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FW: Can XML Schemas do this?

  • To: "[public XML-DEV]" <xml-dev@l...>
  • Subject: FW: Can XML Schemas do this?
  • From: "Alessandro Triglia" <sandro@M...>
  • Date: Tue, 21 Jan 2003 14:52:40 -0500
  • Importance: Normal

entropy of a message


> -----Original Message-----
> From: Alessandro Triglia [mailto:sandro@m...] 
> Sent: Tuesday, January 21, 2003 14:30
> To: 'Eric van der Vlist'
> Cc: 'John Cowan'; 'Jeff Lowery'; 'Bryce K. Nielsen'; 'xml-dev'
> Subject: RE:  Can XML Schemas do this?
> 
> 
> 
> 
> > -----Original Message-----
> > From: Eric van der Vlist [mailto:vdv@d...]
> > Sent: Tuesday, January 21, 2003 14:02
> > To: Alessandro Triglia
> > Cc: 'John Cowan'; 'Jeff Lowery'; 'Bryce K. Nielsen'; 'xml-dev'
> > Subject: RE:  Can XML Schemas do this?
> > 
> > 
> > Hi Alessandro,
> > 
> > > If the order is regarded as a constraint that the schema 
> imposes on
> > > the documents, then the existence of the constraint can be 
> > described
> > > as "less entropy" in the message and the absence of the
> > constraint can
> > > be described as "more entropy" in the message.
> > 
> > That's a nice metaphor.
> > 
> > > A possible question is, Why do we want to put "more 
> entropy" in the
> > > message if we are not interested in the extra "information" that 
> > > corresponds to the increased entropy?  (Or worse, when the extra 
> > > "information" is completely undefined and the readers can 
> infer just
> > > anything?)
> > 
> > Because a limited and controled amount entropy is life! Of
> > course, too much entropy is hell, but entropy is what make 
> > you feel comfortable, warm and cosy in front of your fireplace... 
> > 
> > To switch back to computer related subjects, I always feel
> > more comfortable when there is more than one way to do things 
> > and when no unecessary constraints are imposed to me.
> > 
> > Isn't it why XML 1.0 allows so many syntactical variations?
> 
> 
> I guess the main distinction is between:
> 
> 1) the syntactical variations that (most) people perceive as 
> mere syntactical variations (and therefore are not tempted to 
> attach semantics to), and 
> 
> 2) the variations that are likely to mislead people into 
> inferring some semantics that was not intended.
> 
> Examples of (1) are: attribute order, empty-element tags vs. 
> start-tag end-tag, presence and usage of end-of-line 
> characters, etc.  These variations are inherent in XML 1.0 
> and CANNOT be controlled by a schema. (Other similar 
> variations are inherent in XML Schema and cannot be 
> controlled by a schema either.)
> 
> Examples of (2) are those variations that are really UNDER 
> THE CONTROL of a schema but that the schema fails to exercise 
> control over (thus resulting in both lack of constraint and 
> lack of specific semantics -> "unused" entropy).


Let me see.  If a specification defines a set of possible messages and
*says* that a certain subset of them are just syntactical variations of
a message with identical semantics, nobody should (in theory) dream of
attaching any specific semantics to the different messages of the
subset. 

However, the reality is often that either specifications don't say that
in a sufficiently clear and explicit way, or people just don't listen,
because the temptation is strong.  The result may be misunderstanding,
abuse, interoperability problems, etc.  Reducing the synctactical
variations under (2) above helps to avoid this kind of problems.

Alessandro Triglia


> Alessandro
> 
> 
> > 
> > Eric
> > --
> > Curious about Relax NG? My book in progress is waiting for 
> > your review!
> >                                    
> > http://books.xmlschemata.org/relaxng/
> > 
> > --------------------------------------------------------------
> > ----------
> > Eric van der Vlist       http://xmlfr.org            
> > http://dyomedea.com
> > (W3C) XML Schema ISBN:0-596-00252-1
> > http://oreilly.com/catalog/xmlschema
> > --------------------------------------------------------------
> > ----------
> > 
> > 
> 
> 
> -----------------------------------------------------------------
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