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   Container Tags and Design Principles (WAS RE: [xml-dev] XML/RDF)

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Ok, and no disagreement in principle.  I've used them 
myself and given a document that literally 
has switches in its native structures, found 
them unavoidable.  The topic this impinges 
on is one of DTD/Schema design where the 
design is not decoupled from the processes 
intended to handle instances.  Is it really 
a good idea to design a document type with 
extra tags whose main job is to make the 
XSLT simpler to build?

*Usually* I've tried to avoid them because 
they carry no content.  They act as process 
switches.  In some cases, one can eliminate 
them with multiple DTD/Schemas and proper 
application of namespaces.  Sometimes, 
one discovers the nesting isn't necessary 
or that the owner of the document is willing 
to lose them to simplify (the case I remember 
best is the 38784 tagging).

In X3D/VRML, quite a lot of effort was expended 
over the issues of container tags that existed 
to satisfy some implementers' requirements for 
a one pass parse.  Eventually, the containers 
were taken out.

Mixed content is a different can of worms. 
It is a force of nature.  One can wall around 
it, but it introduces more artificiality. Mixed 
content is a natural part of human documents. 
It just makes it harder to process.


From: Deborah Aleyne Lapeyre [mailto:dalapeyre@mulberrytech.com]

Len said:

>Items like not using mixed content
>or container elements are *usually* decent advice
>in most schema designs.

In some schema languages yes, in some no. In specific,
containers and mixed content are the real strength of XML in
the DTD world. When writing XSLT or quite a few "processing"
systems", containers are a godsend.


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