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

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

-- 
Christoph Lange, Jacobs Univ. Bremen, http://kwarc.info/clange, Skype duke4701