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Re: Closure? (was Re: Request for two new media types submitted)

From: Eric Prud'hommeaux <eric@w3.org>
Date: Wed, 4 Mar 2009 15:03:10 -0500
To: Phil Archer <phil@philarcher.org>
Cc: Stasinos Konstantopoulos <konstant@iit.demokritos.gr>, Ivan Herman <ivan@w3.org>, Public POWDER <public-powderwg@w3.org>, Ralph Swick <swick@w3.org>
Message-ID: <20090304200309.GD28462@w3.org>
* Phil Archer <phil@philarcher.org> [2009-03-04 16:42+0000]
> Eric, Ivan, all,
> I've been looking at the semantics issue again and have discussed it
> further with Stasinos. This is my best shot at finding a resolution - by
> arguing that what we have now is correct.
> My understanding is that the debate is between:
> 1. Applying the extension at the RDF layer
> 2. Applying the extension at the application layer.

Debate is between:
  1. RDF semantic extension (ericP)
  2. RDF core extension (Stasinos)

This came up when I sought to clarify
 We extend RDF with the datatype properties ...
]] — http://www.w3.org/TR/2008/WD-powder-formal-20081114/#SE
and found that Stasinos considered it to not be a semantic extension.

> In POWDER's case the application is a reasoner and/or query engine:  
> POWDER documents assign metadata to sets of resources circumscribed by  
> IRI patterns. Semantically said, POWDER documents assert IRI  
> pattern-defined classes as being owl:subClassOf classes defined by  
> owl:RestrictionS.
> As currently documented and implemented, the WG took the advice of
> Jeremy Carroll and, lead by Stasinos in this area, followed the first
> option. This does not mean that we have extended RDF core outside the
> context of POWDER. RDF Semantics remain unchanged (so we're within our
> charter which states: "This working group is not chartered to make
> extensions to RDF core, neither is it chartered to research the broader
> development of the Semantic Web." [1]. Furthermore, we state at the very
> end of section 4.3 of the formal semantics doc:
> "Software can distinguish those RDF graphs to which the extended
> semantics apply by testing for the appearance of either the
> wdrs:matchesregex or the wdrs:notmatchesregex resource as the object of
> a triple. For instance, in Example 4-4 the following class description
> suffices to recognize a document that uses the semantic extension:" [2]
> Therefore we provide a clear and simple means for syntactically
> recognizing RDF graphs that need the POWDER extension to be fully
> understood.
> Furthermore, the POWDER extension monotonically adds meaning to RDF
> semantics, as no RDF vocabulary is affected.
> As a test of this, Stasinos created the SemPP engine using the
> TransOnto library [3]. This uses Jena and the Pellet Reasoner to process  
> POWDER-S documents. The key implementation being that Jena's API through  
> which a resource is added to the graph was overridden so that the  
> matchesregex triples appear in the graph. As a result, the DL reasoner  
> is unaffected, SPARQL queries are unaffected and, of course, other RDF  
> data is unaffected.
> Now, AIUI Eric's contention is that this is the wrong approach. A better
> way is to work at the application (OWL inference) layer. In this
> scenario, existing DL reasoners would not be useful, we'd need more  
> specific software. We are not averse to specific software (we have  
> defined and built two interoperable POWDER Processors that return RDF  
> descriptions of input URIs) but we are also informed by experience.

I'm not arguing that that the implementation is the wrong approach,
but that the specification is actually describing a pretty ordinary
semantic extension. In support of that:
  1. [FPS] uses the language of semantic extesions [RS].
  2. wdrs:matchesregex stated to be an owl:DatatypeProperty (see [OR]).
  3. Defining wdrs:matchesregex as an extension to the RDF model would
     require duplicating the RDF Semantics, adding all possible
     interactions with wdrs:matchesregex .

