Why doesn't XML have Bag?

[Rick Jelliffe]

 From: Roger L. Costello <costello at mitre.org>

>Why doesn't XML support the notion of an unordered list of elements,
>i.e., a Bag?  Perhaps this is a limitation of DTD, not XML?  That is,
>DTDs do not support Bags, but XML has no such inherent limitation?
Does
>DCD support Bags?  /Roger

Answer A: XML does have a way to support Bags: its called RDF.

Answer B: SGML DTDs could support bags, because they had an operator "&"
to mean required, but in any order. XML DTDs do not have this because
everyone said it was so difficult to implement. But then many people
said oops, because it would have been nice for database data.

Answer C: XML elements have order as a property. However, whether that
property is significant in the context of a document type depends on the
document type, and sometimes just on the kind of processing being
performed at that stage in the document's life. So you could just as
easily say that XML has bags but no sets.

Answer D: XML is not a data modeling language. It is a data-model
modeling language. So you decide what semantics you are to put. This
takes place entirely outside the area of what DTD's attempt to do, which
is just to provide a simple grammar for the data-model modeling.

Answer E: XML does have a way to support Bags: its called architectures.
On any element you attach an attribute that ties it to some other
element with known properties. For example, you tie your parent element
to html:ul for a bag and html:ol for a set.

Answer F: There are whole areas of fundamental semantic ways to slice
things: you want sets and bags, I want rhetorical relationships (I would
love if I could point at any element and know what the appropriate
heading for it was; I would love it if that heading was carted around
during cutting and pasting.)  If you think bags and sets are really
important, then encourage the schema working group to include that
information.


Take your pick!

Rick Jelliffe

Author: The XML & SGML Cookbook: Recipes for Structured Information
Prentice Hall, ISBN 0-13-614223-0


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