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   Re: Inheritance in XML [^*]

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  • From: "Rick Jelliffe" <ricko@allette.com.au>
  • To: <xml-dev@ic.ac.uk>
  • Date: Fri, 24 Apr 1998 14:18:45 +1000

From: Frank Manola <fmanola@objs.com>

>At 1:46 PM -0700 4/23/98, Lisa Rein wrote:
>>> What are two properties? Type and class

>Whether there's a distinction between "type" and "class" depends on the
>context.  Loosely speaking, as well as in many contexts, they're generally
>considered synonymous

I think there is another consideration too. The word "type" has a
meaning in markup languages. The way it is typically used is to mean
"a notation defined using some other notation". In other words, when I say
"this entity is a GIF file" then I am giving its notation, but when I say
"this entity is a string with described by the regular expression (abc)* "
then I am giving its type.

When OO programmers see "document type" or "element type" perhaps
they expect there to be some type-system in place (whether inheritance,
classes,  mixins, or even basic typedef aliasing). But they are, to some
extent, being mislead by the word "type". I don't think the SGML/XML
makes this point explicit, unfortunately, but I think the usage is

Similarly, people get confused between "notation" and "encoding".  For
if I take the ISO 8601 date  1998-05-31 and encode it in Base64 and put it
in my XML document, then there are four notations. The date is in ISO 8601
notation, the 8601 notation is in Base64 notation, and the Base64 is in XML
notation, and the XML is in the notation of whichever character set (i.e.
charset "encoding") is used.  These are not "types" because each notation
is not being defined using the previous notation: it is the instance which
is being
encoded through successive notations (notionallY).

Rick Jelliffe

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