Re: a or b or both - mystery..

[Joe English <jenglish@flightlab.com>]

Marcus Carr wrote:

> My understanding was that the requirement for deterministic
> models was a concession made to those who were writing parsers
> at the expense of those who were creating DTDs.

A more important reason is that, in SGML, the rules for 
when start-tags may be omitted don't make any sense unless 
content models are deterministic.

(There is a way to rephrase the rules for start-tag
omissibility that avoids this, but I don't think it will
ever make it into a revision of the SGML Standard.)

At any rate, there's really no need to retain the restriction
in XML Schemas.  The argument that it makes it easier
on parser authors is IMO bogus; there are *plenty*
of well-known, simple algorithms for RE matching that
have no problem with nondeterministic expressions.

A somewhat more compelling argument is that it's easier
for *human beings* to understand deterministic content models,
but that's better left to the discretion of Schema authors.


> There's only one structure that I know of where deterministic models are
> unable to express the desire of the author, but a case can certainly be
> made that non-deterministic models might be more convenient in some cases.

Which structure is that?


--Joe English

  jenglish@flightlab.com