Toby Inkster

NAME

XRD::Parser - parse XRD and host-meta files into RDF::Trine models

SYNOPSIS

  use RDF::Query;
  use XRD::Parser;
  
  my $parser = XRD::Parser->new(undef, "http://example.com/foo.xrd");
  my $results = RDF::Query->new(
    "SELECT * WHERE {?who <http://spec.example.net/auth/1.0> ?auth.}")
    ->execute($parser->graph);
        
  while (my $result = $results->next)
  {
    print $result->{'auth'}->uri . "\n";
  }

or maybe:

  my $data = XRD::Parser->hostmeta('gmail.com')
                          ->graph
                            ->as_hashref;

DESCRIPTION

While XRD has a rather different history, it turns out it can mostly be thought of as a serialisation format for a limited subset of RDF.

This package ignores the order of <Link> elements, as RDF is a graph format with no concept of statements coming in an "order". The XRD spec says that grokking the order of <Link> elements is only a SHOULD. That said, if you're concerned about the order of <Link> elements, the callback routines allowed by this package may be of use.

This package aims to be roughly compatible with RDF::RDFa::Parser's interface.

Constructors

$p = XRD::Parser->new($content, $uri, [\%options], [$store])

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

The $content variable may contain an XML string, or a XML::LibXML::Document. If a string, the document is parsed using XML::LibXML::Parser, which may throw an exception. XRD::Parser does not catch the exception.

$uri the base URI of the content; it is used to resolve any relative URIs found in the XRD document.

Options [default in brackets]:

  • default_subject - If no <Subject> element. [undef]

  • link_prop - How to handle <Property> in <Link>? 0=skip, 1=reify, 2=subproperty, 3=both. [0]

  • loose_mime - Accept text/plain, text/html and application/octet-stream media types. [0]

  • tdb_service - Use thing-described-by.org when possible. [0]

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

$p = XRD::Parser->new_from_url($url, [\%options], [$storage])

$url is a URL to fetch and parse.

This function can also be called as new_from_uri. Same thing.

$p = XRD::Parser->hostmeta($uri)

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

The parameter may be a URI (from which the hostname will be extracted) or just a bare host name (e.g. "example.com"). The resource "/.well-known/host-meta" will then be fetched from that host using an appropriate HTTP Accept header, and the parser object returned.

Public Methods

$p->uri($uri)

Returns the base URI of the document being parsed. This will usually be the same as the base URI provided to the constructor.

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.

$p->dom

Returns the parsed XML::LibXML::Document.

$p->graph

This method will return an RDF::Trine::Model object with all statements of the full graph.

This method will automatically call consume first, if it has not already been called.

$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 ,
    });

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.

NOTE: the behaviour of this function was changed in version 0.05.

$p->consume

This method processes the input DOM and sends the resulting triples to the callback functions (if any).

It called again, does nothing.

Returns the parser object itself.

Utility Functions

$host_uri = XRD::Parser::host_uri($uri)

Returns a URI representing the host. These crop up often in graphs gleaned from host-meta files.

$uri can be an absolute URI like 'http://example.net/foo#bar' or a host name like 'example.com'.

$uri = XRD::Parser::template_uri($relationship_uri)

Returns a URI representing not a normal relationship, but the relationship between a host and a template URI literal.

$hostmeta_uri = XRD::Parser::hostmeta_location($host)

The parameter may be a URI (from which the hostname will be extracted) or just a bare host name (e.g. "example.com"). The location for a host-meta file relevant to the host of that URI will be calculated.

If called in list context, returns an 'https' URI and an 'http' URI as a list.

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:

  • A reference to the XRD::Parser object

  • A reference to the XML::LibXML::Element being parsed

  • Subject URI or bnode (string)

  • Predicate URI (string)

  • Object URI or bnode (string)

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:

  • A reference to the XRD::Parser object

  • A reference to the XML::LibXML::Element being parsed

  • Subject URI or bnode (string)

  • Predicate URI (string)

  • Object literal (string)

  • Datatype URI (string or undef)

  • Language (string or undef)

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:

  • A reference to the XRD::Parser object

  • A reference to the XML::LibXML::Element being parsed

  • An RDF::Trine::Statement object.

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.

WHY RDF?

It abstracts away the structure of the XRD file, exposing just the meaning of its contents. Two XRD files with the same meaning should end up producing more or less the same RDF data, even if they differ significantly at the syntactic level.

If you care about the syntax of an XRD file, then use XML::LibXML.

SEE ALSO

RDF::Trine, RDF::Query, RDF::RDFa::Parser.

http://www.perlrdf.org/.

AUTHOR

Toby Inkster, <tobyink@cpan.org>

COPYRIGHT AND LICENCE

Copyright (C) 2009-2012 by Toby Inkster

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

DISCLAIMER OF WARRANTIES

THIS PACKAGE IS PROVIDED "AS IS" AND WITHOUT ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.




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