[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Resource Description Format and XML-Data
I agree with much of what Eliot said, most particularly that XML validation must continue to be the focus of XML. Replacement syntax and semantics for DTDs should be compiled to DTDs, and/or DTDs plus _additional_ validation processes. I also have seen no effective refutation of Eliot and Rick's analysis of how semantic association can be done _within_ XML as it stands. On the other hand, it does no good to overplay your cards. Eliot's assertion that extended DTD syntaxes will _never_ solve the semantics problems is over-stated in two ways. First, it's not true if we're talking about solveing single, limited problems. For instance, it's clearly true that solving _some_ semantics problems would be possible by adding to the power of DTDs. For instance, the addition of regexp mapping on content would _increase_ the range of validatable conditions on documents, and might even enable some applications to avoid referring to extended semantics. On the other hand, each such single extension solves only a single problem. Extensibility is still likely to be a requirement. Even this limitation doesn't mean that we couldn't solve the schema problem for once and for all. A Turing complete "semantics testing" language would allow the verification of all computable properties of XML documents. There are some serious disadvantages to this as an apporach: Most interesting properties of arbitrary programs are uncomputable. Unlike the kinds of declarative conditions often mentioned for DTD extensions, arbitrary computations can be intractable. Further, it is only rarely that we need the ability to specify validity conditions like "this document is valid if and only if Goldbach's conjecture is true." I think reactions to proposals for schema languages would be more generally positive if they concentrated on how to supplement DTDs and specify things that are impossible in XML (like content restrictions) or extremely difficult (like modular DTDs) rather than also duplicating DTD functionality. The hard thing is deciding what semantic features will be widely useful enough that failing back to arbitrary computations will be unnecessary, or at least extremely rare. Making it clear that such languages are supplements to DTD specification techniques also removes the (valid) criticism that they are duplicating facilities already in DTDs, and the political suspicion that they are an attempt to make an end-run around a delcaration syntax that is accepted and standardized, if not well beloved of all. -- David 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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