Structured attributes

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Date:       02 Mar 2000 22:40:58 +0000
From:       "Henry S. Thompson" <ht@cogsci.ed.ac.uk>
To:         XML Developers' list <xml-dev@xml.org>
Subject:    Structured attributes

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[A moment's thought will lead to realisation that this is displacement activity on the part of the XML Schema: Structures editor, who certainly has better things to do, but I need to get this off my chest.]

Why not structured attributes in XML? Why structured attributes? Well, we all know the comparison between elements and attributes as they stand:

Elements      Attributes
ordered       unordered
non-unique    unique
structured    unstructured

There are a lot of contexts in which the first two properties of
attributes are desireable, but the third is a serious constraint.

Here's a design sketch for adding structured attributes:

  1) Reserve a family of hitherto unusable names as the names of
  structured attributes: On balnace I favour names with an initial
  full-stop (.), but there are arguments in favour of using an initial
  colon (:) instead;

  2) Use element syntax, at the beginning of all element content, with 
     the following restrictions:
     2a) No attributes on structured attributes;
     2b) Contents is either all structured attributes or all text;
     2c) Order doesn't matter;
     2d) Uniqueness of names among sibling structured _and_, for children of
         elements, vanilla old-style attributes.

Example:

  <person age='49'>
   <.name>
    <.first>Henry</.first>
    <.last>Thompson</.last>
    <.middle>Swift</.middle>
  </.name>
  <children otherParent='p33'>
   . . .
  </children>
 </person>

The constraints above are equivalent to allowing 'attribute' children
to the 'attribute' element in XML Schema, along with making the
attribute name uniqueness constraint cumulative:

 <element name='person'>
  <complexType>
   <attribute name='age' type='integer'/>
   <attribute name='name'>
     <attribute name='first'/>
     <attribute name='middle/>
     <attribute name='last/>
   </attribute>
   <element name='children'>...</children>
  </complexType>
 </element>

Back to my day job :-)

ht

  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
          W3C Fellow 1999--2001, part-time member of W3C Team
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@cogsci.ed.ac.uk
		     URL: http://www.ltg.ed.ac.uk/~ht/

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Prepared by Robin Cover for the The XML Cover Pages archive.

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