[Semantic Web]Future Extensions of OWL

读书笔记 Lee 发表于 2006/3/3 23:02:22 Clearly, OWL is not the final word on ontology languages for the Semantic Web. A number of additional features have already been identified in the OWL Requirements Document, and many others are under discussion.   Modules and Imports Importing ontologies defined by others will be the norm on the Semantic Web. However, the importing facility of OWL is very trivial: it only allows importing of an entire ontology, specified by location. Even if one would want to use only a small portion of another ontology, one would be forced to import that entire ontology. Module-constructions in programming languages are based on a notion of information hiding: the module promises to provide some functionality to the outside world (the export clause of the module), but the importing module need not concern itself with how this functionality is achieved. It is an open research question what a corresponding notion of information hiding for ontologies would be, and how it could be used as the basis for a good import construction Defaults Many practical knowledge representation systems allow inherited values to be overridden by more specific classes in the hierarchy, treating the inherited values as defaults. Although this is widely used in practice, no consensus has been reached on the right formalization for the nonmonotonic behaviour of default values. Closed-World Assumption The semantics of OWL currently adopts the standard logical model of an open-world assumption: a statement cannot be assumed true on the basis of a failure to prove it. Clearly, on the huge and only partially knowable World Wide Web, this is the correct assumption. Nevertheless, the opposite approach (a closed-world assumption: a statement is true when its negation cannot be proved) is also useful in certain applications. The closed-world assumption is closely tied to the notion of defaults and leads to the same nonmonotonic behaviour, a reason for it not to be included in OWL. Unique-Names Assumption Typical database applications assume that individuals with different names are indeed different individuals. OWL follows the usual logical paradigm where this is not the case. If two individuals (or classes or properties) have different names, we may still derive by inference that they must be the same. As with the non-closed-world assumption, the non-unique-names assumption is the most plausible one to make on theWorldWideWeb, but as before, situations exist where the unique-names assumption is useful. More subtly, one may want to indicate portions of the ontology for which the assumption does or does not hold. Procedural Attachment A common concept in knowledge representation is to define the meaning of a term not through explicit definitions in the language (as is done in OWL) but by attaching a piece of code to be executed for computing the meaning of the term. Although widely used, this concept does not lend itself very well to integration in a system with a formal semantics, and it has not been included in OWL. Rules for Property ChainingAs explained previously, for reasons of decidability OWL does currently not allow the composition of properties, but of course in many applications this is a useful operation. Even more generally, one would want to define properties as general rules (Horn or otherwise) over other properties. Such integration of rule-based knowledge representation and DL-style knowledge representation is currently an active area of research.

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