Re: Getting involved in Bison

[Akim Demaille]

[Restricted to bison-patches only]

Hi Victor!

Good to hear from you!

> Le 13 avr. 2020 à 12:54, Morales Cayuela, Victor (NSB - CN/Hangzhou) 
> <address@hidden> a écrit :
> 
> Hello!
> 
> First of all, I hope that all you and your relatives/friends are healthy, 
> this coronavirus is wreaking havoc.

As far as I'm concerned, everybody around me is safe.  Best of luck to 
everybody!


> I have started working on the push parser in C++. Since I feel a bit 
> unconfident about how it should be done, I would like you to double-check 
> first if all this is going the correct direction.

Sure!

> I have just added a few things to transform it slowly from C to C++.

I don't think this is the easiest path.  It is good that you have a look at how 
push parsers are implemented in C, but do not try to morph the C push parsers 
into C++.  Rather, once you understood what needs to be done, do it directly in 
lalr1.cc.  Do not start a nice skeleton, just enhance the current one to 
support push parsers.

I strongly recommend that you start from examples/c++/calc++, or whatever 
example you like, but in C++, using lalr1.cc.  Do not start with something 
generated by yacc.c.

I also recommend that you use git to follow your experiments.  You could do 
that in your Bison repo for instance.

The path should be roughly:

- find what are the local variables of yyparse that should become member of the 
parser object

- do it in your C++ parser.  In C++ we don't need an equivalent for yypstate, 
because we already have it: that the parser object itself.

- have your pull-parser (i.e., the good old yy::parser::parse()) work properly 
this way.  Run tests.  That's one of the reason I suggest using 
examples/c++/calc++ as a starting point: it already has tests, you can add more.

That was step 1.

- when this is done, look again at how push parsers are implemented in C to see 
what needs to change in yyparse so that the control is inverted: parse() will 
be *given* the tokens, instead of having to call yylex itself.  When I say 
"look at C", I think your best option are (i) yacc.c (look for b4_push_if) and 
(ii) examples/c/bistromathic which is a push parser.

- rename parse() as push_parse(symbol_type yyla) that takes the symbol as 
argument.  That's the push parser we are looking for.

- define a new parse() function which has the same signature as the usual 
pull-parser, that repeatedly calls the push_parse function.  Something like 
this:

int parse ()
{
  int status = 0;
  do {
   status = this->push_parse (yylex());
  } while (status == YYPUSH_MORE);
  return status;
}

That's the end of step 2.

When we agree on step 2, step 3 will start: implement these changes in 
lalr1.cc.  I will help on this.


> I have added namespace::yy and wrapped the public functions in a class. I 
> substituted "static const" for "constexpr. All that was easy and worked.

Using constexpr in lalr1.cc would be nice, independently of push-parsers, but 
make sure to use #defines to be portable and fall back to static const for 
older versions of C++.



Also, please, make sure to start from the latest version of master.  There were 
quite a few changes in C++ too.  You shall find it easier than before.

Cheers!

PS/  Please, send unified diffs (diff -u), not contextual ones.  And always 
"old new", not "new old": old is the part with - line, new that with + lines.