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Re: UPA and schema handling

• To: xml-dev@l...
• Subject: Re: UPA and schema handling
• From: Burak Emir <Burak.Emir@e...>
• Date: Mon, 18 Oct 2004 15:25:00 +0200
• In-reply-to: <20041018092126.WXZX28128.mta05-svc.ntlworld.com@Turtle>
• References: <20041018092126.WXZX28128.mta05-svc.ntlworld.com@Turtle>
• User-agent: Mozilla Thunderbird 0.7.3 (X11/20040803)

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
>question.
>
>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:-)

cheers,
Burak

http://lamp.epfl.ch/~buraq

• References:
  • RE: UPA and schema handling
    • From: "Michael Kay" <michael.h.kay@n...>

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