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Re: [GRAPH] graph deadlock?

From: Ivan Herman <ivan@w3.org>
Date: Wed, 21 Dec 2011 09:53:56 +0100
Cc: Richard Cyganiak <richard@cyganiak.de>, W3C RDF WG <public-rdf-wg@w3.org>
Message-Id: <81E92B60-0FE5-4A40-A007-9BA011C2F326@w3.org>
To: Pat Hayes <phayes@ihmc.us>

On Dec 20, 2011, at 19:45 , Pat Hayes wrote:

> 
> On Dec 20, 2011, at 2:29 AM, Ivan Herman wrote:
> 
>> Pat,
>> 
>> On Dec 20, 2011, at 05:45 , Pat Hayes wrote:
>> 
>> [skip]
>> 
>>> 
>>> Now, consider the case where a URI  UUU is used as a graph label in a dataset, and also occurs in the RDF inside a graph in that same dataset, where it is interpreted as denoting, say, a human being or a mailbox. OK so far. Now, however, add the dataset some more RDF (perhaps in the default graph used to express some metadata, for example) in which that same URI is intended to be used to refer to the graph that it labels. There are *no* RDF interpretations in which a single URIref can denote two different things. So this dataset as a whole has no satisfying interpretations. So it is formally inconsistent. Moreover, the inconsistency arises directly, and obviously, from this usage in which a URI is used to "name" something other than what everyone agrees it is in fact interpreted to mean (as, vividly, in Ivan's example using an email address). And this is, surely, *obviously* at odds with the basic assumption of the entire Web, that URIs, when considered as names, identify *one* thing. 
>>> 
>> 
>> is 'labeling' and 'identifying' the same?
> 
> Well, maybe not. But I suspect that if we try to say this, nobody will take the slightest notice. They certainly sound like they ought to be very closely related, so closely that only philosophers could distinguish them, and then only when there is an R in the month.

:-)

> And by the way, SPARQL talks about these URIs *naming* the graph, which sounds even more like identifying. 

So we indeed have a naming (sic!) issue. Indeed, SPARQL uses the term 'naming' for what I referred to as datasets. That is mess that, unfortunately, we have to live with it:-(

For the sake of the discussion we may have to use different terms, and let us forget about SPARQL for a while. Although I do not have the Sandro's talent of finding nice terms, let us say that we speak about labelled graphs, i.e., datasets, and identified graphs. Let us not use the term named graphs for a while...

- Labelled graphs are the minimal level, ie, just using URI-s labeling graphs. We may have to add a restriction that, within a dataset, labels are unique, ie, two different graphs must have two different labels. No further assumptions are used. And, to refer to an earlier quote of yours up there, the labels used that way would take no part in any form of RDF interpretation whatsoever. 

- Identified graphs are labelled graphs where there _is_ a relation, through HTTP GET, between the label and the graph.

I fully understand your bad feeling about labelled graphs. I share it. But we have to accept that, out there, lots of applications are based on that notion, ie, they attach labels to graphs without any sort of further check and assumption. What I am saying is that documenting and making clear the limitation of those states have a value. And application may choose to stay on that level. We may (we probably should...) try to promote the notion of identified graphs as being more 'webby', hence we need to define them properly, but that is defining some sort of an ideal world...

> 
>> My non-semantics dataset view talks about labeling only. 'Indexing' may be another term. 
>> 
>> I come back to the quad store example. I do not believe that quad stores make any assumption, by default, to the behaviour of the URI-s in the 4th column, they are just 'there'. 
> 
> Fine. But when they also occur in the (say) 3rd position, do they or do they not then mean the same as they meant when they occur in the fourth position? (Or maybe: does what they mean in the 3rd position have any relationship at all to their role while being-there in the fourth position?)

If the quad store implements a labelled graph than the answer is no. More exactly, an application should have no assumption that they do.

> The answer seems to be, sometimes they do (of course) and sometimes they don't (of course), but nothing records which case is which.

Right. A quad store, or an application thereof, might declare that it uses labelled or identified graphs. Well.. probably should/must and not might.


> And I object to that situation, as it produces faux-RDF which is designed to be systematically ambiguous in meaning.

And I hear your objection. Today an application or a quad store has no means to say which way it goes. Hence the mess... That is the absolute minimal step that I would like to make to try to clarify things. That is all...

Ivan


> And we can blather about "contexts" for ever, but until this notion is made reasonably precise, all such talk is indeed just blather. I have been to several workshops on 'contexts' where *every single speaker* had a different notion of what the word 'context' meant. 
> 
> Pat
> 
>> 
>> Ivan
>> 
>> 
>> 
>> 
>>>> 
>>>> When you say “this is illegal in RDF” then I think this has to be read as “I don't like it”.
>>> 
>>> No, I mean it violates the semantic assumptions of the (normative) RDF model. I might suggest that when you say "It isnt illegal", this has to be read as "I havnt understood the semantics spec."
>>> 
>>>>> We could simply declare that RDF has no semantics, and is simply to be used by programmers to mess around with in ways they find handy. Really, this might be the best way to move forward. But until we do this, we have to take the semantics seriously. 
>>>> 
>>>> Or we could just not bother giving any formal semantics to any new parts that are added to RDF. Several parts of RDF don't have a formal semantics and work pretty much fine anyways, e.g., RDF lists.
>>> 
>>> They have no semantics because they dont need any semantics. If a list had a semantics, it would describe the list. Those are basically LISP S-expressions coded into RDF triples: they *exhibit* the required structure rather than *describe* it. 
>>> 
>>> But I agree, we could indeed not give semantics to new parts. The sticking point, however, is that this particular 'new' part is already using the 2004 RDF semantics, but it is using it incorrectly. 
>>> 
>>> Pat
>>> 
>>>> 
>>>> Best,
>>>> Richard
>>>> 
>>> 
>>> ------------------------------------------------------------
>>> 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
>>> 
>>> 
>>> 
>>> 
>>> 
>>> 
>> 
>> 
>> ----
>> Ivan Herman, W3C Semantic Web Activity Lead
>> Home: http://www.w3.org/People/Ivan/
>> mobile: +31-641044153
>> FOAF: http://www.ivan-herman.net/foaf.rdf
>> 
>> 
>> 
>> 
>> 
>> 
>> 
> 
> ------------------------------------------------------------
> 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
> 
> 
> 
> 
> 


----
Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
mobile: +31-641044153
FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Wednesday, 21 December 2011 08:53:56 GMT

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