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PROPOSAL: Using @resource to define objects that are resources

From: Mark Birbeck <mark.birbeck@x-port.net>
Date: Wed, 23 May 2007 20:19:11 +0100
Message-ID: <640dd5060705231219s3601c291r16b0bb0d9155876a@mail.gmail.com>
To: RDFa <public-rdf-in-xhtml-tf@w3.org>

This is a proposal for the requirement at:

  <http://lists.w3.org/Archives/Public/public-rdf-in-xhtml-tf/2007May/0018.html>

Any discussion about whether this is a legitimate thing to try to do
should be added to that thread. This thread is for a possible solution
that meets the perceived need.


CURRENT SYNTAX

There are two current technique for specifying an object that is a
resource. They are to use @href on elements that are not anchor tags,
and the second is to use a link element.

The first, using '@href everywhere':

  <div about="http://joost.com/some-film">
    <div property="dc:title">A film</div>
    <div property="dc:description">
      Some notes on the film
    </div>
    <span rel="dc:subject" href="http://film-vocab/horror">Category:
Horror</span>
  </div>

There has been some pushback on this technique.

The second is to use a link element:

  <div about="http://joost.com/some-film">
    <link rel="dc:subject" href="http://film-vocab/horror" />
    <div property="dc:title">A film</div>
    <div property="dc:description">
      Some notes on the film
    </div>
    <span>Category: Horror</span>
  </div>

In terms of use in current browsers, we're finding that context
information is lost when using 'link' in the body of the document, so
this doesn't look like it will work. Obviously the elements could be
added to <head> with an @about, but that makes things quite difficult
to manage.


@HREF EVERYWHERE

In my view the idea that authors will be confused by having '@href
everywhere' is not as big a problem as has been posed. However, I'm
always of the view that if we can find an alternative solution that
does as good a job as a solution that people aren't comfortable with,
why not just use it. In this case, I think there is an alternative
solution that is in some ways better than '@href everywhere'.


A SHORT HISTORY OF @RESOURCE

In my earliest drafts of RDFa I used attributes for subject, predicate
and objects, and the one for objects that were resources was
@resource. However, this was never satisfactory, because it meant that
information would often be duplicated--once for a clickable link, and
once for a statement--and it was the big thing that Ben Adida insisted
we should solve. So, after a great deal of juggling things around, I
stumbled upon the fact that @rel and @rev could be used on anchor
tags--maybe I was the only one who didn't, but I had not known that
that--and so it became pretty clear that HTML already gave us what we
needed and we could use @href instead of @resource. This seemed to
meet Ben's crucial requirement that we should only have to express the
URI once, and so 'bridge the clickable and semantic webs'. :)

Now, since XHTML 2 had previously added a new feature that @href could
be used on any element in a document, to create a navigable link, it
seemed obvious that all we had to do was drop @resource, and replace
it with @href.

However, non-XHTML 2 browsers actually have a tough time turning @href
on a span into a clickable link, and although it can be done with some
script, we don't get that out of the box. This means that we can have
@href attributes in a document that are not clickable links, and there
has been some argument that using @href on non-anchor elements could
confuse people.


PROPOSAL

My proposal would therefore be to still _allow_ @href anywhere, but to
play this feature down, and point people towards @resource. I feel
that an RDFa parser should still process @href as an object that is a
resource, wherever it finds it, so that if it encounters an XHTML 2
document, it will still work.

But whilst we still _support_ that feature, in our example code,
tutorials, and so on, we should instead use the resource attribute to
express an object that is a resource. Hopefully this way things will
be clearer to authors.

One way that we could understand this is that @resource is a core RDFa
attribute, whilst @href is not. When we come to use RDFa in a 'host
language' we add further rules, and in the case of the host language
being HTML or XHTML we can say that @href is given the 'RDFa meaning'
of being equivalent to @resource.


SYNTAX

Our previous example would now become:

  <div about="http://joost.com/some-film">
    <div property="dc:title">A film</div>
    <div property="dc:description">
      Some notes on the film
    </div>
    <span rel="dc:subject" resource="http://film-vocab/horror">
      Category: Horror
    </span>
  </div>

(I'll leave how the predicate is expressed out of this, but there are
good arguments for using @property here. I'll start a new thread for
that.)

Regards,

Mark

-- 
  Mark Birbeck, formsPlayer

  mark.birbeck@x-port.net | +44 (0) 20 7689 9232
  http://www.formsPlayer.com | http://internet-apps.blogspot.com

  standards. innovation.
Received on Wednesday, 23 May 2007 19:19:25 GMT

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