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   Re: Inheritance/defaulting of attributes

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  • From: "Rick Jelliffe" <ricko@allette.com.au>
  • To: <xml-dev@ic.ac.uk>
  • Date: Sun, 5 Oct 1997 16:19:15 +1000

> From: Peter Murray-Rust <peter@ursus.demon.co.uk>

> This requires the implementation to provide for LOCATORs (children) to
> 'inherit' attributes from EXTENDED (parent). In my own DTD I would like to
> be able to use this philosophy, as in the following simple example of a
> list of numbers, all of type FLOAT

> Is this seen as a sufficiently general mechanism in XML that it is worth
> creating a DTD-independent engine for this? If so, is there a general
> mechanism for indicating (in the document instance or DTD) that this
> operation is to be carried out? At present it is only prose in the XLL
> spec?  It might be preferable to have a syntactic stement of this
> requirement - do otehrs feel the same? Can AFs be used? 

In your particular example, it can already be done using XLL, with only
a small change in the invocation syntax.

The VAR elements have a link to the first conatining parent element
with a role "datatype".  The LIST element has an XLL href (remapped to 
the name "type") that links to the approporiate type element.
The TYPES container has empty elements for all the different types
you care to make.  An example of these is the float type, which 
in turn has a pointer to some defining documentation.

Perhaps what you need can best be dealt with by defining some
standard values for the XLL role attribute!  (See the values for
role=  below for candidates.)  I think the standard definition
of some role types (i.e. moving "role" into more certain 
architecture") would be highly useful--if no common values are 
defined, role will go the way of PIs and languish in unuseability
due to meaninglessness.

<!DOCTYPE example [

<!-- Dummy top level -->
<!ELEMENT example - - ( TYPES, LIST* )>

<!-- Define container for types -->
<!ELEMENT TYPES	- - ( FLOAT, ... )>

<!--  First, define the element type float, and give documentation -->
		href		CDATA #FIXED "float-definition.htm" 
		role		CDATA #FIXED "lexical-type-specifier" 
		id		ID	#FIXED "FLOAT" > 

<!-- Now, define the LIST element type -->
		role		CDATA #FIXED "data-type-selector">

<!-- Define the VAR element type -->
		href		CDATA #FIXED "#HERE()ANCESTOR(1, *, role, data-type-selector)"
		role		CDATA #FIXED "link-to-data-type-selector"
		title		CDATA #REQUIRED >


<!-- Here is the instance -->


   <VAR TITLE="x" >1.2</VAR>
   <VAR TITLE="y">2.3</VAR>


Rick Jelliffe

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