Graph labels vs. graph names. (was: Re: complete graphs)

On Oct 4, 2011, at 11:08 AM, Richard Cyganiak wrote:

> On 3 Oct 2011, at 03:59, Pat Hayes wrote:
>>> For example, :G1 below is a name for the graph containing triples that state that {:s1 :p1 :o1.} was published yesterday at <http://example.com/>.
>>> 
>>> :G1 {
>>>   [] a :Publishing;
>>>      :date "2011-09-30"^^xsd:date;
>>>      :webAddress <http://example.com/>;
>>>      :triples :G2.
>>> }
>>> :G2 {
>>>   :s1 :p1 :o1.
>>> }
>> 
>> If G2 is only a "label" (or only a pseudo-name, ie not really denoting) then this does not work, because the use of G2 in the triple in G1 has to follow the RDF semantic rules, and it might not refer to the graph in that case.
> 
> Let's look at two URIs in that graph: G2 and <http://example.com/>.
> 
> As far as I can tell, RDF Semantics tells us nothing about the referents of either of these URIs, except that they're in a certain relationship to the blank node in the graph.

Indeed, which is why if we want to ensure that they do relate to something in particular (such as a graph label to the graph it labels), we should add this as a semantic condition.

> I take it you accept that <http://example.com/> denotes a particular web resource that exists out there in the world, external to our graph G1?

Assuming http-range-14, yes.

> 
> Personally I find it easy to accept that :G2 denotes a particular named graph the exists in the same dataset, external to our graph G1.

Oh, I agree that is entirely reasonable. However, nothing in the specs actually state or require this, so it is technically invalid and certainly not something I would want to rely upon when writing software. Any reasoner which relied upon this would be incorrect.
> 
> I certainly don't see anything in RDF Semantics that contradicts this.

There is nothing that contradicts it, but there is also nothing that sanctions it as being correct. And since we have explicitly decided (as I believe we have) that the fourth IRI in a quad is not a 'name' for a graph in the sense that it is required to denote that graph, I take it that the use of G2: is a label in the second expression does *not* establish a naming or referential relationship between the URI and the graph. (If this is wrong, then of course I stand corrected and withdraw my point.)  Since RDF semantics does require that when that same URI is used in an RDF triple, it must be related to what it names by the standard denotation relationship, it follows that there can be no semantic requirement that the use of this URI inside the triple means the same thing as its use as a graph label. They might as well be two different URIs: in fact, your example with the first occurrence of  :G2 replaced by :G3 would be *exactly* identical in meaning, according the semantic model we have at present. As I am sure you will agree, there is nothing in the RDF semantics which prevents :G2 and :G3 from referring to the same entity. Of course, there is nothing to ensure that they do, either: it is, in fact, a complete crap-shoot; just as it is when you use the same name in contexts which are explicitly, normatively, stated to allow different meanings for occurrences of one and the same name. 

Pat



> 
> Best,
> Richard

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Received on Wednesday, 5 October 2011 06:35:25 UTC