I am not a BNF expert, so just a layman's question: Can the problems
you are having be mended by simply changing the BNF in the ASN.1, or
would a mend affect the actual notation being defined?
If the former, do you have (or could you produce) a complete set of
proposed changes to the BNF that would correct the problem?
If the latter, I am afraid you are about twenty years too late!
John L
Patrick Henry wrote:
>
> On Thu, 18 October 2001, Olivier Dubuisson wrote:
>
> >
> > Andrew Sutton wrote:
> > >
> > > hi,
> > >
> > > i've gotten bored so i've been working on an ASN.1 compiler based on the 97
> > > specifications. actually, it was going pretty well until i realized that i
> > > had mistyped the SymbolsImported productions. anyway, i ran into a problem
> > > and had some feedback or could propose an alternative syntax...
> > >
> > > it seems that the way that the SymbolsFromModuleList is constructed, you can
> > > run into a number of shift/reduce conflicts. if AssignedIdentifier attached
> > > to the GlobalModuleReference is a DefinedValue it conflicts with the first
> > > Symbol in the next SymbolList for multiple imports - if the first symbol is
> > > an abstract value (as opposed to type). if i'm just looking at this all
> > > wrong, let me know (like if its changed in a future version).
> > >
> > > otherwise, here's an alternative syntax that i'd propose (and implement)
> > >
> > > Exports ::=
> > > EXPORTS "(" SymbolList ")" ";" |
> > > empty
> >
> > I guess you meant "IMPORTS" here.
> >
> > > and
> > >
> > > SymbolsFromModule ::=
> > > "(" SymbolList ")" FROM GlobalModuleReference
> >
> > I don't understand what kind of problems you have with the SymbolList.
> > It seems to me that your lexer doesn't provide the right lexical items to
> > your parser.
> >
>
> Not exactly. The problem is that the 'valuereference' lexeme is used ambiguously in
>two contexts, both the SymbolList production and the DefinedValue production (which
>is part of AssignedIdentifier and hence GlobalModuleReference). There is no way to
>resolve the conflict in a straight recursive descent parser without resorting to the
>kind of fishing that Paul described in a prior message, q.v.
>
> As for bottom-up parsing, my compliments to Christian for his brilliant display of
>LALR(1) grammar transformations; but I have serious doubts about the sanity of such
>an approach.
>
> Any comments? To what extent has lex/yacc been used in ASN.1 compiler design?
>
> Regards,
>
> Patrick Henry
>
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--
Prof John Larmouth
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