"""

This example shows how a custom evaluation function can be added to
handle certain SPARQL Algebra elements

A custom function is added that adds ``rdfs:subClassOf`` "inference" when
asking for ``rdf:type`` triples.

Here the custom eval function is added manually, normally you would use
setuptools and entry_points to do it:
i.e. in your setup.py::

    entry_points = {
        'rdf.plugins.sparqleval': [
            'myfunc =     mypackage:MyFunction',
            ],
    }

"""

import rdflib

from rdflib.plugins.sparql.evaluate import evalBGP
from rdflib.namespace import FOAF

inferredSubClass = \
    rdflib.RDFS.subClassOf * '*'  # any number of rdfs.subClassOf


def customEval(ctx, part):
    """
    Rewrite triple patterns to get super-classes
    """

    if part.name == 'BGP':

        # rewrite triples
        triples = []
        for t in part.triples:
            if t[1] == rdflib.RDF.type:
                bnode = rdflib.BNode()
                triples.append((t[0], t[1], bnode))
                triples.append((bnode, inferredSubClass, t[2]))
            else:
                triples.append(t)

        # delegate to normal evalBGP
        return evalBGP(ctx, triples)

    raise NotImplementedError()

if __name__=='__main__':

    # add function directly, normally we would use setuptools and entry_points
    rdflib.plugins.sparql.CUSTOM_EVALS['exampleEval'] = customEval

    g = rdflib.Graph()
    g.load("foaf.rdf")

    # Add the subClassStmt so that we can query for it!
    g.add((FOAF.Person,
           rdflib.RDFS.subClassOf,
           FOAF.Agent))

    # Find all FOAF Agents
    for x in g.query(
            'PREFIX foaf: <%s> SELECT * WHERE { ?s a foaf:Agent . }' % FOAF):
        print x