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Re: SERIOUS concerns about implementation



At 6:50 AM -0600 2/21/97, Robert Streich wrote:
>At 09:01 PM 2/20/97 -0500, David Durand wrote:
>>Afraid not. You lose no information in going from SGML to XML, unless you
>>have somehow managed to come up with a way to use SHORTREF that could not
>>be equivalently expressed with tags. It is a cross-translation, since any
>>SGML DTD and any XML DTD should be able to operate at the same level of
>>abstraction.
>
>This seems a strange comment coming from you, David. Any time you reduce a
>grammar, you reduce expressibility. Any time you reduce expressibility, you
>lose information.

   In the information-theoretical sense this is obviously true, but in
terms of the practical problem of preserving information that is usefully
used to control processing, the case is less clear.

   I did make a few exceptions, and James pointed out another, data
attributes, which I aleways forget because I've never used them...

   My point was that almost all of the "extra expressivenes" of SGML
provides syntactic options for things that can be recorded in other ways.
From my experience, only Public and SDATA are things that I've ever had
occcasion to used. Data attributes seem as if they are the kind of thing
that doesn't move well across systems, but I may be wrong there -- it seems
to me that I can get equivalent effects with NOTATION, though maybe not as
elegantly.

    If it's a down-translation, it's not very far down, and I find it hard
to imagine an SGML document that could not be converted into an equivalent
XML document without losing information.

    So if change the definition of information, and instead ask for an
effective information-preserving transformation from SGML->XML, I would say
that it  always exists. So that you need not lose information in moving
from SGML to XML. The same is not true (in a meaningful sense) for RTF, or
Postscript, for instance...


  -- David

_________________________________________
David Durand              dgd@cs.bu.edu  \  david@dynamicDiagrams.com
Boston University Computer Science        \  Sr. Analyst
http://www.cs.bu.edu/students/grads/dgd/   \  Dynamic Diagrams
--------------------------------------------\  http://dynamicDiagrams.com/
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