xml-dev - Re: Proposed process for DTDs in XML (Implementations)

[Paul Prescod <papresco@technologist.com>]

Ron Bourret wrote:
> 
> Trees vs. Events
> ----------------
> It seems like we need to decide early on whether we are interested in getting
> the DTD as events or a tree.  

I think that we are defining a language, not an API. Since the language
will use XML instance syntax, whether you get access to it as a tree or
events will depend entirely on whether your processor uses SAX or the DOM
(or something else altogether).

> Notice that the definitions of a and b are at the same level.  That is, when I
> build a DOM tree from this XML, a and b are siblings, not parent and child.
> When exploring a DTD, the parent-child relationship is far nicer -- I move up
> and down the DOM tree and get the metadata I need at each level.  On the other
> hand, such a tree complicates the DTD (sorry ;) for XSchema/XTD/etc. and I'm not
> sure if representing children with multiple parents would even be possible,
> given the strict nesting requirements of XML.  Comments?

Your last instinct is correct. A DTD is not a tree, but a directed acyclic
graph. There is no way to define any but the most trivial DTD as a tree:

<!ELEMENT Paragraph (#PCDATA|Emph)>
<!ELEMENT Title (#PCDATA|Emph)>
<!ELEMENT Emph (#PCDATA)>

Emph is used twice, so this simple (and typical) DTD is not a tree and
cannot be represented using XML element nesting.

 Paul Prescod  - http://itrc.uwaterloo.ca/~papresco

"You have the wrong number."
"Eh? Isn't that the Odeon?"
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taxation is mortal. You come when you can, and leave when you must. The 
show is continuous. Good-night." -- Robertson Davies, "The Cunning Man"

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