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Jakob Voß > RDF-Lazy > RDF::Lazy::Tutorial


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RDF::Lazy::Tutorial - Tutorial to get started with RDF::Lazy

A short introduction to RDF::Lazy as RDF templating system ^

Three kinds of nodes

The basic object in RDF::Lazy is a node (RDF::Lazy::Node). A node represents an RDF node in an RDF graph. Every nody can be stringified to its value:

    "$x"      Perl syntax
    [% x %]   Template Toolkit syntax
    {$x}      Smarty syntax

There are three kinds of nodes:

resource nodes

implemented by RDF::Lazy::Resource and stringified to their URI, for instance

literal nodes

implemented by RDF::Lazy::Literal and stringified to their string value. instance hello world.

blank nodes

implemented by RDF::Lazy::Literal and stringified to _: followed by their internal blank node identifier, for instance _:n0815.

Nodes have a couple of useful methods. For instance you can check the kind of a node x with the methods is_resource (or its alias is_uri), is_literal, and is_blank:

    $x->is_literal                         Perl syntax
    [% x.is_literal %]                     Template Toolkit syntax
    {$x->is_literal} or {$x.is_literal}    Smarty syntax

The method str is automatically called to stringify a node, so "$x" and <$x-str>> are equivalent. In HTML templates you can use the method esc to HTML/XML-escape the stringified node value:

    [% x.esc %]                            Template Toolkit syntax
    {$x->esc} or {$x.esc}                  Smarty syntax

No nodes without graph

Each node in RDF::Lazy belongs to a particular RDF graph (RDF::Lazy). You can access a node's graph by its graph method, if needed. Graphs have some factory methods to create new node objects:

    $g->uri("")      A resource node, belonging to $g
    $g->literal("hello world")             A literal node, belonging to $g
    $g->blank("n0815")                     A blank node, belonging to $g

The graph methods ttl and ttlpre are handy o dump the whole graph in Turtle syntax. The latter wraps and escapes Turtle for HTML output:

    [% g.ttlpre %]

For instance you can show the number of triples in a node's graph like this:

    <p>The node's graph contains [% x.graph.size %] triples.</p>

Traversing the graph

One can traverse the RDF graph from any node. For instance, given a foaf:Person node, one can get another resource linked via foaf:knows:

    if ( $x->type('foaf:Person') ) {
        $another_person = $x->foaf_knows;
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