Proposed process for DTDs in XML (Implementations)
papresco at technologist.com
Mon May 25 16:37:17 BST 1998
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
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