[PFS] http://www.w3.org/TR/2008/WD-powder-formal-20081114/#SE
[RS] http://www.w3.org/TR/rdf-mt/#ExtensionalDomRang
[OR] http://www.w3.org/TR/2004/REC-owl-ref-20040210/#Datatype

> In 2004 - that long ago - many of the folk involved with POWDER now came
> up with a thing called RDF Content Labels [4]. It looks very much like
> POWDER, in that it has attribution, a means of putting labels in order
> and so on - all done with what looks superficially like RDF. Dan Bri
> often told me that RDF-CL is OK as long as what consumes it knows what
> to do with it. A general RDF tool kit certainly wouldn't make any sense
> of it. RDF-CL is a vocabulary defined to do a particular job, but is not
> a good citizen of the semantic web.
> Therefore, I would argue that we have in effect tried something very
> much like what Eric is suggesting. Indeed, that was what we were working
> towards right up until TPAC 2007 when I was going round asking anyone I
> could grab hold of how we solved the semantics issue. Take a look at the
> editor's note just above [5] where the question is laid out. This was
> the version of our main Description Resources doc we took to that TPAC
> meeting, fully expecting to be at CR by Christmas that year. Oh if only.
> Two people I asked went for the first option, two others for the second
> (from memory those 4 were you, Eric, Dan Bri, Fabien Gandon and Max  
> Froumentin).
> It was Tim who, given a choice of A or B, said "C, none of these"  and
> told us we should be using OWL classes, JJC who showed us how (with the
> semantic extension) and Stas who's proved that it works with minimal
> code.

Did JJC write http://www.w3.org/TR/2008/WD-powder-formal-20081114/#SE ?
I note that it does have the suspicious fragment identifier "SE".

