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Huh? Grammars aren't good at expressing position independence? I'm
having troubling thinking of any grammars for any language (not just
XML) that don't handle constraints similar to the one that the poster
describes. Heck, even attributes are order independent. XML elements
are _all_ about structure, so having a schema that let's someone
describe that structure should have been a higher priority. XML
Schema was a classic case of design by committee with individual
features grafted onto it by the different participants. Heck, even
attributes are order independent.
Don Bate
At 5:45 AM +1100 2/17/05, Rick Jelliffe wrote:
> > John Graybeal wrote:
>>> We are writing XML of the form:
>>>
>>> <element>
>>> <required_element/>
>>> <optional_element1/>
>>> <optional_element2/>
>>> </element>
>>>
>>> We would like to specify (for example), that there can be only one of
>>> required_element, only one of optional_element1, but as many as desired
>>> of optional_element2. Plus, we would like to be able to specify the
>>> elements in any order.
>
>XML Schemas is a grammar-based schema language, which brings with it
>a lot of limitations as far as modeling capabilities. It has some
>extensions (as does RELAX NG) to make it easier to declare some
>simple structures which are otherwise cumbersome to express in regular
>grammars. In particular, grammars typically use element position to key
>occurrence, which makes it difficult to model when occurrences are
>position-independent.
>
>The advantage of using a grammar is that validation and processing can
>be performed using automata and other techniques, which are theoretically
>well understood. (How much this actually helps implementations to be good
>is another matter.)
>
>Many people who use XML Schemas and DTDs (& RELAX NG to a lesser extent)
>come to the realization that they are better off only modelling in
>grammars the kinds of constraints that grammars lend themselves to, and to
>add additional constraints in a different language. For example, you might
>model in your XML Schema the equivalent (in DTD notation) of
> <!ELEMENT element
> (required_element | optional_element | optional_element3)*>
>then add the following Schematron schema to validate the other constraints"
>
><schema xmlns="http://www.ascc.net/xml/schematron";>
>
> <title>Example for John Graybeal</title>
>
> <pattern>
> <rule context="element">
>
> <assert test="count(required_element)=1">
> There must be one required_element</assert>
>
> <assert test="count(optional_element)<=1">
> There may be one optional_element</assert>
>
> </rule>
> </pattern>
></schema>
>
>See Schematron.com for info on Schematron.
>
>Cheers
>Rick Jelliffe
>
>
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--
Don Bate | Specializing in Consulting and Mentoring in
Bate Consulting, Inc | Object-Oriented Technologies,
| Software Architecture, and Software Process
(972) 618-0208 voice
(972) 618-0216 fax
donbate@iadfw.net
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