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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