SWI-Prolog RDF parser
Jan Wielemaker
SWI,
University of Amsterdam
The Netherlands
E-mail: jan@swi.psy.uva.nl
| RDF (Resource Description Format) is a W3C standard for expressing meta-data about web-resources. It has two representations providing the same semantics. RDF documents are normally transferred as XML documents using the RDF-XML syntax. This format is unsuitable for processing. The parser defined here converts an RDF-XML document into the triple notation. |
RDF is a promising standard for representing meta-data about documents on the web as well as exchanging frame-based data (e.g. ontologies). RDF is often associated with `semantics on the web'. It consists of a formal data-model defined in terms of triples. In addition, a graph model is defined for visualisation and an XML application is defined for exchange.
`Semantics on the web' is also associated with the Prolog programming language. It is assumed that Prolog is a suitable vehicle to reason with the data expressed in RDF models. Most of the related web-infra structure (e.g. XML parsers, DOM implementations) are defined in Java, Perl, C or C++.
Various routes are available to the Prolog user. Low-level XML parsing is due to its nature best done in C or C++. These languages produce fast code. As XML/SGML are at the basis of most of the other web-related formats we will benefit most here. XML and SGML, being very stable specifications, make fast compiled languages even more attractive.
But what about RDF? RDF-XML is defined in XML, and provided with a Prolog term representing the XML document processing it according to the RDF syntax is quick and easy in Prolog. The alternative, getting yet another library and language attached to the system, is getting less attractive. In this document we explore the suitability of Prolog for processing XML documents in general and into RDF in particular.
We realised an RDF compiler in Prolog on top of the sgml2pl package (providing a name-space sensitive XML parser). The transformation is realised in two passes.
The first pass rewrites the XML term into a Prolog term conveying the same information in a more friendly manner. This transformation is defined in a high-level pattern matching language defined on top of Prolog with properties similar to DCG (Definite Clause Grammar).
The source of this translation is very close to the BNF notation used by the specification, so correctness is `obvious'. Below is a part of the definition for RDF containers. Note that XML elements are represented using a term of the format:
element(Name, [AttrName = Value...], [Content ...])
memberElt(LI) ::=
\referencedItem(LI).
memberElt(LI) ::=
\inlineItem(LI).
referencedItem(LI) ::=
element(\rdf(li),
[ \resourceAttr(LI) ],
[]).
inlineItem(literal(LI)) ::=
element(\rdf(li),
[ \parseLiteral ],
LI).
inlineItem(description(description, _, _, Properties)) ::=
element(\rdf(li),
[ \parseResource ],
\propertyElts(Properties)).
inlineItem(LI) ::=
element(\rdf(li),
[],
[\rdf_object(LI)]), !. % inlined object
inlineItem(literal(LI)) ::=
element(\rdf(li),
[],
[LI]). % string value
|
Expression in the rule that are prefixed by the \
operator acts as invocation of another rule-set. The body-term is
converted into a term where all rule-references are replaced by
variables. The resulting term is matched and translation of the
arguments is achieved by calling the appropriate rule. Below is the
Prolog code for the
referencedItem rule:
referencedItem(A, element(B, [C], [])) :-
rdf(li, B),
resourceAttr(A, C).
|
Additional code can be added using a notation close to the Prolog DCG notation. Here is the rule for a description, producing properties both using propAttrs and propertyElts.
description(description, About, BagID, Properties) ::=
element(\rdf('Description'),
\attrs([ \?idAboutAttr(About),
\?bagIdAttr(BagID)
| \propAttrs(PropAttrs)
]),
\propertyElts(PropElts)),
{ !, append(PropAttrs, PropElts, Properties)
}.
|
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, 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.
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. (1)
A test-suite and driver program are provided by rdf_test.pl
in the source directory. To run these tests, load this file into Prolog
in the distribution directory. The test files are in the directory
suite and the proper output in suite/ok.
Predicates provided by rdf_test.pl:
suite/tN.rdf and
display the RDF source, the intermediate Prolog representation and the
resulting triples.
suite/tN.rdf and store the resulting triples in
suite/ok/tN.pl for later validation by test/0.
It took three days to write and one to document the Prolog RDF parser. A significant part of the time was spent understanding the RDF specification.
The size of the source (including comments) is given in the table below.
| lines | words | bytes | file | function |
| 109 | 255 | 2663 | rdf.pl | Driver program |
| 312 | 649 | 6416 | rdf_parser.pl | 1-st phase parser |
| 246 | 752 | 5852 | rdf_triple.pl | 2-nd phase parser |
| 126 | 339 | 2596 | rewrite.pl | rule-compiler |
| 793 | 1995 | 17527 | total |
We also compared the performance using an RDF-Schema file generated by Protege-2000 and interpreted as RDF. This file contains 162 descriptions in 50 Kbytes, resulting in 599 triples. Environment: Intel Pentium-II/450 with 384 Mbytes memory running SuSE Linux 6.3.
The parser described here requires 0.15 seconds excluding 0.13 seconds Prolog startup time to process this file. The Pro Solutions parser (written in Perl) requires 1.5 seconds exluding 0.25 seconds startup time.
Installation on Unix system uses the commonly found configure,
make and make install sequence. SWI-Prolog should be
installed before building this package. If SWI-Prolog is not installed
as pl, the environment variable PL must be set to
the name of the SWI-Prolog executable. Installation is now accomplished
using:
% ./configure % make % make install |
This installs the Prolog library files in $PLBASE/library,
where
$PLBASE refers to the SWI-Prolog `home-directory'.
Run the file setup.pl by double clicking it. This will
install the required files into the SWI-Prolog directory and update the
library directory.
process_rdf(+File,
+BaseURI, :OnTriples(process_rdf(+File, +BaseURI, :OnTriples)
This order is still recognised for compatibility.