NAME

RDF::RDFa::Parser - flexible RDFa parser


SYNOPSIS

 use RDF::RDFa::Parser;
 
 $parser = RDF::RDFa::Parser->new(undef, $uri)->consume;
 $graph  = $parser->graph;


VERSION

1.00_02


DESCRIPTION

Constructor

$p = RDF::RDFa::Parser->new($xhtml, $baseuri, \%options, $storage)

This method creates a new RDF::RDFa::Parser object and returns it.

The $xhtml variable may contain an XHTML/XML string, or a XML::LibXML::Document. If a string, the document is parsed using XML::LibXML::Parser, which will throw an exception if it is not well-formed. RDF::RDFa::Parser does not catch the exception.

The base URI is needed to resolve relative URIs found in the document. If $xhtml is undef, then RDF::RDFa::Parser will fetch $baseuri to obtain the document to be parsed.

Options (mostly booleans) [default in brackets]:

The default options attempt to stick to the XHTML+RDFa spec as rigidly as possible.

$storage is an RDF::Trine::Store object. If undef, then a new temporary store is created.

Public Methods

$p->consume

The document is parsed for RDFa. Triples extracted from the document are passed to the callbacks as each one is found; triples are made available in the model returned by the graph method.

This function returns the parser object itself, making it easy to abbreviate several of RDF::RDFa::Parser's functions:

  my $iterator = RDF::RDFa::Parser->new($xhtml,$uri)
                 ->consume->graph->as_stream;
$p->set_callbacks(\%callbacks)

Set callback functions for the parser to call on certain events. These are only necessary if you want to do something especially unusual.

  $p->set_callbacks({
    'pretriple_resource' => sub { ... } ,
    'pretriple_literal'  => sub { ... } ,
    'ontriple'           => undef ,
    'onprefix'           => \&some_function ,
    });

Either of the two pretriple callbacks can be set to the string 'print' instead of a coderef. This enables built-in callbacks for printing Turtle to STDOUT.

For details of the callback functions, see the section CALLBACKS. set_callbacks must be used before consume. set_callbacks itself returns a reference to the parser object itself.

$p->graph( [ $graph_name ] )

Without a graph name, this method will return an RDF::Trine::Model object with all statements of the full graph. As per the RDFa specification, it will always return an graph containing all the statements of the RDFa document. If the model contains multiple graphs, all statements will be returned unless a graph name is specified.

It will also take an optional graph URI as argument, and return an RDF::Trine::Model tied to a temporary storage with all statements in that graph. This feature is only useful if you're using named graphs.

It makes sense to call consume before calling graph. Otherwise you'll just get an empty graph.

$p->graphs

Will return a hashref of all named graphs, where the graph name is a key and the value is a RDF::Trine::Model tied to a temporary storage.

This method is only useful if you're using named graphs.

It makes sense to call consume before calling graphs. Otherwise you'll just get an empty hashref.

$p->dom

Returns the parsed XML::LibXML::Document.

$p->xhtml

Returns the XHTML/XML source of the document being parsed.

$p->uri( [$other_uri] )

Returns the base URI of the document being parsed. This will usually be the same as the base URI provided to the constructor, but may differ if the document contains a <base> HTML element.

Optionally it may be passed a parameter - an absolute or relative URI - in which case it returns the same URI which it was passed as a parameter, but as an absolute URI, resolved relative to the document's base URI.

This seems like two unrelated functions, but if you consider the consequence of passing a relative URI consisting of a zero-length string, it in fact makes sense.

Utility Method

$structure = RDF::RDFa::Parser::keywords(@bundles)

Without any parameters, gets an empty structure for keywords. Passing additional strings adds certain bundles of predefined keywords to the structure.

  my $keyword_structure = RDF::RDFa::Parser::keywords(
        'xhtml', 'xfn', 'grddl');

This is useful to create a keyword structure may be provided as an option to the RDF::RDFa::Parser constructor. You probably want to leave this alone unless you know what you're doing.

Bundles include: rdfa, html5, html4, html32, iana, grddl, xfn.

Constants

These are not really constants, but functions which consistently return the same thing. You can mostly think of them as constants though.

RDF::RDFa::Parser::OPTS_XHTML

Suggested options hashref for parsing XHTML.

RDF::RDFa::Parser::OPTS_HTML4

Suggested options hashref for parsing HTML 4.x.

RDF::RDFa::Parser::OPTS_HTML5

Suggested options hashref for parsing HTML5.

RDF::RDFa::Parser::OPTS_SVG

Suggested options hashref for parsing SVG.

