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; }