Questions on DCD

Arnold, Curt Curt.Arnold at
Thu Oct 1 17:05:47 BST 1998

Olivo Miotto [mailto:olivo at] Wrote:
>1) First of all, I'm very excited about DCD. I think it really does
>XML into a new realm. I'm a little unclear about the long term
>strategy though: assuming that DCD is approved, is the aim to be
>an alternative to (or even replace perhaps) DTDs? Or be a
>mechanism for defining DTDs? In any case, the triangular
>relationship XML Document <-> DTD <-> DCD needs some
>clarification for me.

1. It appears that DCD will contribute to a W3C effort that will
supercede DTDs.  However, DCD (and XSchema) are both currently useful as
a higher level language for authoring DTDs.  We are using an internal
tool that takes DCD-like source (with additional documentation elements)
and creates a DTD and accompanying HTMLHelp file.  If you would like us
to run your DCD description of DCD through our converter and send you
the resulting DTD and help files.  I'll send you are modifications to
the DCD DTD in a private message.

>2) I can understand that compatibility with RDF is a noble target,
>but I think the interchangeability of Elements and Attributes (DCD
>section 2.1.2) clouds the water, making it more difficult to process
>DCD files. I would rather the syntax="explicit" form be the only way
>of specifying DCD. The remainder of my questions assume this

2. I'm not particularly fond of some of the forms used.  There are a lot
of things XSchema does more elegantly.

>3) A philosophical question: has anyone drawn up a clear functional
>distinction between attributes and elements? DCD astutely refers
>to both as properties, and indeed they have things in common.
>Saying they are interchangeable (or at least that attributes can be
>changed into elements) is however an oversimplification. It seems
>to me that attributes are modifiers of the functionality of the element
>they describe, while elements describe its content. I am not able to
>formalize this very clearly though. Can someone help?

3. If you go back in the logs for a topic "Newbie Q" in the end of
August, you will find an extensive discussion on this point.  Since you
indicated that you have problems accessing the archive server, I'll try
to forward some of the key discussions.

>4) Related to 3): DCD allows elements to also have default and
>fixed values like attributes. Am I correct in thinking that this is
>when producing a DTD from DCD?

4. I think that you are correct.

>7) The boolean type should be "True" or "False", not "1" or "0".

Typical SGML usage is 1 and 0.

>8) If in your content you have a single optional element, you must put
>it in a group. Is this correct?

>9) I don't really understand why we need ID-role when we have
>datatypes of id, idref and idrefs

I've made the same comment.

>10) I'm not really sure what is meant by open/closed content. How is
>it different from ANY and Mixed content?

>11) The <Values> tag has a space-separated list of values. I know this
> is a good thing to do if values must be an attribute, but from a DTD
>viewpoint it would be better as a sequence of <Value> tags, e.g.

11. Your suggestion is similar to the way XSchema did enumerations.

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