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   Re: Ambiguity in XML spec

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  • From: "David Brownell" <david-b@pacbell.net>
  • To: "Eric Bohlman" <ebohlman@netcom.com>, "Michael Champion" <Mike.Champion@softwareag-usa.com>
  • Date: Fri, 19 May 2000 12:23:42 +0200

> > > Actually I think the XML spec would have been substantially improved,
> > > in the technical sense, were it to have been directly validated by an
> > > implementation -- using only the standardized productions.
> >
> > I strongly agree, and hope this is addressed in the corrected version of
> > 1.0 if it ever comes out, or in XML 1.1 / 2.0.  A colleague implementing
> > XML parser "from scratch" --  that is, without previous exposure to all
> > folklore that one picks up from SGML, XML-DEV, deconstructing other
> > code, etc. -- stumbled over all sorts of little problems with the
> > standardized productions.

Symptomatic of the problem is the comment I hear not all that
infrequently:  "XML is really SGML, so you just use the SGML
spec to resolve any issues."  Ah, sorry no -- there's no normative
reference to SGML, and there should be no need to know SGML.

> I have to at least partially disagree.  Many practical, widely used
> languages cannot be specified *entirely* by EBNF productions and thus
> the formal definition of their syntax includes prose in the language
> specification.  For example, a context-free grammar cannot specify that
> all identifiers in a programming language be declared before they're used,
> or that a function must be called with the exact number of parameters
> specfied in its declaration (Aho & Ullman, p.179).  Thus processors for
> more-than-toy languages generally enforce such syntactic constraints via
> "ad-hoc" code rather than through their parsing tables, and only the prose
> in the spec can inform the processor's author how to write that code.

... which is beside the point of the problems I called attention to.

Those sorts of constraints are often viewed as semantic constraints,
and as we ll know XML is syntax not semantics.

- Dave

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