xml-dev - Re: Must DTDs constrain order?

[Joe English <jenglish@flightlab.com>]

Michael Rossi wrote:

> Evan Lenz wrote:
> > 
> > In a DTD, is it possible to not constrain order on a given set of possible
> > elements, while still constraining quantity?
> > <!ELEMENT foo (bar, bat?, bang*)>
> > Can I somehow allow bar, bat, and bang to be in any order?  I suspect not
> > but would love to be corrected.
> 
> 	In an SGML DTD you would do this with the "&" operator.
> Unfortunately, XML has dropped the "&" in the interest of simplicity (that
> is, simplicity of parser implementation). Maybe now that we're all
> sophisticated enough to handle XML Schemas, we should bring back the "&" in
> DTDs. :-) But I digress.
> 
> 	It might still be theoretically possible to model this in XML DTDs
> if you did something more sophisticated like:
> 
> <!ELEMENT foo ( (bar, bat?, bang*) | (bat?, bar, bang*) | (bang*, bar, bat?)
> ... )>
> 
> But I SERIOUSLY doubt this could be done unambiguously for all possible
> combinations, and as you can see would quickly become far to complex a
> solution for all but the simplest content models. Sorry.

Actually, it is always possible to derive an unambiguous
content model, but like you said it quickly becomes too
complex to be practical without '&'.

Note that the SGML '&' connector doesn't work quite the right
way here either.  With

    <!ELEMENT foo (bar & bat? & bang*)>

you can have any number of 'bang' elements, but they
have to appear consecutively.  To get the desired behaviour,
you have to do something like:

    <!ELEMENT foo 
    	(bang*, ((bar, bang*) & (bat, bang*)?)) >

What's really needed is an "interleave" connector.


--Joe English

  jenglish@flightlab.com