Re: Getting involved in Bison

[Akim Demaille]

Hi Victor!

> Le 20 févr. 2020 à 04:07, Morales Cayuela, Victor (NSB - CN/Hangzhou) 
> <address@hidden> a écrit :
> 
> Hi!
> 
>> It really depends what you'd like to do.  You reported you're fluent in C++. 
>>  Something quite interesting would be to enable push-parser in lalr1.cc.  
>> It's more ambitious than the subcomplain task, but it will probably take you 
>> less time, because it's more compact, it will impact much less code.
> 
> Ok, seems very interesting! I use C++ (currently C++17) in my daily job as a 
> developer and I always try to keep up with the latest of the standard.
> I will familiarize myself first with push/pull parsers and then I will start 
> with it 😊 I will also take a look at TODO to check if there is already any 
> specification about how it should look like.

I think all the information you need is in the manual, in the push example in C 
(see examples/c/pushcalc), and in yacc.c's implementation of push.  Actually, 
Java also has push-parser support, and is probably a better source of 
inspiration than C.

I recommend that you first build by hand a push parser before trying to get 
this done by m4.  For instance take examples/c++/calc++, and change the 
generated parser so that it works as a push parser.  Keep this preciously 
somewhere to make sure "make" doesn't kill it.  When it works, you can show us 
a diff between the regular pull parser and the push parser, and when we agree, 
we can proceed to make it work in m4.

Actually, maybe examples/c++/calc++ is too bloated: first create a very simple 
example in C++, similar to examples/c/calc.  Start from this one, it is much 
simpler, and yet sufficient.

Most of the effort, I guess, is finding what local variables must be turned 
into member variables, to survive across calls to yyparse (amusingly enough 
coroutines would make this easy, but we can't afford them yet ;-).  But 
lalr1.java should help.

Note that C++ allows to consider other options than the C parsers.  Since the 
C++ parser is an object, maybe we can have push and pull use the same object.  
I mean, *maybe*, we can *always* move most local variables into member 
variables, even for regular pull-parsing.  Of course it's technically possible, 
the "maybe" refers to the runtime cost: *if* benchmarks show that it is not a 
regression to move to a model with more member variables and less local 
variables, then the m4 part will be much easier.

Maybe you should start with that: find a means to benchmark two pull parsers: 
one off-the-shelf, generated by today's lalr1.cc, and another one where the 
local variables that need to be member variables in push-mode are made member 
variables.

How does that sound?