>       It feels to me as if one reading of Eric's proposal would be to
> revisit a version of that original discussion. You'll understand my
> reluctance to do so.
> The reason it has taken us so long to get from there to where we are now
> is precisely because we've been trying to fit the square peg of matching
> URIs against patterns into the round hole of RDF with a minimum of  
> geometric distortion. I genuinely believe we have achieved that in the  
> current documentation and implementation.
> A new OWL datatype property is easier to document but it pushes POWDER
> into a silo where all software is specialist. The aim has always been to
> devise a means whereby a lot of triples that describe Web resources can
> be generated easily and processed as far as possible by existing
> software - hence the use of a barely-adapted Jena and wholly unchanged
> Pellet in SemPP.
> I hope I've understood both sides of the argument correctly?
> Taking all this into account, I am strongly inclined to leave the
> document as is when seeking the transition to PR.
> Cheers
> Phil.
> [1] http://www.w3.org/2007/02/powder_charter
> [2] Just above
> http://www.w3.org/2007/powder/Group/powder-formal/20090205.html#emptyIRIsets
> [3] http://transonto.sourceforge.net/
> [4] http://www.w3.org/2004/12/q/doc/content-labels-schema.htm
> [5] http://www.w3.org/2007/powder/Group/powder-dr/20071102.html#basicQueries
> -- 
> Phil Archer
> http://philarcher.org/
> i-sieve technologies                |      W3C Mobile Web Initiative
> Making Sense of the Buzz            |      www.w3.org/Mobile
> Eric Prud'hommeaux wrote:
>> * Stasinos Konstantopoulos <konstant@iit.demokritos.gr> [2008-12-23 10:00+0200]
>>> On Mon Dec 22 21:26:34 2008 Eric Prud'hommeaux said:
>>>> * Stasinos Konstantopoulos <konstant@iit.demokritos.gr> [2008-12-21 07:14+0200]
>>>>> On Dec 20, 2008, at 6:38 PM, Eric Prud'hommeaux wrote:
>>>>>> I'm not sure which of the following you are arguing:
>>>>>>  1 "Extends RDF" could not be interpreted as "extends the RDF 
>>>>>> data  model"
>>>>>>  2 my proposed clarification is incorrect
>>>>>>  3 my proposed clarification is not an improvement
>>>>> #2
>>>> OK, here is the proposed wording:
>>>> "POWDER-S uses an <a href=
>>>> "http://www.w3.org/TR/2004/REC-owl-semantics-20040210/syntax.html#owl_DatatypeProperty_syntax"
>>>>> OWL DatatypeProperty</a> to relate a resource to a regular expression
>>>> which that resource matches. While POWDER-S uses OWL classes to group
>>>> resources, any engine determining if a resource belonged in one of
>>>> these OWL classes would need to be able to test a resource against a
>>>> regular expression."
>>>> What are you arguing is incorrect?
>>> this bit here:
>>>> "any engine determining if a resource belonged in one of these OWL
>>>> classes would need to be able to test a resource against a regular
>>>> expression."
>>> Although that is one possible way to go about implementing a POWDER-S
>>> processor, it is not the only one, so it is not that case that "any
>>> engine ... would need".
>>> One counter-example to the universal quantification in your wording
>>> is the SemPP processor (http://transonto.sourceforge.net/).
>>> In this approach the "engine determining if a resource belonged in one
>>> of these OWL classes" (which in SemPP's case is vanilla Pellet) knows
>>> nothing about regexps; the regexp matching is done at the RDF
>>> layer where nothing is known about OWL classes or any other OWL
>>> vocabulary.
>> I understand your point that the implementation seems like it is doing
>> matching resources against regex patterns at the core level. I argue
>> that the programmatic boundries don't coincide with the logical
>> boundries, and that the behavoir is best described as a semantic
>> extension. To wit, the POWDER Formal Semantics [PFS] asserts that
>> [[
>> <x, reg> is in IEXT(I(wdrs:matchesregex)) if and only if:
>>     * reg conforms with regular expression syntax, AND ...
>> ]]
>> all of which is in the language set aside in RDF Semantics [RS] for
>> semantic extensions (in fact, everything in the semext class in the
>> document appears to be just that, a semantic extension). It's not that
>> you *couldn't* define it as an extension to RDF core, it's just that
>> it would be painful, and the behavoir of two such extensions would not
>> be defined.
>>>>> Machinery further up the application stack (RDFS and OWL reasoners)
>>>>> can remain happily ignorant about what's happening underneath.
>>>> Ahh, I believe it is customary to treat DatatypeProperies like
>>>> wdrs:matchesregex or my:isEvenInteger as extensions to the inference
>>>> layer. 
>>> Design choices are best made per application depending on each
>>> application's specific needs. POWDER extends RDF and not RDFS or OWL.
>> To some degree, though RDF has some text which favors one path over
>> another.
>>>>>> Equivalent, sure, but it's distracting for the reader because the
>>>>>> they start looking for an intersection where there is none, and.
>>>>> As already noted, there are good technical reasons for this   
>>>>> inconvenience.
>>>> The main reason I see for this is that the xml representation
>>>> expresses intersections, but not other logical constructs such as
>>>> unions or complements. I expect this represents the far majority of
>>>> use cases, as regular expressions can already express both unions
>>>> and if you feel like compiling them into a regex, complements.
>>>> Thus, all patterns can be reduced to a pure conjunction, so there's
>>>> less pressure for working group to include a step for simplification.
>>> That's yet another interesting alternative for implementing POWDER-S.
>>> But I don't think you would prefer to have to wade through
>>> the single-regexp representation of a POWDER/XML <ol> element--even
>>> with just two branches--in the document.
>> Were you to be earlier in your process, I'd argue for a
>> post-processing XSLT with a single rule for
>> owl:class/owl:intersectionOf/owl:Restriction[count(/*) == 1]
>> to change
>> [[
>> <owl:Class>
>>   <owl:intersectionOf rdf:parseType="Collection">
>>     <owl:Restriction>
>>       <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
>>       <owl:hasValue  rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">(porn\.example)\/?</owl:hasValue>
>>     </owl:Restriction>
>>   </owl:intersectionOf>
>> </owl:Class>
>> ]]
>> into
>> [[
>> <owl:Restriction>
>>   <owl:onProperty rdf:resource="http://www.w3.org/2007/05/powder-s#matchesregex" />
>>   <owl:hasValue  rdf:datatype="http://www.w3.org/2001/XMLSchema-datatypes#string">(porn\.example)\/?</owl:hasValue>
>> </owl:Class>
>> ]]
>> but I'm content that the cost of a change like this would exceed the
>> benefits.
>>>> I was just thinking that these details could go into an issues list.
>>>> As editor, I found the value of the issues list was not just to
>>>> document outstanding issues, but to serve as a bit of a FAQ. It's
>>>> possible that others besides me will be struck by the complexity
>>>> and search for the design decision.
>>> Please see the relevant paragraph in Section 3.1 [1], right after the
>>> first occurrence of a singleton intersection, and make an editorial
>>> comment if you feel the explanation is not sufficient.
>> I wasn't arguing for the spec's sake, just for ease of tracking
>> important controversial points.
>>>>> [1] http://www.w3.org/TR/2008/WD-powder-formal-20081114/#multiDRsemantics
>> [PFS] http://www.w3.org/TR/2008/WD-powder-formal-20081114/#SE
>> [RS] http://www.w3.org/TR/rdf-mt/#ExtensionalDomRang


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Received on Wednesday, 4 March 2009 20:03:21 UTC

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