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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@metalab.unc.edu | Writer/Programmer |
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| The XML Bible, 2nd Edition (Hungry Minds, 2001) |
| http://www.ibiblio.org/xml/books/bible2/ |
| http://www.amazon.com/exec/obidos/ISBN=0764547607/cafeaulaitA/ |
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