SGML, XML and SML

[Kragen Sitaker]

Don Park writes:
> SML is a subset of XML and is not to be considered, in any sense,
> as conforming to the full XML 1.0 specification.
> 
> If you have better wordings, I'll be obliged.  It is not my
> intention to sabotage XML in-flight.

Usually, when people talk about languages in set-theoretic terms, they
are referring to the sets of strings that belong to the languages.  If
language A is a subset of language B, that means every string that is a
valid sentence (or document, or program, or whatever) in language A is
also valid in language B.  Usually they also mean that strings valid in
both languages have the same meanings in both.

Obviously there are SML "documents" that are not valid XML documents,
such as "<x/><y/>", so the first sense doesn't apply.  I'm not clear
about whether the second applies or not.

You might mean that the specification of SML consists of pieces of the
specification of XML, but that's not true either.

Perhaps you could say SML is a simplified version of XML.

Also, a nitpick: language specifications don't conform to the XML spec,
unless they are written in XML; XML documents do.  Perhaps you should
say that SML documents do not necessarily conform to the XML
specification.

A note: I am enthusiastic about the idea of a stream-based XML
variant.  I've always thought of it as a flaw that XML documents cannot
concatenate to form other XML documents -- i.e. XML is not closed under
concatenation -- and, due to the fact that <?xml version="1.0"?> is not
a PI, you can't include an arbitrary XML document inside another XML
document -- another flaw, in my view.

I think it would be very nice, from a processing point of view, to have
an XML variant that didn't have CDATA sections or DTDs (although entity
definitions are useful!).  I suspect CDATA sections are hard to live
without if you're writing XML documents about HTML or XML, though.

-- 
<kragen at pobox.com>       Kragen Sitaker     <http://www.pobox.com/~kragen/>
The Internet stock bubble didn't burst on 1999-11-08.  Hurrah!
<URL:http://www.pobox.com/~kragen/bubble.html>


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