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At 4:39 PM -0800 12/7/01, Jeff Lowery wrote: >Speaking of XML that is not: > >Today I've been staring at an industry spec whose XML data model is not >definable by any schema language that I know of. Validation is currently >done by what I call "little gray boxes": open-source executables that >validate the XML document in nice, lengthy, idiosyncratic C++. > There are some quite reasonable constraints that are never going to be able to be expressed in anything less than a Turing complete language (e.g. verifying that the content of an element is a prime number). There are other constraints that can't be expressed without access to the full API to access data external to the document itself. (e.g. looking up the contents of the a record in a remote database to make sure the price matches the PRICE element in the document.) And there are still other constraints that aren't going to be solved by anything short of actual intelligence (Verify that the text of the document doesn't contain anything that's blatantly offensive.) So far Schematron's the most powerful language, but it still hasn't reached Turing completeness. Using actual program code lets your schemas verify criteria of types one and two. Type three is still an area of active research. But existing schema languages seem mostly stuck at Type 0. -- +-----------------------+------------------------+-------------------+ | Elliotte Rusty Harold | elharo@m... | Writer/Programmer | +-----------------------+------------------------+-------------------+ | The XML Bible, 2nd Edition (Hungry Minds, 2001) | | http://www.ibiblio.org/xml/books/bible2/ | | http://www.amazon.com/exec/obidos/ISBN=0764547607/cafeaulaitA/ | +----------------------------------+---------------------------------+ | Read Cafe au Lait for Java News: http://www.cafeaulait.org/ | | Read Cafe con Leche for XML News: http://www.ibiblio.org/xml/ | +----------------------------------+---------------------------------+
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