RE: Single-sourcing, who's doing it?

["Mark Baker" <mbaker -at- ca -dot- stilo -dot- com>]

Bonnie Granat asks

> > That's a big statement. SGML may be the grandfather of
> > markup, but I think it died a death some years ago.
> > This does not mean that it is without merit. Just that
> > it is dead as far as the technology curve goes.
> >
>
> How can this be, if XML is a subset of SGML, which is what W3C says it is?

Bonnie,

The statement that XML is a subset of SGML is often misunderstood and rarely
explained.

SGML and XML are identical in the basic structures they support.

XML contains a subset of the markup features of SGML (though SGML had to be
revised to keep this statement true). By markup features, I mean the rules
that say whether, and how, a particular character in the data stream is to
be recognized as markup or as data. In SGML these rules are sophisticated.
In XML they are very basic and unsophisticated.

Because XML did not allow markup minimization (for example, the elimination
of end tags) it had to introduce the empty tag markup (eg: <hr/>). SGML was
then amended to allow this also.

The other area in which XML was originally a subset of SGML was in the area
of content models. A content model is a set of rules about what elements can
be in a document and what elements other elements can contain and in what
order. DTD's express the content model of a document (its "document type").
A document is considered valid if it matches a designated content model.

The original XML DTD specification is a subset of the allowable types of
content models that are supported by SGML DTDs. (This means that if you want
to transform an SGML DTD into an XML DTD, you have to remove any of the
not-supported content models.)

However, XML now has at least five different ways of expressing content
models: XML DTDs, XML Schema, Biztalk, Schematron, and Relax NG. Each one of
them has its own set of rules for specifying content models so that a
content model expressed in one may not be compatible with another. Not all
of these are subsets of the capabilities of SGML DTD.

XML has become a huge and unwieldy beast, far more complicated than SGML
ever was, and in most respects it doesn't make much sense to describe XML as
a subset of SGML any more.

However, it remains true that it is a trivial operation to transform any
SGML document into well-formed XML (there are free tools that do it for
you). This means that you can use XML tools to process content created in
SGML. And that is a good thing.

---
Mark Baker
Stilo Corporation
1900 City Park Drive, Suite 504 , Ottawa, Ontario, Canada, K1J 1A3
Phone: 613-745-4242, Fax: 613-745-5560
Email mbaker -at- ca -dot- stilo -dot- com
Web: http://www.stilo.com

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