RDF::Lazy::Tutorial(3) 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 "http://example.org/foo".
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("http://example.org/foo")      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;