一日一段 －0713 Another important difference is that RDF Schema descriptions are not necessarily prescriptive in the way programming language type declarations typically are. For example, if a programming language declares a class Book with an author attribute having values of type Person, this is usually interpreted as a group of constraints. The language will not allow the creation of an instance of Book without an author attribute, and it will not allow an instance of Book with an author attribute that does not have a Person as its value. Moreover, if author is the only attribute defined for class Book, the language will not allow an instance of Book with some other attribute.
另外一个重要的区别是：RDF Schema的描述不一定像程序语言类型声明那样是规约性的(prescriptive).例如，如果在程序语言中，类 Book定义为有一个属性author,且author的类型是Person,这常会解释为一些约束。这个语言不会允许Book的一个实例没有author属性，也不会允许Book的一个实例的book属性值的类型不是Person。更为重要的是，如果author的定义为类Book的唯一属性，这个语言不会允许Book的实例具有其他的属性。
RDF Schema, on the other hand, provides schema information as additional descriptions of resources, but does not prescribe how these descriptions should be used by an application. For example, suppose an RDF schema states that an ex:author property has an rdfs:range of class ex:Person. This is simply an RDF statement that RDF statements containing ex:author properties have instances of ex:Person as objects.
相反，RDF Schem提供的模式信息是作为对资源的额外描述(descriptions)，并不会限制这些描述怎样被用于一个应用。例如，假设一个RDF Schema中定义了属性ex:author的值域(rdfs:range)是ex:Person,这仅仅是一个简单的RDF陈述: 谓词是ex:author的陈述的客体是类ex:Person的实例。
This schema-supplied information might be used in different ways. One application might interpret this statement as specifying part of a template for RDF data it is creating, and use it to ensure that any ex:author property has a value of the indicated (ex:Person) class. That is, this application interprets the schema description as a constraint in the same way that a programming language might. However, another application might interpret this statement as providing additional information about data it is receiving, information which may not be provided explicitly in the original data. For example, this second application might receive some RDF data that includes an ex:author property whose value is a resource of unspecified class, and use this schema-provided statement to conclude that the resource must be an instance of class ex:Person. A third application might receive some RDF data that includes an ex:author property whose value is a resource of class ex:Corporation, and use this schema information as the basis of a warning that "there may be an inconsistency here, but on the other hand there may not be". Somewhere else there may be a declaration that resolves the apparent inconsistency (e.g., a declaration to the effect that "a Corporation is a (legal) Person").
这个Schema提供的信息可能以多种方式得到利用。一个应用可能把这个陈述解释为创建RDF数据的模板的一部分，且用它来限制任意ex:author的值都是ex:Person的实例。也就是说，这个应用和程序语言一样,把模式信息解释为约束(constraint). 然而，另外一个应用可能把这个陈述解释为获取的数据的额外信息，这个信息并没有在原数据中显式表示。例如，第二个应用可能收到一些RDF数据，这些数据包含了一个值为未知类型的ex:author属性，则可以利用RDF Schema中的那个陈述推导出这个ex:author属性的值是类ex:Person的实例。 第三个应用可能收到一些RDF数据, 这些数据包含了一个值为ex:Corporation的实例的ex:author属性，利用RDF Schema的信息，可以发出警告：“数据可能不一致，也可能不是”。在其他地方，可能存在一个声明消除了这个数据不一致性（例如，声明一个“Corporation”也是一个“Person”）。
Moreover, depending on how the application interprets the property descriptions, a description of an instance might be considered valid either without some of the schema-specified properties (e.g., there might be an instance of ex:Book without an ex:author property, even if ex:author is described as having a domain of ex:Book), or with additional properties (there might be an instance of ex:Book with an ex:technicalEditor property, even though the schema describing class ex:Book does not describe such a property).
此外，根据应用解释属性描述的方式，一个实例可以被认为是合法的，即使没有RDF Schema指定的属性（如，可能有一些ex:Book的实例没有ex:author属性，即使ex:author的定义域为ex:Book）,或者具有其他的属性（如，可能有一些ex:Book的实例具有ex:technicalEditor属性，即使描述ex：Book的Schema没有对这个属性的描述）。
In other words, statements in an RDF schema are always descriptions. They may also be prescriptive (introduce constraints), but only if the application interpreting those statements wants to treat them that way. All RDF Schema does is provide a way of stating this additional information. Whether this information conflicts with explicitly specified instance data is up to the application to determine and act upon.
换句话说，RDF Schema中的陈述通常是描述(descriptions),他们可能是规约性的,但只有当应用这样解释时。RDF Schema做的事情时提供额外的信息。这些信息是否和显式指定的实例数据有冲突，取决于具体的应用。
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2004/7/12 15:38:00
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