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RE: Can XML Schemas do this?

  • To: "Bryce K. Nielsen" <bryce@s...>,"xml-dev" <xml-dev@l...>
  • Subject: RE: Can XML Schemas do this?
  • From: "Dare Obasanjo" <dareo@m...>
  • Date: Tue, 21 Jan 2003 14:05:05 -0800
  • Thread-index: AcLBlUpWv/Dd+KyRTRyb27hwe9UcbAAAwxFA
  • Thread-topic: Can XML Schemas do this?

xml schema unordered elements
It seems my last mail on this didn't get through or was ignored. Anyway
here goes again, there are serious implementation concerns with
supporting unordered elements in combination with minOccurs/maxOccurs in
W3C XML Schema because the FSM would become quite large. 

The W3C XML Schema recommendation created the xs:all compositor which
allows one to create unordered content models in limited scenarios. It
turns out that after some implementation experience on the part of
schema processor developers we have realized that these rules were
conservative and something akin to RELAX NG's <interleave> is workable.
However general unorderedness is still difficult to validate in
combination with all the other rules of W3C XML Schema especially
derivation. 

-- 
PITHY WORDS OF WISDOM 
An expert is one who knows more and more about less and less until he
knows absolutely everything about nothing.                            

This posting is provided "AS IS" with no warranties, and confers no
rights. 

>  
>  
> -----Original Message-----
> From: Bryce K. Nielsen [mailto:bryce@s...] 
> Sent: Tuesday, January 21, 2003 1:35 PM
> To: 'xml-dev'
> 
> > Plus, this HTML example should also highlight issues about 
> streaming, 
> > buffering and incremental processing.
> >
> 
> Yes, but I think HTML highlights the perfect reason for 
> having both ordered and unordered elements. So we say the 
> <HTML> node needs to be ordered, but the <BODY> node 
> shouldn't be. The browser shouldn't care if it receives a <P> 
> node before or a <BR>, or a <TABLE> in front of a <UL> node. 
> So, in XSDs I suppose you'd have to describe this like this:
> 
> <sequence minOccurs="0" maxOccurs="unbounded">
>   <element name="P"/ minOccurs="0" maxOccurs="1"/>
>   <element name="BR"/ minOccurs="0" maxOccurs="1"/>
>   <element name="TABLE"  minOccurs="0" maxOccurs="1"/>
>   <!-- repeat for the other HTML tags --> </sequence>
> 
> That just seems really backwards to me that when order isn't 
> important, why should the schema language make it important? 
> Again, I understand that at times, ordered-elements is very 
> necessary, I just also think there should be allowances for 
> unordered-elements (and IMHO feel flexible XML should be the default).
> 
> -BKN
> 
> P.S. Maybe I misunderstood the XML spec, but I didn't get the 
> impression that element-order was important to just XML. I 
> thought it was something that schema languages have introduced?
> 
> 
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