- From: Frank Manola <fmanola@mitre.org>
- Date: Fri, 26 Apr 2002 07:15:15 -0400
- To: w3c-rdfcore-wg@w3.org
Dan asked me to post this (originally sent off-list) to the list: Dan-- Here are some comments on "RDF Vocabulary Description Language 1.0: RDF Schema" http://www.w3.org/2001/sw/RDFCore/Schema/200203/ Under Status: "The Working Group specifically solicit feedback from reviewers regarding the title of this specification. During CR review and subsequently, many commentators expressed concern about the relationship between RDF Schema and XML Schema." The relationship between RDF Schema and XML Schema may need to be clarified; but I hope that's not because both use the term "schema". "RDF Schema" is fine! Keep in mind that the Web community is not the only relevant community here. The database community in particular is quite used to the idea of a "schema" as precisely the sort of thing being described here (and was using the terminology first). Both RDF and XML schemas are variants of this same theme. We can't possibly invent new terms each time we find some kind of terminology clash (or similarity) when different communities come together. We're supposed to be supporting the Semantic Web, for heaven's sake! If people can't figure out how to distinguish "RDF Schema", "XML Schema", and "database schema", they're in the wrong business. "The RDF Schema class and property system is similar to the type systems of object-oriented programming languages such as Java. However, RDF differs from many such systems in that instead of defining a class in terms of the properties its instances may have, an RDF schema will define properties in terms of the classes of resource to which they apply. This is the role of the rdfs:domain and rdfs:range mechanisms described in this document. For example, we could define the author property to have a domain of Book and a range of Literal, whereas a classical OO system might typically define a class Book with an attribute called author of type Literal." This doesn't really clarify what the difference between these two approaches really is (I tried to do this in the Primer, but it, or an amplification, belongs here). "This specification does not attempt to enumerate all the possible forms of vocabulary description that are useful for representing the meaning of classes and properties in RDF data..." This and the following paragraph are awfully "lofty" sounding. What exactly do they mean? In particular, what is the "common practice" that is being "established", and does RDF Schema have any special status in establishing that practice? Under the RDF Classes table: rdfs:Container is introduced, but then Bag, Seq, and Alt are referred to as "collections". If Bag, etc. are types of Container, this should be stated explicitly. The descriptions of rdfs:Literal, rdfs:Class, and rdfs:Property should explicitly identify them as being classes. Under rdfs:subClassOf: this says the property is "transitive". The meaning of "transitive" should be defined. Under rdfs:range and rdfs:domain: These descriptions still don't say anything about the effect of specifying multiple domain and range properties. The WG has made a decision on this, and it needs to be in the Schema spec. A note could explain any misgivings anyone might have about this. Under "RDF Container Classes and Properties": in each case, the classes are described as "representing" things; e.g., "rdf:Bag class represents RDF's 'Bag" container construct". It's not really very clear what this means. (What actually happens, at least according to the Primer, is that a resource is defined as being a Bag container by having a type property whose value is rdf:Bag; it's not clear this is the same thing). Under rdfs:member: explicitly say what a "super-property" is. The description of rdf:value should explicitly identify it as being a property. The descriptions rdf:subject, rdf:predicate, and rdf:object should explicitly identify them as being properties right away, e.g., something like "rdf:subject A property that defines the subject of an RDF statement." Regarding the Extensibility Mechanisms, speaking as a Primer Editor, I don't see this material going into the Primer if it *isn't* going to be in the Schema spec (and maybe not even then!). "The phrase 'RDF vocabulary' is used here to refer to those resources which evolve over time" Huh? Lots of resources evolve over time. "RDF vocabulary" doesn't refer to all of them. (To add the confusion here, schemas evolve over time too). "The resources defined in RDF schemas are themselves Web resources, and can be described in other RDF schemas." This should be more explicit about what "the resources defined in RDF schemas" are (specifically, they are user-defined classes and properties, right?) --Frank -- Frank Manola The MITRE Corporation 202 Burlington Road, MS A345 Bedford, MA 01730-1420 mailto:fmanola@mitre.org voice: 781-271-8147 FAX: 781-271-875
Received on Friday, 26 April 2002 07:05:02 UTC