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Re: questions about element declarations



At 07:31 PM 9/23/96 CDT, Michael Sperberg-McQueen wrote:
>The proposals on the table seem to me to raise several questions about
>element declarations which we haven't yet discussed here:
>
>* Should XML retain SGML's prohibition on multiple declarations for the
>same element (11.2.1)?

I assume that the purpose of this restriction is to make it somewhat
possible to limit author's abilities to modify centrally-defined
declaration sets.  If that's so, then I think we should keep the
restriction in XML.

The issue of whether or not this is an appropriate restriction should be
left for the SGML revision, I think (I personally think it's not a useful
restriction, but it's largely a matter of policy, not a technical issue, so
we probably shouldn't argue it here).

>*  Should XML restrict the use of #PCDATA to content models of the form
>(#PCDATA), or of the form (#PCDATA | A | B | ...)*, as a way of avoiding
>the mixed-content trap (11.2.4) and/or of simplifying RE handling
>(7.6.1)?

I think this would be a good thing (assuming we allow mixed content at
all).  SGML effectively requires this anyway, so why not formalize it?  

>* Should XML prohibit the use of inclusion and exclusion exceptions in
>element declarations? (11.2.4, 11.2.5)?

Yes.  Exclusions are, with very rare exceptions, a bad thing, causing much
more trouble than they're worth. Inclusions, while expressing a useful
semantic, also cause more trouble then they're worth and require DTD
processing.

>* Should XML forbid use of the '&' connector in content models
>(11.2.4.1)?

How much does AND complicate validation? I've seen some statements to the
effect that it complicates it quite a bit, but I have no way to evaluate
these claims.

I think AND expresses a useful semantic, so I would say keep it unless the
validation cost is too high.

>* Should XML allow nondeterministic content models (11.2.4.3)?

Again, how much does this complicate validation? I'm not ambiguity expert,
but could the problem be solved simply by stipulating that a token is
always matched to the first place in the content model it can match,
without lookahead? 

Cheers,

E.
--
W. Eliot Kimber (kimber@passage.com) 
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