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Re: ISSUE-30: How does SPARQL's notion of RDF dataset relate our notion of multiple graphs?

From: Steve Harris <steve.harris@garlik.com>
Date: Tue, 19 Apr 2011 09:12:34 +0100
Cc: Pat Hayes <phayes@ihmc.us>, Ivan Herman <ivan@w3.org>, RDF Working Group WG <public-rdf-wg@w3.org>
Message-Id: <3A7FFC97-D099-4BF1-9837-CBA3F1E3ECEF@garlik.com>
To: Richard Cyganiak <richard@cyganiak.de>
On 2011-04-18, at 23:06, Richard Cyganiak wrote:

> Hi Pat,
> 
> I'll push back. I understand the value of the RDF Semantics document in that it defines the valid entailments of an RDF graph. I still do not understand what role it has beyond that, and how it is relevant to operations that do not involve entailment.
> 
> (Just to keep the underlying question in mind: We disagree on the question whether the RDF Semantics document needs to mention aspects of RDF that do not involve entailment. I think it doesn't have to, and shouldn't. You disagree.)
> 
> On 17 Apr 2011, at 00:10, Pat Hayes wrote:
>>> My understanding is that the RDF Model Theory exists to define which inferences are valid, given an RDF graph. What other purpose does it serve?
>> 
>> It defines what RDF means.
> 
> No, it doesn't, and I think this is not a very honest statement. If the MT defined what the RDF language means, then it would have to define what an utterance in that language means. The meaning of RDF is in all the weird and wonderful technological, social and economic processes and conventions that establish, more or less reliably, the referents of URIs. Those processes are not described in the Model Theory (or, for the most part, in any other W3C Recommendation), to the contrary, the MT explicitly punts on most of them. The MT provides some icing on top of that magnificent mess.
> 
>> Or to be achingly precise, it puts constraints upon what RDF can possibly mean.
> 
> I guess that's a fair characterization.
> 
>> To give just two examples, it implies that the truth of an RDF triple cannot depend upon the form of a URI (other than by this form changing what the URI denotes) and it specifies that any URI must be interpreted as referring to the same entity every place it occurs.
> 
> The first of those I don't understand. The second I don't think is quite true, as the spec is only concerned with *single RDF graphs*, and AFAIK specifies nothing regarding the interpretation of URIs in different graphs.
> 
>> These constraints on meaning apply to any RDF processing, not just to entailment checking. SPARQL for example satisfies semantic conditions which are related to the RDF semantics. 
> 
> Can you give me an example of such a semantic condition satisfied by SPARQL that is not covered by entailment?
> 
>> And I insist that this - the semantics of the triples - is not something that can be ignored while conforming to the RDF specs.
> 
> You say that to conform to the RDF specs, one must not ignore the semantics of the triple. What does this mean, ignoring the semantics of the triple? How can I tell wether I'm ignoring the semantics of a triple or not?
> 
> Does the Model Theory gives rise to any test cases or conformance criteria?
> 
> This is a honest question.
> 
>> Of course, the specs can be ignored, and no doubt often are. But our job is to write the specs., so we are rather obliged to take them seriously.
> 
> Rest assured that I am taking the specs seriously. But I am also doing my best to take the users of RDF seriously. It is my belief that if they often ignore the specs, then we should be open to the possibility that something is wrong with the specs, and maybe it can be fixed.
> 
>> Well, if that is your view, then by all means let us as a WG declare that RDF has no normative model theory, and is simply a meaningless notation.
> 
> XML and JSON and CSV and the relational model don't have normative model theories. Does that make them “meaningless notation”?

Arguably Relational Algebra is the model theory of the Relational Model. Not SQL however, which doesn't strictly follow either.

>> I will be happy to go along with this, which might surprise you. But we should not give our notation a normatively defined semantics and ALSO say that this semantics should be ignored in practice.
> 
> I pointed out that it *is* often being ignored in practice. I am not saying that it *should* be ignored in practice.

It's not necessarily a bad thing for the model to be ignored by users. That happens in SQL too, but it's not a disaster.

> I am, however, saying that we should have agreement about the role that the Model Theory in the Semantic Web project. I do understand that the MT allows us to derive the entailment rules. I do understand that it allows us to do semantic extensions in a principled and formally correct way. I do understand that it serves as the underpinning of OWL (whether they like it or not). And from this understanding of the role of MT I do not see how it follows that not mentioning RDF datasets in the MT would be a problem.
> 
>> I suggest, in all seriousness, that you put this forward as a WG issue: propose that RDF be declared to have no normative semantics at all. At the very least, the resulting debate might get some issues out into the open air. 
> 
> Look, Pat. I like having normative entailment rules written down. I like having normative axiomatic triples. I like having normative text about the treatment of blank nodes. I like having normative text that explains how datatypes work. All of these things give rise to conformance criteria and can be written down in test cases and lead to observable behaviour in software implementations and validatable criteria in published data. But I don't believe that writing down all these nice things in a certain mathematical notation imbues magical properties on RDF and constitutes the difference between RDF being “meaningful” and “meaningless”. As I see it, RDF is a data model like any other, but it comes with quite a sweet set of inference machinery that happens to be written down in a manner that is rather quirky and impenetrable (for the intended target audience, which should be implementers of RDF-based systems and authors of RDF data).

I honestly don't believe that authors of RDF data should be reading, or have to read, any of the formal specs. I wouldn't stop anyone, and it's how I learnt RDF (there wasn't much else), but I just don't believe that it's the most efficient way to learn this stuff.

- Steve

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Received on Tuesday, 19 April 2011 08:13:05 GMT

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