RDF::RDFa::Parser::OPTS_ATOM

Suggested options hashref for parsing Atom / DataRSS.

RDF::RDFa::Parser::OPTS_XML

Suggested options hashref for parsing generic XML.


CALLBACKS

Several callback functions are provided. These may be set using the set_callbacks function, which taskes a hashref of keys pointing to coderefs. The keys are named for the event to fire the callback on.

pretriple_resource

This is called when a triple has been found, but before preparing the triple for adding to the model. It is only called for triples with a non-literal object value.

The parameters passed to the callback function are:

The callback should return 1 to tell the parser to skip this triple (not add it to the graph); return 0 otherwise.

pretriple_literal

This is the equivalent of pretriple_resource, but is only called for triples with a literal object value.

The parameters passed to the callback function are:

Beware: sometimes both a datatype and a language will be passed. This goes beyond the normal RDF data model.)

The callback should return 1 to tell the parser to skip this triple (not add it to the graph); return 0 otherwise.

ontriple

This is called once a triple is ready to be added to the graph. (After the pretriple callbacks.) The parameters passed to the callback function are:

The callback should return 1 to tell the parser to skip this triple (not add it to the graph); return 0 otherwise. The callback may modify the RDF::Trine::Statement object.

onprefix

This is called when a new CURIE prefix is discovered. The parameters passed to the callback function are:

The return value of this callback is currently ignored, but you should return 0 in case future versions of this module assign significance to the return value.


FEATURES

HTML Support

The constructor function parses incoming markup as well-formed XML and will croak if given tag-soup HTML (or even valid HTML in most cases). However, you can pass the constructor an XML::LibXML::Document object instead of markup: this object could have been constructed from an HTML document.

The HTML::HTML5::Parser module is able to create an XML::LibXML::Document from tag-soup HTML; and the HTML::HTML5::Sanity module may be able to assist you in fixing namespace and other oddities in the resulting document. Here's an example of using both with RDF::RDFa::Parser:

  use HTML::HTML5::Parser;
  use HTML::HTML5::Sanity qw(fix_document);
  use RDF::RDFa::Parser;
  
  my $html_parser = HTML::HTML5::Parser->new;
  my $document    = $html_parser->parse_string($html);
  my $fixed_doc   = fix_document($document);
  
  my $rdfa_parser = RDF::RDFa::Parser->new(
                      $fixed_doc,
                      'http://example.com/doc.html',
                      RDF::RDFa::Parser::OPTS_HTML5);

Note that the RDF::RDFa::Parser::OPTS_HTML4 and RDF::RDFa::Parser::OPTS_HTML5 constants provide suggested settings for parsing HTML. In particular, they make CURIE prefixes case-insensitive, add support for the HTML lang attribute (instead of or as well as the xml:lang attribute), and bring in support for additional rel/rev keywords.

Atom / DataRSS

When processing Atom, if the 'atom_elements' option is switched on, RDF::RDFa::Parser will treat <feed> and <entry> elements specially. This is similar to the special support for <head> and <body> mandated by the XHTML+RDFa Recommendation. Essentially <feed> and <entry> elements are assumed to have an imaginary "about" attribute which has its value set to a brand new blank node.

If the 'atom_parser' option is switched on, RDF::RDFa::Parser fully parses Atom feeds and entries, using the XML::Atom::OWL package. The two modules attempt to work together in assigning blank node identifiers consistently, etc. If XML::Atom::OWL is not installed, then this option will be silently ignored.

The RDF::RDFa::Parser::OPTS_ATOM constant provides suggested settings for parsing Atom. It switches on the 'atom_elements' option (but not 'atom_parser'), adds support for IANA-registered rel/rev keywords, switches off support for some XHTML-specific features, enables processing of the xml:base attribute, and adds support for embedded chunks of RDF/XML.

Generally speaking, adding RDFa attributes to elements in the Atom namespace themselves can result in some slightly muddy semantics. It's best to use an extension namespace and add the RDFa attributes to elements in that namespace. DataRSS provides a good example of this. See http://developer.yahoo.com/searchmonkey/smguide/datarss.html.

SVG

The SVG Tiny 1.2 specification makes the use of RDFa attributes within SVG images valid.

The RDF::RDFa::Parser::OPTS_SVG constant provides suggested settings for parsing SVG. It switches off support for some XHTML-specific features, enables processing of the xml:base attribute, and adds support for embedded chunks of RDF/XML.

Embedded RDF/XML

Though a rarely used feature, XHTML allows other XML markup languages to be directly embedded into it. In particular, chunks of RDF/XML can be included in XHTML. While this is not common in XHTML, it's seen quite often in SVG and other XML markup languages.

