The parser is designed to operate in various environments and
therefore provides interfaces at various levels. First we describe the
top level defined in library(rdf), simply parsing a RDF-XML
file into a list of triples. Please note these are not asserted
into the database because it is not necessarily the final format the
user wishes to reason with and it is not clean how the user wants to
deal with multiple RDF documents. Some options are using global URI's in
one pool, in Prolog modules or using an additional argument.
load_rdf(File, Triples,[]).[], local identifiers
are not tagged.share (default), blank-node
properties (i.e. complex properties without identifier) are reused if
they result in exactly the same triple-set. Two descriptions are shared
if their intermediate description is the same. This means they should
produce the same set of triples in the same order. The value noshare
creates a new resource for each blank node.true, expand rdf:aboutEach
into a set of triples. By default the parser generates
rdf(each(Container), Predicate, Subject).xml:lang
declaration in an enclosing element).true, xml:lang declarations in the document
are ignored. This is mostly for compatibility with older versions of
this library that did not support language identifiers.rdf:datatype=Type
attribute, call ConvertPred(+Type, +Content, -Literal).
Content is the XML element contentas returned by the XML
parser (a list). The predicate must unify Literal with a
Prolog representation of Content according to
Type or throw an exception if the conversion cannot be made.
This option servers two purposes. First of all it can be used to
ignore type declarations for backward compatibility of this library.
Second it can be used to convert typed literals to a meaningful Prolog
representation. E.g. convert’42’to the Prolog integer 42 if
the type is xsd:int or a related type.
xmlns:NS=URL
declaration found in the source.
The Triples list is a list of rdf(Subject,
Predicate, Object) triples. Subject is either a plain
resource (an atom), or one of the terms each(URI) or prefix(URI)
with the obvious meaning. Predicate is either a plain atom
for explicitely non-qualified names or a term
NameSpace:Name. If NameSpace is
the defined RDF name space it is returned as the atom rdf.
Finally, Object is a URI, a Predicate or a term of
the format literal(Value) for literal values. Value
is either a plain atom or a parsed XML term (list of atoms and
elements).
The Object (3rd) part of a triple can have several different
types. If the object is a resource it is returned as either a plain atom
or a term NameSpace:Name. If it is a
literal it is returned as literal(Value), where Value
takes one of the formats defined below.
xml:lang qualifier.
lang(LanguageID, Atom)xml:lang qualifier
LanguageID specifies the language and Atom the
actual text.
element(Name, Attributes, Content) and atoms for CDATA
parts as described with the SWI-Prolog
SGML/XML
parser.
type(Type, StringValue)rdf:datatype=Type a term
of this format is returned.
XML name spaces are identified using a URI. Unfortunately various
URI's are in common use to refer to RDF. The rdf_parser.pl
module therefore defines the namespace as a multifile/1
predicate, that can be extended by the user. For example, to parse the
Netscape
OpenDirectory
structure.rdf file, the following declarations are used:
:- multifile
rdf_parser:rdf_name_space/1.
rdf_parser:rdf_name_space('http://www.w3.org/TR/RDF/').
rdf_parser:rdf_name_space('http://directory.mozilla.org/rdf').
rdf_parser:rdf_name_space('http://dmoz.org/rdf').
The initial definition of this predicate is given below.
rdf_name_space('http://www.w3.org/1999/02/22-rdf-syntax-ns#').
rdf_name_space('http://www.w3.org/TR/REC-rdf-syntax').
The above defined load_rdf/[2,3] is not always suitable. For example, it cannot deal with documents where the RDF statement is embedded in an XML document. It also cannot deal with really large documents (e.g. the Netscape OpenDirectory project, currently about 90 MBytes), without huge amounts of memory.
For really large documents, the sgml2pl parser can be
programmed to handle the content of a specific element (i.e. <rdf:RDF>)
element-by-element. The parsing primitives defined in this section can
be used to process these one-by-one.
dialect(xmlns) output option. XML is either a
complete <rdf:RDF> element, a list of RDF-objects
(container or description) or a single description of container.
Exploits the call-back interface of sgml2pl, calling
OnTriples(Triples, File:Line) with the list of
triples resulting from a single top level RDF object for each RDF
element in the input as well as the source-location where the
description started.
Input is either a file name or term stream(Stream).
When using a stream all triples are associated to the value of the
base_uri option. This predicate can be used to process
arbitrary large RDF files as the file is processed object-by-object. The
example below simply asserts all triples into the database:
assert_list([], _).
assert_list([H|T], Source) :-
assert(H),
assert_list(T, Source).
?- process_rdf('structure,rdf', assert_list, []).
Options are described with load_rdf/3.
The option
expand_foreach is not supported as the container may be in
a different description. Additional it provides embedded:
rdf:RDF
elements. If this option is false (default), it gives a
warning on elements that are not processed. The option embedded(true)
can be used to process RDF embedded in xhtml without warnings.