Why doesn't XML have Bag? Uh, "set"

[David Megginson]

Roger L. Costello writes:

 > Thanks Dave for clarifying terminology.  It is "set" that I meant, not
 > "bag".  Just to make certain that I understand, an XML DTD cannot
 > express the following:
 > 
 > "A <Kitchen> element contains exactly three child elements: one instance
 > of <Sink>, one instance of <Stove>, and one instance of <Refrigerator>,
 > and these child elements can appear in any order."
 > 
 > Correct?  /Roger

More or less.  Technically, you *can* express this constraint with an
XML DTD:

  ((sink, ((stove, refrigerator) | (refrigerator, stove))) |
   (stove, ((sink, refrigerator) | (refrigerator, sink))) |
   (refrigerator, ((sink, stove) | (stove, sink))))

Obviously, things get unmanageable if the set grows a little bigger.

In an SGML DTD, you would use

  (sink & stove & refrigerator)

but in practical use, this never worked that well for documents except
in the special case of legacy-data conversion (it confused people
using authoring tools and generally made processing unnecessarily
difficult), and most SGML gurus strongly deprecated it.  XML is
hitting a slightly different usage domain (less emphasis on documents,
more on data), so perhaps it might be worthwhile including this in the
new schema standard.

 > P.S. Attributes can be listed in any order in an XML document,
 > regardless of the order that they are listed in the DTD.  Right?

Right -- order and repetition are properties of elements but not of
attributes.


All the best,


David

-- 
David Megginson                 david at megginson.com
           http://www.megginson.com/

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