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   Why doesn't XML have Bag?

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  • From: David Megginson <david@megginson.com>
  • To: "Roger L. Costello" <costello@mitre.org>
  • Date: Fri, 26 Mar 1999 08:42:33 -0500 (EST)

Roger L. Costello writes:

 > 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

XML DTDs can constrain the content of a bag just fine:


XML DTDs cannot constrain the content of a set (where each element may 
appear exactly once, in any order).  This is not an SGML DTD
limitation, since in SGML you can use


You can simulate this in XML DTDs, but the content models become
absurdly large.

This is not to say that you cannot have a set in XML even *with* DTD
validation; it's just that DTD validation will not catch the errors.
For example, either


or even


will allow a set, but they will not catch the error where the same
element appears twice.

All the best,


David Megginson                 david@megginson.com

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