be able to *use* the IRI to *refer to* the graph, inside some RDF triples. And this simple definition of named graph does not cut it for that use.

What does RDF Semantics have to do with that? RDF Semantics isn't a theory of reference.

People use the URI <http://dbpedia.org/resource/Malta> to refer to the mediterranean island nation in RDF triples. Even though RDF Semantics doesn't say anything about countries. You know, it actually works.

> You have to also specify that the IRI denotes the graph (a semantic condition.)

Where's the semantic condition that specifies that the IRI denotes the country?

> Just being a handy organizing tag isn't enough to get the intended meaning of this 'tag' into RDF. 

RDF Semantics isn't about meaning. It's about entailment relationships between RDF graphs. RDF graphs acquire meaning through social contracts and conventions.

Best,
Richard




> 
> I confess that I am now rather puzzled about what position the WG has actually taken. On the one hand, there seems to be a strong feeling that this kind of labeling flexibility must be permitted: on the other, there seems to be a resolution that the labeling IRI in a SPARQL store does indeed denote the graph. Taken together, these directly contradict the current RDF semantics, so *something* has to be changed. 
> 
> Pat
> 
>> 
>> Best,
>> Richard
>> 
>> 
>> 
>> 
>>> 
>>> Pat
>>> 
>>>> Knowing who can't live with this minimalist approach would be a form of progress IMO.
>>>> 
>>>> Best,
>>>> Richard
>>>> 
>>>> 
>>>> [1] http://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#section-multigraph
>>>> 
>>> 
>>> ------------------------------------------------------------
>>> IHMC                                     (850)434 8903 or (650)494 3973   
>>> 40 South Alcaniz St.           (850)202 4416   office
>>> Pensacola                            (850)202 4440   fax
>>> FL 32502                              (850)291 0667   mobile
>>> phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>> 
>> 
>> 
> 
> ------------------------------------------------------------
> IHMC                                     (850)434 8903 or (650)494 3973   
> 40 South Alcaniz St.           (850)202 4416   office
> Pensacola                            (850)202 4440   fax
> FL 32502                              (850)291 0667   mobile
> phayesAT-SIGNihmc.us       http://www.ihmc.us/users/phayes
> 
> 
> 
> 
> 
> 
Received on Wednesday, 14 December 2011 23:52:53 GMT

This archive was generated by hypermail 2.3.1 : Tuesday, 26 March 2013 16:25:46 GMT