Must XML be SGML compatible? (Was: An incompatible CData idea)

[Jim Harmon]

Jarle Stabell wrote:
> 
> First of all, thanks for the quick and helpful replies I got about 
> CData.
> 
> I knew about the CData marked section (soon I may also be able to 
> remember its syntax :-)), but it had slipped my notice that CData 
> isn't allowed in XML content models.
> 
> David Durand wrote:
> 
> >The marked section approach (SGML name for above ugly syntax) has 
> >all the advantages except that it's ugly (and thus arguably not as 
> >easy as it could be). But it is SGML compatible this way, and your 
> >proposal cannot be made compatible, so is not suitable for XML.
> 
> (My "ranting" below is of course not meant "against" Mr. Durand in 
> any way, who wrote a very clear and helpful answer to my original 
> posting)
> 
> I find the requirement that XML must be SGML compatible a bit hard to 
> swallow.

While I'm completely novitiate in this discussion, I think something
that stands out against your argument, at least from the limited
research I've done to understand SGML and XML thus far, is the XML is a
subset of SGML, therefore, compatibility MUST be an issue.

Unless I completely misunderstand the information thus far, SGML as a
specification contains all the constraints that will allow XML to exist,
in the same way that HTML is a subset of SGML.

Now, in my mind the IMPLEMENTATION of the language can be dialectually
divergent from the language of SGML, but the concept must remain
compatible (enough) so that anyone with access to the specified DTDs can
present an XML document in the author's intended format.

Am I wrong on that?

Isn't the point of XML itself the greater realization of the potential
of SGML on the Internet?  Wasn't HTML supposed to be the quick-and-dirty
get-it-on-the-Web-FAST subset of the SGML concept, and now XML the
enlightened "let's-do-it-RIGHT-this-time" tool?

Did I miss something?

> We all know the drawbacks of being compatible, that XML will not be as
> clean/beautiful and easy to read/write/learn/teach as it could be if 
> this requirement was not present.
> If XML becomes a great success, most XML users in five (?) years time 
> will likely not care about SGML compatibility (today they probably 
> don't know about XML and SGML at all, although many of them probably 
> know (about) HTML.)
> 
> I don't see many benefits of being SGML (syntax) compatible (except 
> for some obscure "political reasons"), nearly the only thing I (not 
> being a SGML guru!) can think of are:
> 
> * The ability of using non-XML-aware SGML tools on XML documents.
> 
> How long will this benefit be of any substantial value?
> 
> If XML becomes very popular, I guess all SGML vendors will upgrade 
> their tools in a very short time, as explicitly supporting XML will be 
> a nice thing to have on the feature list (and is easy to implement if 
> you already have the SGML machinery).
> One of the benefits may be that parsing XML documents will run 
> noticeably faster with a XML-specfic parser than a general SGML 
> parser.
> 
> Perhaps most of the SGML vendors will do something similar to:
> 
> If This_is_a_XML_document Then //Aha!
>    Parse_it_with_our_speedy_new_XMLparser
> Else
>    Parse_it_with_our_good_old_SGMLparser
> 
> irrespective of whether XML is SGML compatible or not?
> 
> Cheers,
> Jarle
> 
> (considering running for cover... :) )
> 
> ----
> Just to point out a small point re my suggestion:
> 
> <Element1><Element2*>This is a CData section.<Test>.</Element2></Element1>
> 
> is equivalent to
> 
> <Element1><Element2><![CDATA[This is a CData
> section.<Test>.]]</Element2></Element1>
> 
> and not to the shorter:
> <Element1><![CDATA[This is a CData section.<Test>.]]</Element1>
> 
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-- 
   Jim Harmon                           The Telephone Connection
jim at telecnnct.com                          Rockville, Maryland

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