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   Re: Access Languages are Tied to Schemas

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  • From: "Rick Jelliffe" <ricko@allette.com.au>
  • To: <xml-dev@ic.ac.uk>
  • Date: Fri, 21 Nov 1997 13:09:11 +1100

> From: Mark Baker <markb@iosphere.net>

> On Thu, 20 Nov 1997, Joe Lapp wrote:
> > When we evaluate the capabilities of SGML/XML to support object models,
> > I think we need to take client behavior into account.  The repository is
> > acting more like a file system for the state information of objects, and
> > the clients are more like applications that use the file system.
> No, I think that's what we should be trying to stay away from.
> XML is self-describing structured storage - for anything you want to shove 
> in it.  Implementation, state, properties, events, behavioural semantics, 
> whatever.

I think this is a good point.

XML/SGML is a markup language (it is concerned with the mechanics of
constraining, labelling and pointing to user-defined hierarchical information) 
not a data modeling language.  This neutrality is its weakness, in that 
may will be suboptimal for any specific job, compared to what you might 
do if you have all the resources and brains to tailor a specific notation
and train everyone up in it.  

However, most people can only learn a small handful of languages, so
having a standard markup language frees people's brains to concentrate
on the distinguishing specifics of their information, rather than juggling
many different notations in their brains.

This neutrality also explains why XML's content model system is so simple.
SGML has a more complex content model system (inherited inclusions and exclusions,
and a "required anywhere" connector "&"), but they have been found in practise
to complicate matters more than seems warranted.  So I think it is useful 
to not think of the "poverty" of XML content models, but rather their "modesty"
and "neutrality". 

Rick Jelliffe

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