Prolog code often contains references to constant resources with a
known
prefix (also known as XML namespaces). For example,
http://www.w3.org/2000/01/rdf-schema#Class refers to the
most general notion of an RDFS class. Readability and maintainability
concerns require for abstraction here. The RDF database maintains a
table of known prefixes. This table can be queried using rdf_current_ns/2
and can be extended using rdf_register_ns/3.
The prefix database is used to expand prefix:local terms
that appear as arguments to calls which are known to accept a resource.
This expansion is achieved by Prolog preprocessor using expand_goal/2.
Explicit expansion is achieved using the predicates below. The
predicate rdf_equal/2 performs
this expansion at compile time, while the other predicates do it at
runtime.
- rdf_equal(?Resource1,
?Resource2)
- Simple equality test to exploit goal-expansion.
Namespace handling for custom predicates
If we implement a new predicate based on one of the predicates of the
semweb libraries that expands namespaces, namespace expansion is not
automatically available to it. Consider the following code computing the
number of distinct objects for a certain property on a certain object.
cardinality(S, P, C) :-
( setof(O, rdf_has(S, P, O), Os)
-> length(Os, C)
; C = 0
).
Now assume we want to write labels/2
that returns the number of distinct labels of a resource:
labels(S, C) :-
cardinality(S, rdfs:label, C).
This code will not work because rdfs:label is not
expanded at compile time. To make this work, we need to add an rdf_meta/1
declaration.
:- rdf_meta
cardinality(r,r,-).
The example below defines the rule concept/1.
:- use_module(library(semweb/rdf_db)). % for rdf_meta
:- use_module(library(semweb/rdfs)). % for rdfs_individual_of
:- rdf_meta
concept(r).
%% concept(?C) is nondet.
%
% True if C is a concept.
concept(C) :-
rdfs_individual_of(C, skos:'Concept').
In addition to expanding calls, rdf_meta/1
also causes expansion of
clause heads for predicates that match a declaration. This is
typically used write Prolog statements about resources. The following
example produces three clauses with expanded (single-atom) arguments:
:- use_module(library(semweb/rdf_db)).
:- rdf_meta
label_predicate(r).
label_predicate(rdfs:label).
label_predicate(skos:prefLabel).
label_predicate(skos:altLabel).
