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   Off topic (RE: XSchema Spec, Sections 2.0 and 2.1 (Draft 1))

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  • From: "Rick Jelliffe" <ricko@allette.com.au>
  • To: "XML Dev" <xml-dev@ic.ac.uk>
  • Date: Tue, 9 Jun 1998 22:59:13 +1000

> From: Matthew Gertner

> It is extremely confusing to
> someone learning XML that referencing some external binary data and
> transcluding external text use the same mechanism.

In my experience, this happens when people expect an entity reference to be
like a macro call, which it is not. Entity references are really kinds of
links, with particular access semantics (in XML, these are basically two:
"parse" and "don't parse").

> On top of this, an entity
> reference syntax is simply too terse. They is no way to document or
> parameterize the use of an entity reference (without resorting to
> comments),
> which is why it was necessary to invent XLink in the first place.

Well, they are things that were left out of XML on purpose to simplify
things. Just for people's information, since it helps understand "why is
this not there?", SGML allows attributes on entity declarations (well,
actually on things that have notations). So the reference is not
parameterized (with attributes) but the declaration is. SGML even lets you
chain entity declarations, to say "this entity is inside this entity is
inside this entity" (Formal System Identifiers), for example for compressed
archive files or patch systems.

In any case, you can parameterize an entity reference: wrap it in an element
with the appropriate attributes and write some code that can use that
structure. Or use an ENTITY attribute even.

It is a question of style more than substance, I think. SGML style was
"remove constants to declararions in the header" and "declare something
before you use it" while HTML is "declare and use in the same place".  XML
tries to allow both styles, whichever is appropriate for the scale of your

XML was not designed to be a complete solution to all markup problems. It
was meant to be simple, in the full knowledge that this simplicity would
make it unsuitable for many real-life uses. Personally, I expect that as
people get more used to the problems of electronic publishing and document
construction, many of SGML's discarded features will get re-adopted (though
in nice disciplined layers) under different guises, and branded with an "X"
so people can pretend it is not SGML.

Rick Jelliffe

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