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Re: rev and the costs of inverses/aliases in SPARQL

From: Richard Cyganiak <richard@cyganiak.de>
Date: Mon, 8 Mar 2010 17:10:56 +0000
Cc: Dan Connolly <connolly@w3.org>, public-sparql-dev <public-sparql-dev@w3.org>, public-rdfa <public-rdfa@w3.org>
Message-Id: <DA35FF0B-239A-480E-BA50-3921DD93C28A@cyganiak.de>
To: Ivan Herman <ivan@w3.org>
My €0.02: I find @rev handy sometimes because it allows me to be  
clever and reduce markup. But if @rev didn't exist, then I could cope  
by changing the structure of the markup to something a bit more  
verbose (but probably easier to understand -- it's easy to be too  
clever, as Bijan observed).

I think that there will be strong pressures on vocabularies to be RDFa- 
friendly, and that most vocabularies of the future will be designed so  
that typical markup scenarios can be solved without @rev.

I see @rev as a band-aid. Using vocabularies that haven't been  
optimised for RDFa is less painful if you have @rev. With optimised  
vocabularies, it will be rarely or never used.

(What do I mean by “optimised”? I don't mean vocabularies with lots of  
inverses. I mean vocabularies where each property has the direction  
that occurs naturally in page markup. I think that almost all  
relationships have such a “natural” direction if used in web pages.  
Admittedly that's conjecture.)


On 8 Mar 2010, at 16:41, Ivan Herman wrote:

> On 2010-3-8 16:00 , Dan Connolly wrote:
>> I just ran into this message again from an HTML 5 validator:
>> "The rev attribute on the a element is obsolete. Use the rel  
>> attribute
>> instead, with a term having the opposite meaning."
>> This seems to encourage the pattern of minting an inverse
>> for each property, a la:
>>  abridgement
>>  abridgementOf
>> -- http://vocab.org/frbr/core.html
> From an RDFa point of view, if I am an author, I consider using a
> specific vocabulary and I use the attributes as they are defined.
> Authors cannot be expected to mint additional predicates on the fly if
> those are not defined; this is way too much for many of them anyway.
>> Doesn't that just gum up the works when doing SPARQL queries? Which
>> do you query for, abridgement or abridgementOf? Or do you use
>> a UNION?
>> It's one thing to discover, post-hoc, that two properties are
>> inverses of each other, and to write down that relationship.
>> But to make up these inverse-aliases by choice seems like
>> a big waste, to me.
>> (see also http://esw.w3.org/topic/HasPropertyOf bit on inverses and
>> aliases)
>> How are SPARQL users dealing with this in practice?
>> Meanwhile, RDF/XML doesn't have syntax for inverting a relationship
>> (a la is/of in N3), and there's data that says rev="..." is
>> too confusing for HTML authors to use.
> I am not sure RDF/XML is relevant. RDF/XML gives you different ways of
> expressing triples, so one can encode anything freely, there is no  
> real
> constraint. In the case of RDFa there is an additional constraint that
> one wants to follow the HTML structure to include the presentation  
> content.
> It is of course possible, in RDFa to express everything with @rel only
> but, in some cases, the missing @rev makes it very convoluted.
>> "The short answer is unfortunately "NO". Use of "rev" SHOULD be
>> avoided."
>> -- http://microformats.org/wiki/rel-faq
>> "the only <link rev=""> link to appear is rev="made" (to point to the
>> author's page) — and the latter is not used that much more than the  
>> more
>> sensible rel="author". Also, ironically, just off the graph in  
>> position
>> 21 is rel="made", probably showing that the distinction between rel  
>> and
>> rev may be too subtle for many authors."
>> -- http://code.google.com/webstats/2005-12/linkrels.html
>> Would the RDFa authoring community miss a/@rev if it went away?
>> Does anyone have 1st-hand experience to share?
> I think Damian has just posted a good use case example.
> Another example is
> <img rev="foaf:depiction"
>     resource="http://www.ivan-herman.net/foaf#me"
>     src="http://www.ivan-herman.net/Images/me2003-small.png"/>
> That gives me
> <http://www.ivan-herman.net/foaf#me>
>   foaf:depiction <http://www.ivan-herman.net/Images/me2003- 
> small.png> .
> Without a @rev, I have to add a new hierarchy to do the same:
> <div about="http://www.ivan-herman.net/foaf#me" rel="foaf:depiction">
>   <img src="http://www.ivan-herman.net/Images/me2003-small.png"/>
> </div>
> Which unnecessarily complicates the structure.
> There is another issue. There is already deployed RDFa out there.  
> Quite
> a lot, actually. As a consequence, there is a strong requirement of
> backward compatibility in the RDFa WG charter. This also means that if
> the @rev is removed from the core HTML5 document, RDFa will have it
> alongside the RDFa specific attributes like @about or @resource...
> Ivan
> -- 
> Ivan Herman, W3C Semantic Web Activity Lead
> Home: http://www.w3.org/People/Ivan/
> mobile: +31-641044153
> PGP Key: http://www.ivan-herman.net/pgpkey.html
> FOAF   : http://www.ivan-herman.net/foaf.rdf
> vCard  : http://www.ivan-herman.net/HermanIvan.vcf
Received on Monday, 8 March 2010 17:11:31 UTC

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