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  • From: Paul Prescod <paul@prescod.net>
  • To: xml-dev@ic.ac.uk
  • Date: Thu, 18 Mar 1999 13:26:42 -0600

Chris Lilley wrote:
> Unfortunately I came across EBNF long before I came accross DTD syntax,
> so about half an hour after meeting DTDs I was, like, what do you mean
> it can't express that this attribute is a url? Why can't it express that
> this attribute is an ISO standard date?

I can guarantee you today that the XML schema effort will not allow you to
express everything that EBNF will so if that's your standard it will fail.
But even if we use EBNF as our standard: do you know of any programming
languages expressed entirely in EBNF? Or even entirely in *any formalism*?

> Yes, validation is important - and I mean real validation, with no
> critical-path human-readable comments in the DTD and multiple utilities
> to check different aspects of validity (like separate scripts to ensure
> that an attribute is a valid date or customer number).

It will never be the case that it will be possible to write schemas that
are so tight that they remove the need for comments that describe
additional constraints to other human beings. There will always be a need
not only for multiple schema languages but also for the ultimately
flexible schema language: prose text.

Luckily, eliminating all other schema languages is not a goal of the W3C
schema language effort. 

> So what is critically needed is a real, namespace-aware, schema 
> language that can be used to do real validation.

I hear a lot of users saying that. They don't seem to realize that there
is no such thing as "real validation" there is only "the validation I need
to do today." Ten years from now, we'll be griping that XMLSchemas don't
do "real validation" for some other arbitrarily advanced definition of

 Paul Prescod  - ISOGEN Consulting Engineer speaking for only himself

"A year ago, when Ernest Pecounis said he wanted to bring
Linux into the state agency he works for, there was a swell of
laughter from his colleagues. Guess who's laughing now."
 - http://www.zdnet.com/pcweek/stories/news/0,4153,393443,00.html

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