Re: sticking custom meta informations to grammar rules

[Christian Schoenebeck]

On Thursday 30 January 2014 18:03:28 Hans Aberg wrote:
> I recall some others in the past that have written on interactive parsers
> showing completions. So it may be of interest integrating it into Bison.

I am sure this is a commonly requested feature. However my code is C++ and the 
algorithm is calling itself recursively. Advantage is that the amount of code 
is very small, can easily be maintained, however it has the potential to blow 
the program's memory stack, especially on complex languages and on resource 
limited embedded devices. So these two points already disqualify it from being 
accepted of becoming an internal part of Bison itself.

> The parser algorithms throw away all grammar information; Bison only keeps
> in its table information about the states. So that is not possible. If one
> has context dependencies, that is handled in the parser actions,
> interacting with the lexer, if needed.

Not really. I also thought so, but in each state for each upcoming potential 
reduction you get the associated grammar rule index number. With that grammar 
rule index number you can get its full grammar rule definition. I.e.

        std::string sRuleName = yytname[yyr1[rule]];

would give you the left hand side of the grammar rule. And then there are the 
yyprhs and yyrhs tables to get all right hand side symbols of the individual 
grammar rule. So you got all you need. Works fine!

> They are not officially a part of a public API, and may change without
> notice. Some things have changed, for example, the C++ has a different one
> from the C one.

Sure, that could happen at any time. But as far as I can see it, Bison's yyfoo 
tables and the fundamental parser algorithm based on them are treated 
conservatively. Thumbs up! :-) So I am estimating this risk to be low in the 
mid or even long term. And of course minor things change, like defines etc., 
but that's trivial to maintain when new Bison versions are released.

Best regards,
Christian Schoenebeck