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Re: SV: A certain difficulty - lack of action!

From: Gabe Beged-Dov <begeddov@jfinity.com>
Date: Mon, 28 Feb 2000 20:05:23 -0800
Message-ID: <38BB4583.7AB98A5A@jfinity.com>
To: Sergey Melnik <melnik@db.stanford.edu>
CC: martind@netfolder.com, RDF Interest Group <www-rdf-interest@w3.org>
Sergey Melnik wrote:
> 
> Didier PH Martin wrote:
> > [...]
> >
> > <myElement xlink:type="simple"
> >            xlink:href="your resource link here">
> >   <dc:description>your description</dc:description>
> >   <dc:author>the author here</dc:author>
> >                 etc....
> > </myElement>
> 
> So how do you say in XLink
> 
> http://w3.org/somepage.html --dc:Creator--> http://w3.org/people/12345 ?

Good question. See possible answer below. First some background
opinion. I don't see XLink saying or asserting things. Instead, it
creates hyperlinks (RDF OTOH, can both say things and create links).
The Xlink spec has the following description in the introduction:

  Hyperlinks are links that are meaningful to end users, 
  often being presented to them directly for use and actuation.


These hyperlinks are specified as a set of endpoints (locators) along
with an optional set of "arcs" or traversal paths between some subset
of the endpoints. The simple link (as in Didier's example above) can
be considered a degenerate case of the extended link. When unwound,
the simple link has two endpoints, a local endpoint consisting of the
content of the simple link element and the remote endpoint that is
specified in the xlink:href attribute. It also has an arc going from
the local to remote endpoint. Lets rewrite it below:

<myElement xlink:type="extended">
  <locator xlink:type="locator" 
           xlink:role="remote" xlink:href="your resource link here"/>
  <locator xlink:type="resource" xlink:role="local">
    <dc:description>your description</dc:description>
    <dc:author>the author here</dc:author>
  </locator>
  <arc xlink:type="arc" xlink:from="local" xlink:to="remote"/>
</myElement>

Looking at this unwound version you notice that its not exactly
succinct and also that there is alot of extra (unnecessary?)
information like the locator element names. Its also not clear what
the correct canonical unwinding from simple to extended is. The Xlink
spec doesn't have a normative version of the unwinding. 

To get to your question, one possible way to "say" this in XLink is:

<someTag xlink:type="extended">
  <someLocTag xlink:type="locator" xlink:role="someRole"
              xlink:href="http://w3.org/somepage.html"/>
  <someLocTag xlink:type="locator" xlink:role="otherRole"
              xlink:href="http://w3.org/people/12345"/>
  <someArcTag xlink:type="arc" xlink:role="dc:Creator"
              xlink:from="someRole" xlink:to="otherRole"/>
</someTag>

Its interesting to note that the only piece of "semantic" information
here is the value of the xlink:role attribute on the arc element. This
attribute is optional and its interpretation is unspecified. The arc
element itself is optional and is intended to provide "traversal"
information. 

In general, the only straightforward mapping I see from XLink to RDF
is when you use the default extended link (i.e. with no arcs). If all
the locators have roles you can then treat the  extended link like an
rdf:description element. This only works for anonymous or local
resources since the extended link doesn't have an "about".For example:

<anonResource xlink:type="extended">
  <dontcare xlink:type="resource" xlink:role="qpred1"> a literal value
</dontcare>
  <dontcare xlink:type="resource" xlink:role="qpred2"> another
literal</dontcare>
  <dontcare xlink:type="locator" xlink:role="qpred3"
xlink:href="anObjectURI"/>
</anonResource>

The local locators then are equivalent to literal properties and the
remote locators are equivalent to resource properties. The roles map
to property names. Unfortunately, Xlink's emphasis on presentation and
minimization mean that this is not the way extended links are
specified. Locator roles are used as locally scoped ids for arc
matching and arcs are used for traversal/presentation.

Cordially from Corvallis,

Gabe Beged-Dov

http://www.jfinity.com/gabe
Received on Monday, 28 February 2000 23:18:12 GMT

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