When RDF::RDFa::Parser encounters a chunk of RDF/XML in a document it's parsing (i.e. an element called 'RDF' with namespace 'http://www.w3.org/1999/02/22-rdf-syntax-ns#'), there are three different courses of action it can take:

  1. Continue straight through it.

    This is the behaviour that XHTML+RDFa seems to suggest is the right option. It should mostly not do any harm: triples encoded in RDF/XML will be generally ignored (though the chunk itself could theoretically end up as part of an XML literal). It will waste a bit of time though.

  2. Skip the chunk.

    This will skip over the RDF element entirely, and thus save you a bit of time.

  3. Parse the RDF/XML.

    The parser will parse the RDF/XML properly. If named graphs are enabled, any triples will be added to a separate graph. This is the behaviour that SVG Tiny 1.2 seems to suggest is the correct thing to do.

You can decide which path to take by setting the 'embedded_rdfxml' option in the constructor. For HTML and XHTML, you probably want to set embedded_rdfxml to '0' (the default) or '1'. For other XML markup languages (e.g. SVG or Atom), then you probably want to set it to '2'.

If you use the RDF::RDFa::Parser::OPTS_XHTML, RDF::RDFa::Parser::OPTS_SVG etc constants, they should mostly do the right thing.

Named Graphs

The parser has support for named graphs within a single RDFa document. To switch this on, use the 'graph' option in the constructor.

The name of the attribute which indicates graph URIs is by default 'graph', but can be changed using the 'graph_attr' option. This option accepts clark notation to specify a namespaced attribute. By default, the attribute value is interpreted as a fragment identifier (like the 'id' attribute), but if you set 'graph_type' to 'about', it will be treated as a URI or safe CURIE (like the 'about' attribute).

The 'graph_default' option allows you to set the default graph URI/bnode identifier.

Once you're using named graphs, the graphs method becomes useful: it returns a hashref of { graph_uri => trine_model } pairs. The optional parameter to the graph method also becomes useful.

See also http://buzzword.org.uk/2009/rdfa4/spec.

Auto Config

RDF::RDFa::Parser has a lot of different options that can be switched on and off. Sometimes it might be useful to allow the page being parsed to control some of the options. If you switch on the 'auto_config' option, pages can do this.

A page can set options using a specially crafted <meta> tag:

  <meta name="http://search.cpan.org/dist/RDF-RDFa-Parser/#auto_config";
     content="xhtml_lang=1&amp;keywords=rdfa+html5+html4+html32" />

Note that the content attribute is an application/x-www-form-urlencoded string (which must then be HTML-escaped of course). Semicolons may be used instead of ampersands, as these tend to look nicer:

  <meta name="http://search.cpan.org/dist/RDF-RDFa-Parser/#auto_config";
     content="xhtml_lang=1;keywords=rdfa+html5+html4+html32" />

Any option allowed in the constructor may be given using auto config, except 'use_rtnlx', and of course 'auto_config' itself.

It's possible to use auto config outside XHTML (e.g. in Atom or SVG) using namespaces:

  <xhtml:meta xmlns:xhtml="http://www.w3.org/1999/xhtml";
     name="http://search.cpan.org/dist/RDF-RDFa-Parser/#auto_config";
     keywords="iana+rdfa;xml_base=2;atom_elements=1" />


BUGS

RDF::RDFa::Parser 0.21 passed all approved tests in the XHTML+RDFa test suite at the time of its release.

RDF::RDFa::Parser 0.22 (used in conjunction with HTML::HTML5::Parser 0.01 and HTML::HTML5::Sanity 0.01) additionally passes all approved tests in the HTML4+RDFa and HTML5+RDFa test suites at the time of its release; except test cases 0113 and 0121, which the author of this module believes mandate incorrect HTML parsing.

Please report any bugs to http://rt.cpan.org/.

Common gotchas:


SEE ALSO

XML::LibXML, RDF::Trine, HTML::HTML5::Parser, HTML::HTML5::Sanity, XML::Atom::OWL.

http://www.perlrdf.org/.


AUTHOR

Toby Inkster <tobyink@cpan.org>.


ACKNOWLEDGEMENTS

Kjetil Kjernsmo <kjetilk@cpan.org> wrote much of the stuff for building RDF::Trine models. Neubert Joachim taught me to use XML catalogues, which massively speeds up parsing of XHTML files that have DTDs.


COPYRIGHT

Copyright 2008-2010 Toby Inkster

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.