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    1/*  Part of ClioPatria SeRQL and SPARQL server
    3    Author:        Jan Wielemaker
    4    E-mail:
    5    WWW: 
    6    Copyright (C): 2010, VU University Amsterdam
    8    This program is free software; you can redistribute it and/or
    9    modify it under the terms of the GNU General Public License
   10    as published by the Free Software Foundation; either version 2
   11    of the License, or (at your option) any later version.
   13    This program is distributed in the hope that it will be useful,
   14    but WITHOUT ANY WARRANTY; without even the implied warranty of
   16    GNU General Public License for more details.
   18    You should have received a copy of the GNU General Public
   19    License along with this library; if not, write to the Free Software
   20    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
   22    As a special exception, if you link this library with other files,
   23    compiled with a Free Software compiler, to produce an executable, this
   24    library does not by itself cause the resulting executable to be covered
   25    by the GNU General Public License. This exception does not however
   26    invalidate any other reasons why the executable file might be covered by
   27    the GNU General Public License.
   30:- module(rdf_bnode,
   31	  [ bnode_vars/3		% +Graph0, -VarGraph, -BNodeVars
   32	  ]).   33:- use_module(library(semweb/rdf_db)).   34:- use_module(library(assoc)).

RDF graph operations on bnodes

This module operates on RDF graphs represented as rdf(S,P,O) that contain blank nodes (bnodes). Bnodes can be considered existential variables in (sub-)graph, which motivates replacing them by Prolog variables. */

 bnode_vars(+RDF, -RDFWithVars, -Vars) is det
Consistently replace bnodes in RDF with Prolog variable and unify Vars with a list of the variables found. Note that, if we perform matches with such graphs, multiple variables may unify to the same concrete resource. One might consider adding constraints such as dif/2.
RDF- is a list rdf(S,P,O)
Resolved- is a list rdf(S,P,O), where resources may be a variable
NodeIDs- is a list of variables representing the bnodes.
   57bnode_vars(Graph0, Graph, NodeIDs) :-
   58	empty_assoc(Map0),		% BNodeID --> Var
   59	bnode_vars(Graph0, Graph, Map0, Map),
   60	assoc_to_values(Map, NodeIDs).
   62bnode_vars([], [], Map, Map).
   63bnode_vars([rdf(S0,P0,O0)|T0], Graph, Map0, Map) :-
   64	(   rdf_is_bnode(S0)
   65	;   rdf_is_bnode(P0)
   66	;   rdf_is_bnode(O0)
   67	), !,
   68	Graph = [rdf(S,P,O)|T],
   69	bnode_var(S0, S, Map0, Map1),
   70	bnode_var(P0, P, Map1, Map2),
   71	bnode_var(O0, O, Map2, Map3),
   72	bnode_vars(T0, T, Map3, Map).
   73bnode_vars([Triple|T0], [Triple|T], Map0, Map) :-
   74	bnode_vars(T0, T, Map0, Map).
   77bnode_var(R0, BNodeID, Map0, Map) :-
   78	rdf_is_bnode(R0), !,
   79	(   get_assoc(R0, Map0, BNodeID)
   80	->  Map = Map0
   81	;   put_assoc(R0, Map0, BNodeID, Map)
   82	).
   83bnode_var(R, R, Map, Map)