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   Re: [xml-dev] Can we treat XML elements and attributes as sets

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Anne Thomas Manes wrote:

> If you go with the reasoning that elements with no namespace are
> elements in the namespace with no name, then the following two
> elements would be the same kind of element, when obviously they are
> not:
> <a>pi</a>
> <a>
>   <b>3.14156</b>
> </a>

No, I think they are the same kind of element; the element that has the 
content model (if there's a DTD) something like this:

<!ELEMENT a (#PCDATA | b)>

Anything designed to process arbitrary a elements had better be ready to 
handle both cases. This isn't uncommon. Many elements in many XML 
vocabularies can have different content models in different places.

The tricky bit here that I've struggled with while writing this is what 
does it mean for two elements to be the same. In this context, that 
we're talking about the element "kind" rather than the element instance. 
Otherwise the first element <a>1</a> is not the same as the second 
element <a>1</a>. At the same time, I want to avoid the word "type" 
which has just way too many connotations. However, I don't really want 
to say that the names are the same either, since that doesn't really 
reflect how namespaces are used in practice.

I'm still not perfectly happy with my language here. Improvements are 
solicited and appreciated. The difficulty we have explaining this stuff 
clearly and correctly I think reflects some underlying problems in the 
whole namespace model.

Maybe this isn't even a namespace problem. Ignoring namespaces and going 
back to XML 1.0, can we say that two FOO elements share some part of 
their nature? Maybe not, Clearly an HTML table is not a furniture table, 
but isn't this exactly the problem namespaces were supposed to solve? 
i.e. a {http://www.w3.org/199/xhtml}table is always one kind of thing, 
never some other kind of thing?

´╗┐Elliotte Rusty Harold  elharo@metalab.unc.edu
XML in a Nutshell 3rd Edition Just Published!


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