xml-dev - Re: DTD vs DCD vs Schema

["Steven R. Newcomb" <srn@techno.com>]

> At 11:36 AM 1/17/99 -0800, David LeBlanc wrote:

> What i'm asking, is it the intent that, in the fullness of time, one
> of these will supplant DTDs as a document type notation?

This is really two questions:

(1) Will the set of semantics expressible by DTDs be changed?  (This
    is the primary question, although it's only implicit in your
    question.)

(2) Will the existing syntax of DTDs be replaced by another syntax?

The answer two both questions is "Yes, and certainly within the next
millenium."

The important thing is to go forward, and to avoid going backward,
with respect to the set of semantics that are expressible using DTDs.

I think today the first questions to ask are, "What's the goal?",
"What's the direction toward the goal?", and "How can we minimize the
likelihood of having to backtrack later, when huge investments in
information assets would be jeopardized?"  

Then we need to ask, "What are the semantics we need to represent,
in order to achieve the goal or move toward the goal?"

Once we've decided these basic semantic issues, the syntax issues
become a lot more straightforward.  Here, just for example, are some
as-yet-unsupported modeling features that it might be good to support:

* Subtyping of element types.

* Lexical modeling of strings in attribute values and in element
  content.

* Association of *arbitrary* semantic properties, constraints,
  etc. with element types and attribute values.  I'm using the term
  "arbitrary" here as the opposite of "built into the formalism", in
  the way that, for example, in today's DTD formalism, content models
  impose certain well-defined (and not at all arbitrary) *kinds* of
  constraints.  Maybe this is just a matter of providing a means of
  associating comments unambiguously with the constructs that they're
  about, but maybe it can be more rigorous than that.

-Steve

--
Steven R. Newcomb, President, TechnoTeacher, Inc.
srn@techno.com  http://www.techno.com  ftp.techno.com

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