Category Archives: SemWeb

http://www.w3.org/2001/sw/
skos:broader http://danbri.org/words/?author=2/skos.rdf#c9 Web Technology

Transforming RDF/XML with XSLT

HTML, RDF/XML, Redland, Vocabularies, XSLT

A couple of months ago, while working on a project that will hopefully see the light of day soon, I realised I needed terms for singular and plural labels for properties and classes. Even with the help of SchemaWeb I couldn’t find existing terms, so I decided to cook my own, resulting in the label vocabulary with two properties:

plural
A relation between a term and its label in literal plural form.
singular
A relation between a term and its label in literal singular form.

This was not the only vocabulary I was working on at that moment, and I needed to be able to get an overview, a human-readable version. Last year I did the RDFS Explorer for basically the same purpose, but since I was entering OWL territory, it wasn’t really up to the task. Back to square one.

Continue reading Transforming RDF/XML with XSLT →

FOAF Explorer update

FOAF Explorer, RDF, Vocabularies

It has bothered me for a while, that the FOAF Explorer wasn’t able to handle duplicate statements. It would either repeat the entire property/value pair, or in some situations “just” show the values next to each other without whitespace or other separators in between.

That last issue really isn’t fixed yet, but at least it now only happens with different values – I managed to remove duplicate statements with some crude PHP hacking.

I was already doing a parse and custom (re-)serialise with Redland/Raptor (the PHP source is available) before passing it on to the XSLT, so it was “just” a matter of making sure the same statement wasn’t serialised twice.

Since part of the point of reserialising was to group statements by subject, I had an index in the form of an array of statements per subject. Even though it can be optimized, I simply added a loop to check for the presence of the current predicate/object pair:

$found=0; 
  $os=librdf_node_to_string($object); 
  reset($Nodes[$node]); 
  while(!$found && list(,$pso)=each($Nodes[$node])) { 
    $p=key($pso); 
    list($s,$o)=current($pso); 
    $found=($p==$predicate && librdf_node_to_string($o)==$os); 
  }

It works (try it!), and while the entire FE processing should now take longer, it actually helps somewhat that the XSLT doesn’t have to cope with too much…

While I was at it, I added support for the use of XFN as an RDF vocabulary, with the namespace http://gmpg.org/xfn/1#. It is now treated the same way as the Relationship vocabulary and the Trust vocabulary, which means that it’s handled as if all the properties are rdfs:subPropertyOf foaf:knows. It’s not perfect, the display could use some collapsing, but it works (try it!).

Oh, almost forgot: Also added support for the Quaffing vocabulary by Leigh Dodds.

If only FE really knew about rdfs:subPropertyOf…

FOAF output from WordPress

FOAF, WordPress

Note: Please see FOAF Output Plugin for newer versions of WordPress.

A few days ago, Christopher Schmidt mentioned in a comment that he had been hacking on FOAF export for WordPress. He kindly let me take a look at his work, and in addition posted a message to rdfweb-dev.

In short, he had created a set of dynamic stand-alone profile pages, one with HTML output and one with RDF/XML output using FOAF, in a single file, /profile2.php.

Seeing that, it dawned on me: WordPress already has a “profile” page for each of the authors, likely at /archives/author/<login>/ (depending on the permalink structure defined) — and it even has a template name: the_author_posts_link. In its default incarnation it’s simply a list of the posts by that author, but turning it into a profile page by also showing the basic information and linked friends didn’t seem too hard.

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Improving RSS output from WordPress

FOAF, WordPress

The default syndication output from WordPress is okay, but does leave room for improvement – even though it “defaults” to RSS 2.0, there’s RSS 1.0 hidden behind the scenes.

The first order of action is to fix the default page template to point to the RDF/XML output instead of RSS 2.0. In the file /index.php there are link elements in the head section of the HTML output, switching to RSS 1.0 is quite easy, simply change the RSS 2.0 line to the following (and remove the link to the 0.92 version):

<link rel="alternate" type="application/rdf+xml" title="RSS 1.0" href="<?php bloginfo('rdf_url'); ?>" />

In the meta section there is also a couple of links, one for RSS 2.0 that will be replaced by a link to RSS 1.0, and one for comments that is just dropped (an RDF/XML version may be added at a later date):

<li><a href="<?php bloginfo('rdf_url'); ?>" title="<?php _e('Syndicate this site using RSS'); ?>"><?php _e('<abbr title="RDF Site Summary">RSS</abbr> 1.0'); ?></a></li>

Now to the actual content of the RSS file: The default template includes, for each post, a dc:creator property that looks like this:

<dc:creator>mortenf (mailto:mort&#101;n&#64;&#119;as&#97;b.dk)</dc:creator>

That’s not terribly helpful for the Semantic Web, so I’ve hacked the template wp-rdf.php to include the following FOAF structure instead:

<foaf:maker>
  <foaf:Person>
    <foaf:name><?php the_author_firstname(); print ' '; the_author_lastname() ?></foaf:name>
    <foaf:nick><?php the_author_login() ?></foaf:nick>
    <foaf:mbox_sha1sum><?php print(bin2hex(mhash(MHASH_SHA1,'mailto:'.$authordata->user_email))) ?></foaf:mbox_sha1sum>
    <foaf:weblog rdf:resource="<?php bloginfo_rss('url') ?>"/>
  </foaf:Person>
</foaf:maker>

In addition to this improvement, the indication of categories could also use a little tune-up. The default output shows categories through the use of dc:subject, which is quite allright but somewhat too vague – how could anyone know that my category SemWeb/FOAF is essentially the same as Danny Ayers‘ Knowledge/FOAF category? To make up for this, it is possible to use the Description field in WordPress’ category system to assign a URI to each (or some) of the categories, and then generate appropriate statements by adding two lines to the the_category_rss template function in /wp-includes/template-functions-category.php:

if(preg_match('|^w+:.+/|',$category->category_description))
    $the_list .= '<foaf:topic rdf:parseType="Resource"><dc:title>'.$category->cat_name.'</dc:title><foaf:page rdf:resource="'.$category->category_description.'"/></foaf:topic>';

The results of these tweakings should show up nicely in the RSS feed – check it out here.

Yet another semweb blogger

Blogging, SemWeb

In the words of Mark Nottingham:

Everybody seems to have one, so here’s mine.

I’ve been biding my time, mostly because I’m still not sure that blogs provide an advantage over other means of community communication such as mailing lists and wikis, but also because I, like a lot of other people, want to roll my own tool. Time does not come cheap, and progress in that area is slow, so here it is – powered by WordPress…

The first posts will likely be on exactly that subject, WordPress, trying to make it fit my needs better, but as the title – Binary Relations – of my new weblog suggests, the writing here will focus on the Semantic Web with it’s social and technical implications. Subjects in that area will be Redland, FOAF, names in FOAF and image description, tying together my digital photos with the currently upcoming SWAD-Europe Image Description Workshop.

This will likely not be a link blog, as people like Danny Ayers and the collective efforts by the #rdfig community do a good job in that area. My vision for this blog is more along the lines of the work by Matt Biddulph and Mark Pilgrim – longish posts trying to generate forward motion in an interesting area.

We’ll see…