![](https://csdnimg.cn/release/download_crawler_static/4983739/bg4.jpg)
– restrictions to a subset of datatype values (e.g., a
gale is a wind whose speed is in the range from 34
to 40 knots);
– relationships between values of data properties on
one object (e.g., a square table is a table whose
breadth equals its depth);
– relationships between values of data properties on
different objects (e.g., people who are older than
their boss); or
– aggregation functions (e.g., the duration of a pro-
cess is the sum of the durations of its subpro-
cesses).
Another important limitation of the datatype
support in OWL 1 is the lack of a suitable set of
built-in datatypes. OWL 1 relies on XML Schema
11
for the list of built-in datatypes. The design of OWL
1, however, did not involve a thorough analysis of
which XML Schema datatypes were appropriate for
OWL 1. OWL 1 only requires the implementation
of xsd :string and xsd :integer , and leaves the im-
plementation of other XML Schema datatypes as
optional. A subsequent analysis has revealed that
datatypes in XML Schema and OWL 1 are used in
different ways, so not all datatypes of one formal-
ism are appropriate for the other and vise versa.
In particular, many XML Schema datatypes can
be difficult to implement in OWL 2. The datatypes
xsd:double and xsd :float are finite and are thus
equivalent to very large disjunctions, which can
be a significant source of inefficiency. Furthermore,
datatypes that represent time intervals, such as
xsd:gDay and xsd:time, require the representation
of infinite sets of irregular intervals; currently, it is
not clear whether and how such datatypes should
be implemented in practice.
2.1.4. Keys
OWL 1 DL does not provide means for express-
ing key constraints on data properties, which are a
core feature of database technologies. For example,
in OWL 1 DL it is not possible to state that “US
citizens are uniquely identified by their social se-
curity number.” Such statements can be expressed
in OWL 1 Full by means of inverse-functional data
properties; however, since no implementations of
OWL 1 Full are available (see Section 2.7), there
was no practical reasoning support for such state-
ments. Furthermore, no variant of OWL 1 supports
compound key constraints on either data or object
11
http://www.w3.org/TR/xmlschema-2/
properties, such as that “each address is uniquely
identified by its street, street number, postcode,
and country.”
The lack of keys in OWL 1 has been recognized as
an important limitation in expressive power. Unfor-
tunately, adding keys in its full generality to OWL
1 would harm the computational properties of the
language and could even lead to undecidability[23].
2.2. Syntax Issues
OWL 1 comes with two normative syntaxes: the
Abstract Syntax
12
and OWL 1 RDF.
13
The stan-
dard also defines an XML syntax, but this syntax
is not normative and it has not been widely used,
so we do not discuss it here. The Abstract Syntax
serves as the actual definition of the language, and
it has often been used as basis for the design of
OWL 1 APIs. Certain design choices taken in OWL
1 Abstract Syntax, however, have made the syntax
confusing for developers, which resulted in the sub-
optimal design of OWL APIs. Furthermore, both
syntaxes have proved difficult to parse correctly. Fi-
nally, the relationship between the two syntaxes is
rather complex, which causes problems when trans-
forming an ontology from one syntax into the other.
We next discuss these problems in more detail.
2.2.1. Frame-Based Paradigm
The design of the OWL 1 Abstract Syntax
has been heavily influenced by the tradition of
frame-based ontology languages. The frame-based
paradigm was already familiar to many users, and
has proved natural and popular. Consider the OWL
1 DL axiom (1), which declares a class Tiger and
states that it is a subclass of the class Cat:
Class(Tiger partial Cat) (1)
In a frame-based system, this axiom is usually inter-
preted as a frame specification for the class Tiger ,
which typically acts as a declaration—a statement
that a class exists in an ontology—and groups all
relevant properties of the class in one place.
Although the frame-based paradigm is some-
times useful for modeling, the logical underpinning
of OWL 1 is actually provided by the somewhat
different paradigm of description logics (DLs). The
fundamental modeling concept in DLs is not a
frame, but an axiom—a logical statement about
12
http://www.w3.org/TR/owl-semantics/
13
http://www.w3.org/TR/owl-ref/
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