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Re: Default value for @about depending on language [Re: Non-XHTML host languages for RDFa]

From: Ivan Herman <ivan@w3.org>
Date: Tue, 01 Dec 2009 09:35:39 +0100
Message-ID: <4B14D55B.8040900@w3.org>
To: Christoph LANGE <ch.lange@jacobs-university.de>
CC: RDFa Developers <public-rdf-in-xhtml-tf@w3.org>
Hi Christoph,

I am trying to remember history but my memory is fading with age:-( But
I seem to remember that we did have the similar mechanism on the table
for XHTML using simply @id instead of @xml:id (SVG came fairly late into
the picture when most, if not all, of RDFa was already defined).

AFAIR the issue was the fear of generating unwanted triples (eg, by
breaking the assignment of subject if there is another @about somewhere
up in the tree). Indeed, @xml:id (or @id) could be used for many
different purposes, like internal linking or, in the case of HTML, as a
natural anchor for CSS styling, and the RDFa mechanism may get in the
way. But, I believe, for a general XML language you may have similar
issues. Eg, what if one wants to use such an XML file as an input to
some sort of XSLT processing in future? Just as for CSS, using @xml:id
is a fairly natural anchor point XSLT patterns...

I also have another, slightly more general issue. As you have documented
yourself, there are already some unfortunate differences between
RDFa+XHTML and RDFa+XML. Putting my implementer's hat on, I would love
to make some of those (like the usage or not of xml:base) disappear and
reduce the differences to the strict minimum (essentially to the
existence of head and body and even those differences are questionable
to me). In this sense, I am a bit reluctant to differentiate between
semantic and presentational languages for RDFa...

Anyway. We will have to discuss that, that is for sure!



Christoph LANGE wrote:
> Dear all,
>   one more thought on RDFa in different languages.  Depending on the host
> language, it might make sense to allow an RDFa host language to influence the
> choice of a subject.  I see a difference between semantic and presentational
> languages.  In presentation-oriented languages like XHTML and SVG, the role of
> RDFa is allowing for semantic annotations.  One could argue that
> semantic-oriented languages don't need RDFa, but for our semantic markup
> language OMDoc we actually found RDFa very useful, as it enabled us to reuse a
> lot of existing RDF-based metadata vocabularies without inventing further
> idiosyncratic markup.
> The rule how XHTML+RDFa establishes a new subject, now only considering @about
> for simplicity, is that either @about exists on an element, then it defines
> the new subject, or the parent subject will be reused.
> In OMDoc, however, the situation is different.  There, RDFa metadata are
> attached to elements, and the metadata are always metadata of these elements,
> and it it recommended to give elements an @xml:id.  Therefore, we have in most
> cases the situation
> <element xml:id="i" about="#i">
>   <meta property="onto:foo" content="bar"/>
>   ...
> </element>
> i.e. a redundant @about attribute that one has to give, as otherwise the RDFa
> (according to the XHTML+RDFa parsing rules) would not be parsed correctly.
> Comparing that to XHTML+RDFa, where it does not occur that frequently, I could
> imagine that the following difference explains this; correct me if I'm wrong:
> XHTML is a largely presentation-oriented language, and RDFa in XHTML is used
> to present in a human-readable way knowledge whose original location, if any,
> is some formalization (imagine the case of using XHTML to render ontologies),
> but not the XHTML document.  In host languages that are semantic in itself, I
> suppose that, in contrast to XHTML, the things to be annotated are resources
> whose original location is the respective XML document.  In the latter case I
> think it would make sense to introduce the above-mentioned default mechanism
> for the @about attribute.
> Note that so far I'm only arguing why this would be good to have in certain
> languages.  I have not yet considered all side-effects this has on RDFa's
> other ways of establishing a subject.
> But what do you think about this in general?
> Cheers, and thanks for your feedback,
> Christoph


Ivan Herman, W3C Semantic Web Activity Lead
Home: http://www.w3.org/People/Ivan/
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Received on Tuesday, 1 December 2009 08:36:17 UTC

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