xml-dev - Re: XSchema Spec Section 2.2, Draft 1

[Paul Prescod <papresco@technologist.com>]

On Wed, 10 Jun 1998, John Cowan wrote:
> 
> I think that this suggestion (loosening what is allowed in a content
> model) has some merit to it, although IMHO it would need to be
> a hypothetical AnyElement rather than true ANY, to preserve
> unambiguous parsing.

First: XSchemas do not drive parsing, but verification.

Second: What is the importance of unambiguous verification? The SGML 
world has been trying to get rid of that albatross for years, and XML 
would have done so had it had the opportunity. 

http://www.w3.org/TR/REC-xml#determinism

> What "festering hole"?  XML's solution is simple: all whitespace is
> meaningful, just as all non-whitespace is meaningful.  

That is not true. Some whitespace is "insignificant." You can only find
out which whitespace falls into this category with a validating parser, 
however. 

http://www.w3.org/TR/REC-xml#sec-white-space

This is a mess, because the validator's idea of what is ignorable is
necessarily different from that of applications built on top of
non-validating parsers. Whitespace which is NOT considered PCDATA in one
program (especially validating parsers) will be considered PCDATA in
another.

Although whitespace in XML is gross, I don't claim that there is a better 
solution. Merely removing the concept of significance may be simpler, but 
in my opinion that "solution" has its own problems.

> I don't claim familiarity with SGML theory or practice, but I note
> the existence of something called the "SGML mixed content problem".
> As far as I can see, the standard solutions to this problem are
> exactly what XML mandates: the simple (#PCDATA | foo | bar)* content
> models.

The mixed content problem is related to the grossness of whitespace. 
Since our role is merely to verify markup, and not to interpret it for an 
application, I think that we can get around it easily. Whitespace 
verifies as #PCDATA if #PCDATA is allowed at a particular point. 
If #PCDATA is NOT allowed at a particular point, the whitespace is 
ignored for the purposes of verifying.

 Paul Prescod


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