Weak DTDs

Paul Prescod papresco at technologist.com
Sat Oct 18 15:51:53 BST 1997

Peter Murray-Rust wrote:
> Yesterday evening I converted a typical chemical manuscript into CML
> including RDF and DC metadata, images, spectra, molecules, bibliography,
> XML-LINKs to several related XML and non-XML documents, and so on. I found
> the freedom of NOT having a 'conventional' DTD was very liberating. I
> believe that (with the latest JUMBO) it displays quite attractively and
> meaningfully to human readers.

Being without DTDs *is* very liberating for individuals. So is being
without written laws. But I don't want to live in a community without

In your case, how can you guarantee that the RDF and DC metadata
conforms to those systems? How do you make sure that your images specify
their required attributes? How do you make sure that your molecules nest
in the right places? Are you really willing to make software (even just
simple stylesheets) that properly format RDF data in the middle of a
molecule, a molecule in the middle of an image, a bibliography entry in
the middle of your spectra? Or are you going to fill your code (and
stylesheets) with hundreds of ASSERT statements that raise error
messages when these elements are found out of their expected context?

Admittedly, depending on your exact needs, SGML DTDs may or may not be
able to express them, (and XML DTDs are less likely to be able to
express them), but in most cases they are the most efficient way of
expressing these constraints. But their goal is to protect your code
from bogus documents. You can dump them, but now you must do all of the
checking yourself.
 Paul Prescod

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