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Re: [xml-dev] annotations outside validation

From: "Simon St.Laurent" <simonstl@simonstl.com>

> On Tue, 2001-11-13 at 08:21, Eric van der Vlist wrote:
> > > All this discussion of IDs is making me wonder if maybe it isn't time to
> > > consider creating a class of attributes which aren't considered for
> > > validation of any kind, DTD or schema.
> > 
> > <!--    why not using comments    -->
> > <?or even="processing instructions"?>
> > 
> > ???
> > 
> > These are one of the few constructions which escape to validation (even 
> > though there is no reason why one wouldn't invent a schema language 
> > which could validate them as well)!
> Sure!  Those are good examples of the kind of flexibility I'd like to
> see.  The only problem is that there are no clear rules for how those
> items relate to particular elements or attributes.  
The problem is not with needing new kinds of attributes.

The problem is caused by using schema languages which are closed by
default. Closed schema languages are a hangover from DBMS, where
indexing by constant offsets into data structures is an important
optimization.  And perhaps from compression systems where invariance
can be used for optimised compression. Why should this be an important 
consideration for generic, ordinary XML? 

Imagine a schema language that took the namespace mechanism seriously
enough to let you put elements or attributes from any different namespace
anywhere: open by default unless the schema specifically denies it for some
elements.   And imagine if this schema language also aided this by not forcing 
any spurious decisions about element ordering, where none was strictly required.   
If you used such a schema language it would be easy to write your schemas 
to maintain openness. 

Schematron is one such language. (XML Schemas is not, because you have
to do extra work to get openness rather than it being the default.)

Schema languages which default to closed stifle freeform modularity 
using namespaces.

Rick Jelliffe