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Subject: Re: [relax-ng] close schema language

David RR Webber - XMLGlobal wrote:

> Message text written by Eric van der Vlist
> Schematron is the only language I know where "everything which is not 
> forbidden is allowed", the main principle of the other languages being 
> rather that "everything which is not allowed is forbidden" which, by 
> itself, is "closed".
> <<<<<,<''
> Eric - excellent points.


> Having worked for many years in Prolog - there's another facet to
> this declarative / assetive world which can be fun - ie - somethings
> that you never even realized can also be allowed - together. 
> It teaches you that you must always explicitly assert the
> "Otherwise - do nothing" facts - which is definately a good habit
> to get into.
> Java - throw / catch users note - weird behavioural treatment of
> fringe error conditions relates!!

Yes. There is a difference, though between a schema language (especially 
when it is used to validate documents) and a development language and I 
think that most of the people see a XML schema as a kind of firewall 
which will protect them from the unexpected -these "otherwise" facts-.

Foreign namespaces (and "open" schemas in general) are then seen as 
potentially dangerous since it's like removing a part of the firewall 
exposing the applications to more unexpected facts and requiring a 
different design.

Among the RSS 1.0 Working Group, for instance, everyone agreed to accept 
foreign namespaces elements, but the majority decided to accept them 
only when the content model (simple/complex) was not changed showing 
that the notion of open/close is relative ;=) ...

Rendez-vous  Paris pour le Forum XML.
Eric van der Vlist       http://xmlfr.org            http://dyomedea.com
http://xsltunit.org      http://4xt.org           http://examplotron.org

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