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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] Re: Accepting non-deterministic content models
 * Roger L. Costello | | I believe that the origin of this "problem" is that in the XML spec | it states that parsers *should* reject non-deterministic content | models. I am wondering if perhaps "should" would be better replaced | with "must"? In my opinion the best fix would be to replace "should" with "must not". The only reason to reject non-deterministic content models is because the SGML standard requires it, and so for backwards compatibility XML did the same. This is also why the requirement is so vague. It doesn't say "should", it says "for compatibility, it is an error if...". When implementing an XML parser, however, you can build a finite state automaton representing the content model. This is fairly easy, and gives you a structure that can be traversed very quickly in order to validate the contents of an element against its content model. If this approach is followed, you have to do extra work to detect whether the original content model was "non-deterministic". In fact, the reason why xmlproc accepts such content models (as Tom Passin reports) is that I haven't implemented this check yet. I don't think there's all that much point in doing so, either. Now that we've ended up with a spec that is the way it is, I think the best course to follow for an implementation is to accept such content models, but to warn about them. There is nothing problematic about them, except that they vioalate SGML backwards-compatibility. --Lars M. 
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