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Re: Requiring semantic extensions to be AZ-proper?

From: Antoine Zimmermann <antoine.zimmermann@emse.fr>
Date: Wed, 12 Sep 2012 22:52:36 +0200
Message-ID: <5050F614.4030104@emse.fr>
To: Richard Cyganiak <richard@cyganiak.de>
CC: public-rdf-wg@w3.org
Le 12/09/2012 22:04, Richard Cyganiak a écrit :
> Antoine,
>
> On 12 Sep 2012, at 18:47, Antoine Zimmermann wrote:
>> I don't know if the phrase "proper semantic extension" is standard,
>> but I guess it maps to something defined by logicians.
>>
>> What I consider a "proper" semantic extension X of an entailment
>> regime Y has to satisfy the following property:
>>
>> "For any dataset D1 and D2, if D1 X-entails D2, then D1 Y-entails
>> D2."
>
> Okay, so let's define some working terminology and call that
> “AZ-proper”.
>
> <snip detailed explanation of “AZ-proper” semantic extensions>
>
> I get this now. Thanks, it was very helpful.
>
>>> I can't quite figure out what these conditions are just from
>>> reading RDF Semantics. The “general monotonicity lemma” [2] seems
>>> relevant, but is phrased as a result of something being a
>>> semantic extension, rather than as a condition on semantic
>>> extensions.
>>
>> Well, the lemma, which I was not aware of, seems to say that the
>> semantic extensions defined by RDF semantics are satisfying the
>> property that I mentioned (modulo the use of RDF graphs instead of
>> datasets).
>
> Okay, right.
>
>> But it does not define in general a condition on semantic
>> extensions.
>>
>> I don't think RDF Semantics 1.0 specifies what's a "proper"
>> semantic extension and does not require that any future extension
>> must be monotonic.
>
> Well, read the second-to-last paragraph here:
> http://www.w3.org/TR/rdf-mt/#intro

Interesting, I certainly overlooked and even skip parts of the text of 
the introduction. This should be made more apparent in the new spec.


> And this: http://www.w3.org/TR/rdf-mt/#vocabulary_entail

Well, this talk about adding conditions on extra vocabularies but does 
not mandate anything particular about them.


> And this: http://www.w3.org/TR/rdf-mt/#MonSemExt

This indeed confirms what is said in introduction and seems to impose 
that /any/ extension monotonically extends its base entailment regime. I 
consider it's good news and that this should be followed in dataset 
semantics too.


> I've read all that stuff multiple times, and am still muddled on what
> it tries to do. But my impression is that it attempts to define a
> clean extension mechanism for the semantics, where new semantic
> extensions can be defined and associated with new vocabulary terms,
> and with scope beyond those entailment regimes defined in the spec
> itself. So, there could have been FOAF-entailment and SKOS-entailment
> and so on (perhaps defined simply as: In FOAF-entailment, the triples
> in http://xmlns.com/foaf/spec/index.rdf are considered axiomatic).
> Maybe I'm reading too much into this?

I read the same thing. These are sections that are easily overlooked by 
people who implement the semantics of the normative languages (RDF, 
RDFS, D-entailment, OWL) since you don't need to know that to produce a 
conformant reasoner. But to us who are defining a new semantic 
extension, it obviously matters. I was one of those who overlooked this 
before. But I think it's consistent with what I intuitively assumed in 
AZ-proper, right?


>> And even though it did say so about extensions of RDF semantics, it
>> would not apply to the semantic of dataset, which does not deal
>> with a set of triples alone.
>
> But the distinction between “normal” interpretations and “dataset”
> interpretations could easily be removed, so that every interpretation
> can interpret RDF terms, RDF triples, RDF graphs and RDF datasets,
> no?

Certainly. But I'd prefer that we have a description of the semantics of 
RDF graph that does not consider multiple graphs in the first place, and 
the dataset semantics builds on top of it for the systems who need to 
manage several graphs simultaneously (yet I'm not firm on this for the 
moment).


> This seems very tempting to me because it would make everything
> nicely uniform. (I said earlier in the thread that this is probably
> just an editorial issue.)

It seems to me that the current dataset proposal makes the whole thing 
much more uniform already. Sure we could go one step further.


>>> By the way, do we agree that RDF Semantics should be very clear
>>> and specific about what it means to define semantic extensions to
>>> the various entailment regimes? I think making that normatively
>>> crystal clear is part of why that spec exists. And in fact, one
>>> way how one can conform to RDF Semantics is by defining a
>>> “conforming semantic extension of XYZ-entailment”, IMO.
>>
>> Why not, but first we have to agree that we want to prescribe
>> extensions that do not extend monotonically (I want to avoid the
>> phrase "non-monotonic extension" which can have a different
>> meaning). Perhaps there are some cases where it makes sense to do
>> so (e.g., define an extension for working on closed databases).
>
> Well, we can't stop anyone from doing that, it would just not be an
> extension that conforms to the requirements of RDF Semantics.

Given the passages that you cite before, it seems indeed that such 
extensions would not be conformant.


>> Personnaly, I'm in favour of restricting all normative extensions
>> to be monotonical extension.
>
> I've heard that it's possible to cleanly combine multiple such
> extensions, especially if the new entailments are “triggered” by the
> use of vocabulary terms that are specific to the extension. That
> sounds like a pretty good argument in favour of “AZ-properness” to
> me.
>
> I think Pat also said that he'd like to shuffle things around in the
> RDF Semantics document to make the existing entailment regimes work
> that way, that is, RDFS-entailment would be “triggered” by the use of
> a term in the RDFS namespace. That kind of makes sense to me.

Yes, and IIRC, Pat said that we could rewrite the semantics in such a 
way that each vocabulary defines its own constraints that reasoners 
could pick individually to build their own "entailment regime". That 
would be nice, but there are some issues. For instance, the extra 
semantic conditions of RDFS also apply to the RDF vocabulary. So 
implicitly, RDFS requires that the RDF conditions are also in place. But 
apart from this, yes the actual regime in place in a reasoner could be 
made modular, so to speak.


>
> Best, Richard
>

Cheers,
-- 
Antoine Zimmermann
ISCOD / LSTI - Institut Henri Fayol
École Nationale Supérieure des Mines de Saint-Étienne
158 cours Fauriel
42023 Saint-Étienne Cedex 2
France
Tél:+33(0)4 77 42 83 36
Fax:+33(0)4 77 42 66 66
http://zimmer.aprilfoolsreview.com/
Received on Wednesday, 12 September 2012 20:52:44 GMT

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