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   RE: [xml-dev] Attribute Order

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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 

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 |
|          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/      |
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