Re: Suggestion for dynamic grammar/parser - pls advise

Hans-Peter Diettrich wrote:
> John Sasso wrote:
>
>> The grammars are such that grammar
>> G_k+1 (for language L_k+1) is like G_k except new productions and
>> terminals (and, of course, non-terminals) are added in order to
>> recognize new features in version k+1 of the language that are not
>> present in grammar G_k of L_k.
>
> Why would you need different grammars, when the parser of the most
> derived language will accept the other languages as well? This would
> be required only when the languages form an tree, with incompatible
> extensions in e.g. the G_k_1 and G_k_2 branches.
>


Thank-you for responding to my post. Well, one thing I neglected to
note (for the sake of simplicity of the post, but on retrospect I should
have) is that a new version of the language may have components of the
prior version of the language removed from the grammar. That is, L_k
may have certain keywords or statement constructs removed (made
obsolete) which were present in L_k-1.


> If the purpose is the rejection of something like non-ANSI extension to
> a language, I'd assign version number attributes to the added rules.
> Then one can determine, during the application of a rule, whether it is
> part of the given language version. Or one can determine, after an
> parse, what's the minimal required language version, from the maximum of
> the processed rule version numbers.
>


OK, that's interesting, I was thinking of something similar (i.e. adding
version # attributes to the production rules), but it seemed a bit
complex. However, if it is doable, can you point me to any literature
that describes how to do such?


>
>> I would like to develop a parser which is scalable and flexible such that:
>>
>> 1. When a new version of the language becomes available, the parser can
>> be easily extended to recognize a program written in the new language
>> w/o a lot of time-consuming rework.
>
> Doesn't this mean that you'll have to establish the guidelines or
> restrictions, that will apply to such extensions?
>
> Extending a grammar requires knowledge of the meta language, and of how
> to deal with errors detected by the parser generator. There exists no
> way to "easily" extend a grammar, unless errors can not occur at all ;-)
>


Lets just assume I have full access to the "meta-language" for new
versions of the language. Not the parser or grammar w/ production
rules, mind you, but with the meta-language (i.e. syntax rules and the
like) I can construct the production rules.


>
>> *** Assume someone else, not I, creates new versions of the language.
>
> This might result in a language tree, with different extensions inserted
> by multiple people. If nobody has to care about the extensions, added by
> other people, everybody can maintain his own (flat) language branch.
>
>
>> 2. If the input to the parser is a program written in L_k, and the
>> version of the language the program was written in (i.e. k), then the
>> parser will parse the program based on grammar G_k.
>
> This is where I'd tell the parser to treat expansions of rules above
> level k as syntax errors. The restriction will have to be encoded in the
> semantical part of the grammar.
>
> DoDi


Is this w.r.t the language tree? I apologize that I am not familiar
with such (I know syntax trees, but I gather you are not referring to
the same thing).


--john

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