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 maintability
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.
rdf_current_prefix(Prefix, Expansion), atom_concat(Expansion, Local, URI),
true, replace existing namespace alias. Please note that
replacing a namespace is dangerous as namespaces affect preprocessing.
Make sure all code that depends on a namespace is compiled after
changing the registration.true and Alias is already defined, keep the original
binding for Prefix and succeed silently.Without options, an attempt to redefine an alias raises a permission error.
Predefined prefixes are:
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.
Note that this predicate is a meta-predicate on its output argument. This is necessary to get the module context while the first argument may be of the form (:)/2. The above mode description is correct, but should be interpreted as (?,?).
existence_error(rdf_prefix, Prefix)existence_error(rdf_prefix, Prefix)
Terms of the form Prefix:Local that appear in TermIn
for which
Prefix is not defined are not replaced. Unlike rdf_global_id/2
and
rdf_global_object/2, no
error is raised.
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 distict 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).