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   Re: Notations

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  • From: John Cowan <cowan@locke.ccil.org>
  • To: XML Dev <xml-dev@ic.ac.uk>
  • Date: Mon, 14 Dec 1998 16:31:31 -0500

Joel Bender wrote:

> From this I get the impression that the DTD contains a NOTATION that refers
> to some specification of content, i.e., image/jpeg.

Yes.  To specify that a particular element contains an image/jpeg,
you must give that element an attribute of type NOTATION with the
value being the (local) name of the notation, e.g. "image_jpeg".

> This does not provide
> a mechanism to describe how it is encoded, you need a content transfer
> encoding description as well.

True.  The only useful encodings for XML purposes are "implicit"
and "base64", where "implicit" means a stream of characters in the
prevailing character set.

> Humm.  This gives me the impression that a notation doesn't say how the
> contents are encoded, only that its a 'picture'.

A notation can say whatever you want it to, depending on the document
that is referenced by the notation's external identifier.  It can
be as vague as "picture" or as specific as "text/html;charset='8859-9'"
for Turkish web pages.

One problem with notations as currently designed is that you either
understand a notation or you don't; there's no reliable way to get the kind
of partial understanding that the hierarchical nature of MIME types
gives you (i.e. knowing that "text/foobar" is a "text").
Full SGML has a solution to this, but it depends on mechanisms that
aren't in XML.

> To continue with my analogy <box><width>30</width></box>, I must be able to
> say that the contents will be a decimal encoded integer.  Not hex, octal,
> bit string, base64 encoded, etc.

Just so.  Write documentation explaining the format, and create a
DTD containing a NOTATION declaration referring to the documentation.
You can use full URI references, so "http://dom.ain.net/whatever#tag"
will work fine.

John Cowan	http://www.ccil.org/~cowan		cowan@ccil.org
	You tollerday donsk?  N.  You tolkatiff scowegian?  Nn.
	You spigotty anglease?  Nnn.  You phonio saxo?  Nnnn.
		Clear all so!  'Tis a Jute.... (Finnegans Wake 16.5)

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