xml-dev - Re: Integrity in the Hands of the Client

["Rick Jelliffe" <ricko@allette.com.au>]

> From: Paul Prescod <papresco@technologist.com>

>
> 
> "Sometimes the actual claims for markup-based systems are overstated; 
> the claim that SGML results in portable documents, for example, 
> falls afoul of the observation that it is possible to put angle 
> brackets around troff tags, supply a simple document type descrip- 
> tor,and thereby achieve anSGML-compliant document, without gaining 
> any portability or descriptiveness for the information. True 
> portability requires not only that informa- tion be transportable 
> from one machine to another,but that the semantics of that informa- 
> tion be the same on either machine. SGML, in particular,claims to 
> transfer no semantics, so it surely cannot guarantee portability."
> 
> [1] "Markup Reconsidered" http://www.sil.org/sgml/raymmark.ps

Without wishing to disagree in any way with Paul, the quote is perhaps not
quite true, I think.  

Sticking angle brackets on troff code may give you a document that is 
syntactically *valid* SGML but, because to the extent that it uses elements
to markup processing instructions, the document does not *conform* to 
SGML.  Such conformance cannot be judged mechanically, but by looking at the
definitions in ISO 8879 for processing instructions and elements.

People often seem to think "SGML is a grammar; I can markup all sorts of 
sloppy things; therefore SGML is a bad grammar".  But SGML is more than 
a queer grammar, it is a language: the terms "element" and 
"processing instruction" (etc) have broad but useable meanings.

I think one problem with XML is that these definitions of what an element,
etc., actually mean are not present.  XML *is* just a grammar, more or less.
But to convert it to a useful language, we often have to plug in SGML's
definitions.  

And again, we shouldn't then think that in all cases "SGML conformance=good;
SGML non-conformance=bad".  But that is separate from "do I need
SGML validity? do I need XML well-formedness? do I need a custom syntax?".


Rick Jelliffe 

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