Re: Proposal Announcement - XML DTDs to XML docs
Simon St.Laurent wrote: > > I'd like to announce the posting of an unofficial and incomplete proposal for > the representation of XML DTDs as XML documents at: We discussed this in the XML working group. The idea does have the benefits you list in your proposal. But there are more disadvantages than you have listed, and some hard problems to be solved. Nobody has ever done the work to flesh it out to the point where an XML-document notation is as expressive as the current DTD notation. I believe that people who have not been using the DTD notation for a long time underestimate the extent to which common use depends on weird stuff like parameter entities specifying partial content models, element types in element type declarations and so forth. It might be possible to build an XML-document notation that was as expressive as XML DTDs (or at least expressive enough) and didn't use any text substitution wizardry, but nobody has done that yet. This is not a problem if you are content having your XML-syntax used only by those who build simple DTDs. There are other, more subtle problems. Having a single notation appeals mathematically to those who like consistent, recursive structures, but it is not clear that end-users fall into that category. A user interface (markup languages *are* user interfaces) can be too consistent, if it obscures the differences between things. In the case of documents and DTDs, I expect many users would get confused about the distinction between documents and DTDs if DTDs *were* documents. There is also the issue of compatibility. If the DTD for DTDs is extensible and open, as most proponents argue it should be, then Microsoft, Netscape and Sun can all take shots at "extending" it in the way that they "extended" HTML. If the DTD DTD was specifically designed to be extensible, then we could not complain about that. Depending on the level of extensibility, XML documents could actually parse differently depending on which browser you were using. If it was designed NOT to be extensible, then we have to cross one of the benefits of this alternate notation off of the list. Another problem is "specification encapsulation." XML 1.0 is specifically designed NOT to depend on XLink or XPointer. Your proposal depends on them. It seems to me that there is some sort of circularity problem there. Several of my complaints about your proposal stem from the fact that DTDs both change the parse and validate the document. In other words, they are both schemata and "parse information providers". If your XML-instance DTDs only validated, then many of the complaints would go away. But if they only validated, I don't think it would be accurate to call them DTDs anymore. Then they would be just "schemas" since they would accomplish only one of the DTD's two functions. I suspect that these functions will, in fact, become more and more distinct as time goes by. So proposals that keep them together should probably not succeed. Our current DTD syntax is quite reasonable and efficient for the task of declaring entities, setting default attributes and so forth. If we want extensible, XML-instance notation schemata, then we should probably forget about replacing DTDs and just define extensible, XML-instance schemata (i.e. XML-Data) and leave DTDs to do the other tasks. Paul Prescod - http://itrc.uwaterloo.ca/~papresco "A writer is also a citizen, a political animal, whether he likes it or not. But I do not accept that a writer has a greater obligation to society than a musician or a mason or a teacher. Everyone has a citizen's commitment." - Wole Soyinka, Africa's first Nobel Laureate 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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