[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Inheritance in XML (was Re: Problems parsing XML)
Matthew Gertner: >Some of the basic tenets of OO (encapsulation, polymorphism) are only >applicable when behavior is modelled, but I would maintain that others >(inheritance, identity) are equally applicable to data. The two last >examples would both be of huge benefit to XML and are both currently >lacking. I agree absolutely. I have found identity and subtyping to be the two biggest benefits in using an object database over a relational database. >Nevertheless, inheritance of some sort is absolutely vital if XML is to >fulfill its promise. If we can't produce standard DTDs which can be >extended, *without* modifying the base DTD, then many of the advantages of >XML go out the window. I agree that this is central. Let's leave identity out of the discussion, as that does, I think, fall into the XML Linking domain, and concentrate on what I prefer to call subtyping. There's a lot of stuff in the SGML culture that one could fall back on: architectural forms etc, but I for one find it extremely arcane and difficult to relate to my own domain of object modelling and database design, which I think is familiar to a much wider community. I know some people will disagree, but the way I use XML, a DTD is a schema, an element definition in a DTD is a class, a document is a database, and an element within a document is an instance of a class. What is missing is that we can't define one class (element type) as a subtype of another. Since we are only concerned with structural subtyping and not with behaviour, I don't think it would actually be difficult to define this concept. The main thing that's tricky is that you can get the "is-a" the wrong way round. If a PREFACE is-a-kind-of CHAPTER, that means you can find anything (elements, attributes) in a PREFACE that you can find in a chapter, and more besides. It also means you can reduce a PREFACE to a CHAPTER by removing these extra bits. I'm not entirely sure what "removing the extra bits" means: for example should it remove elements that cannot occur in a CHAPTER, or should it just remove the tags that surround those elements? This tends to show up the lack of semantics in the object model underlying XML. Just some thoughts... Mike Kay, ICL xml-dev: A list for W3C XML Developers. To post, mailto:xml-dev@i... Archived as: http://www.lists.ic.ac.uk/hypermail/xml-dev/ To (un)subscribe, mailto:majordomo@i... the following message; (un)subscribe xml-dev To subscribe to the digests, mailto:majordomo@i... the following message; subscribe xml-dev-digest List coordinator, Henry Rzepa (mailto:rzepa@i...)
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