OASIS Mailing List ArchivesView the OASIS mailing list archive below
or browse/search using MarkMail.


Help: OASIS Mailing Lists Help | MarkMail Help



   Re: [xml-dev] UPA and schema handling

[ Lists Home | Date Index | Thread Index ]

Michael Kay wrote:

>>It's also obvious that a processor using schema to build a 
>>code model is 
>>  able to ignore UPA and still get its work done.
>Only if its work is confined to giving a yes/no answer to the validity
>If the work is to associate types with individual element instances, then
>UPA is rather important.
There are two things that deserve to be mentioned.

--> Firstly, a simple example involving derivation-by-extension Suppose 
you were allowed to use arbitrary regular expressions (I use short 
notation instead of XML Schema).
type Foo = A*
type Bar extends Foo = A*.

An element of type Bar contains something like A*A*, because derivation 
by extension means "append it at the right end".
And now, there is no way to know which of the A's in a valid instance 
belong to Foo and which to Bar.

This looks strange at first but may turn into a problem if somebody 
extends Foo without knowing (or having control over) what its content 
model is.

--> Secondly, it is possible to rewrite every regular expression in a 
(huge and incomprehensible) form that goes through that so-called UPA 
constraint. This can of course only happen on one level, i.e. no 
derivation by extension involved.

Document-afficionados and everybody who hates UPA, have a party now!  
(at least those who did not yet switch to Relax NG:-)




News | XML in Industry | Calendar | XML Registry
Marketplace | Resources | MyXML.org | Sponsors | Privacy Statement

Copyright 2001 XML.org. This site is hosted by OASIS