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   Re: [xml-dev] [schema] which assertions can't you express ?

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Chiusano Joseph wrote:
> Philippe Poulard wrote:
>>An XML Schema is the expression of some assertions expected on an XML
>>document class. 
> That's an excellent definition, one of many possible definitions.
>>Assertions on XML documents ensure that applications
>>will process them without causing faults. 
> If by "them" you mean XML documents,


then that would not be true, as
> there are many other factors - for example, did the schema

i wasn't talking about a schema, but about assertions expected, that 
some may be express in a schema, others in programs

cover all of
> the required validations per the requirements (those that XML schema is
> capable of covering)? And then there may be requirements that XML schema
> cannot cover, some of which you discuss below.
>>Expressing assertions with
>>schemas ensure that applications developpers will spend most of their
>>time in designing data process and few of their time in controlling them.
> Again, it depends on the requirements and what is required beyond what
> XML schema can handle and therefore may have to be enforced in
> application code, database triggers, etc.

well, i don't talk yet about implementations, just about kinds of 
assertions, in the aim to draw up the list of those that could be 
expressed in a schema, and those that is a matter for applications

however, when one thinks about xml documents all along their process 
chain, one may expect that the schema may cover some assertions at a 
time and others at another time, according to the life-cycle of the 
document, given by some accessible datas ; that's a requirement, i think 
; an alternative solution is to have an external schema selector, that 
chooses the right schema from a set

>>Any schema technology is designed to cover numbers of assertions
>>expressed. However, if think that the existing schema technologies (DTD,
>>W3C XML Schema, Relax NG, Schematron) can't express some constraint types.
> Speaking only for XML Schema, that is true.
>>I have some ideas of what these constraint types might be, but I'd like
>>to know, helped by your own experiences, which kind of useful assertions
>>can't be expressed with these schema technologies (so frustrating).
>>For example :
>>-i'd like that the number of occurrences of an element is equal to a
>>value of a given attribute (Schematron could do that)
> Cannot be done with XML Schema.
>>-i'd like that the value of a given attribute exists in my RDBMS
> Cannot be done with XML Schema, but OASIS CAM can handle this.


> Hope that helps--


           (. .)
|   Philippe Poulard    |


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