#!/usr/bin/perl
use strict;
use warnings;
no warnings 'redefine';
use utf8;
use Data::Dumper;
use Scalar::Util qw(reftype blessed);
use RDF::Query;
my $la          = RDF::Query::Node::Literal->new( 'a' );
my $l3          = RDF::Query::Node::Literal->new( '3', undef, 'http://www.w3.org/2001/XMLSchema#integer' );
my $ra          = RDF::Query::Node::Resource->new( 'http://example.org/a' );
my $rb          = RDF::Query::Node::Resource->new( 'http://example.org/b' );
my $va          = RDF::Query::Node::Variable->new( 'a' );
my $vb          = RDF::Query::Node::Variable->new( 'b' );
# construct a BGP with two triples { ?a :b "a" ; :b ?b }
my $triplea     = RDF::Query::Algebra::Triple->new( $va, $ra, $la );
my $tripleb     = RDF::Query::Algebra::Triple->new( $va, $rb, $vb );
my $bgp         = RDF::Query::Algebra::BasicGraphPattern->new( $triplea, $tripleb );
# now add a filter, and wrap it in a GGP { ?a :b "a" ; :b ?b . FILTER(?b < 3) }
my $expr        = RDF::Query::Expression::Binary->new( '<', $vb, $l3 );
my $filter      = RDF::Query::Algebra::Filter->new( $expr, $bgp );
my $ggp         = RDF::Query::Algebra::GroupGraphPattern->new( $filter );
# add a LIMIT clause
my $limit       = RDF::Query::Algebra::Limit->new( $filter, 5 );
# and a PROJECT to get the variable list
my $proj        = RDF::Query::Algebra::Project->new( $limit, [map {RDF::Query::Node::Variable->new($_) } qw(a b)] );
# you still need to add the method (SELECT) yourself. at some point, the code
# from RDF::Query::describe, RDF::Query::construct and RDF::Query::ask will be
# moved into ::Algebra subclasses, and then we won't need to do this manually.
# until then, just tag on 'SELECT ' onto the front of the query string.
print 'SELECT ' . $proj->as_sparql;
# should print:
# SELECT * WHERE {
#       ?a <http://example.org/a> "a" .
#       ?a <http://example.org/b> ?b .
#       FILTER (?b < 3) .
# }
# LIMIT 5