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

From: Mark Birbeck <mark.birbeck@webbackplane.com>
Date: Tue, 1 Dec 2009 10:31:53 +0000
Message-ID: <640dd5060912010231w762e2521u59a58c29c37f5af1@mail.gmail.com>
To: Ivan Herman <ivan@w3.org>
Cc: Christoph LANGE <ch.lange@jacobs-university.de>, RDFa Developers <public-rdf-in-xhtml-tf@w3.org>
Hi Ivan,

There has been discussion about @id being 'special' over the years,
but it's been quite piecemeal. So it's probably not your memory that
means you don't recall this. :)

However, when the proposition has come up, I believe a few
'principles' have emerged about why we should avoid giving @id special
status, and it might help the discussion if I explain some of them.

The main 'principle' that has been repeated a few times over the
years, is that it's important to be able to talk separately about the
data in the document, and the document itself.

The 'data in the document' might be an address or vCard.

The 'document itself' might be not only the document, but a footer,
sidebar, etc.

That's no different to the usual 'information resource' issue that
crops up a lot in RDF, where we want to be able to talk about the
web-page itself (when was it published, who by, etc.), and the the
information it represents (the item for sale, the person, etc.).

Note that RDF doesn't require there to be anything at the end of a
URI, which means that we don't actually need to use the pattern that
many people are using in their FOAF files:

  <div id="me" about="#me">

Instead, the following is sufficient:

  <div about="#me">

Anyway, if we agree that there are important uses for being able to
talk about the document itself, distinct from its content, then we can
also see that making @id automatically provide a subject would
completely mess up the normal RDFa parsing -- one moment you are
talking about a person's FOAF file, and then suddenly your statements
are about the elements in the document.

(Note that RDFa provides an RDF 'interpretation' of existing semantic
content, such as @rel and @href. But with @id, it's difficult to see
the semantic content that is added, that could then be 'interpreted'
as RDF.)

But Christoph seems to be saying that his language needs to make lots
of statements about the document itself, resulting in a common pattern

  <x id="y" about="#y">

I don't have any immediate suggestions about how to work around this,
although I'm sure we could come up with something. But I would caution
against using @id in a language, to automatically generate a subject,
since I think it will cause problems for that language in the longer



Mark Birbeck, webBackplane



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On Tue, Dec 1, 2009 at 8:35 AM, Ivan Herman <ivan@w3.org> wrote:
> 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!
> Thanks
> Ivan
> 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/
> mobile: +31-641044153
> PGP Key: http://www.ivan-herman.net/pgpkey.html
> FOAF: http://www.ivan-herman.net/foaf.rdf
Received on Tuesday, 1 December 2009 10:32:38 UTC

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