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and datatype functions and operators are based on the functions and op-
erators of XQuery [86]. Finally, WSML defines an XML syntax and an
RDF syntax for exchange over the Web. When the RDF syntax is used,
WSML can be seen as an extension of RDFS.
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Frame-based syntax.
Frame logic [72] allows the use of frames in logical
expressions. This allows the user to work directly at the level of concepts,
attributes, instances, and attribute values, instead of at the level of predi-
cates. Furthermore, variables are allowed in place of concept and attribute
identifiers, which enables metamodeling and reasoning over a signature in
the rule-based WSML language variants.
7.6 Relation to RDF(S) and OWL
In Section 7.4 we showed that parts of WSML can be expressed in RDF
and OWL, the most prominent language recommendations in the context of
the Semantic Web. Now let us take a closer look at the ontological modeling
capabilities that WSML offers in comparison with these two languages.
We recall that RDFS (see Section 3.2.1) allows the expression of classes,
properties, class hierarchies, property hierarchies, and domain and range re-
strictions, all of which are provided similarly by WSML. The following two
factors make the layering of more expressive ontology languages on top of
RDFS di
cult:
1. RDFS allows the use of the language vocabulary as subjects and objects
in the language itself.
2. RDFS allows the use of the same identifier to occur at the same time in
place of class, individual, and property identifiers.
We believe that the number of cases where the first feature is used, namely
the use of language constructs in the language itself, is limited. However, the
use of the same identifier as class, individual, and property identifiers (also
called metamodeling) is deemed useful in many cases. WSML does not allow
the use of the language constructs in arbitrary places in an ontology, but does
allow metamodeling in its Flight, Rule and, Full variants.
WSML is an extension of a significant part of RDFS. However, it does not
allow the use of language constructs in the language itself and does not allow
full treatment of blank nodes because this would require reasoning with exis-
tential information (which is not allowed in the rule-based WSML variants).
WSML provides a significant extension of RDFS through the possibility of
specifying local attributes, range and cardinality constraints for attributes,
and attribute features such as symmetry, transitivity and reflexivity. Further-
more, WSML (in its rule-based variants) provides an expressive rule language
which can be used for the manipulation of RDF data.
As for OWL (see Section 3.3), we can easily verify that the logical ex-
pressions in WSML-Core form a semantic subset of OWL Lite. WSML-DL is
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