Re: how to get left hand side symbol in action

[Derek Clegg]

> On May 10, 2019, at 10:43 AM, Akim Demaille <address@hidden> wrote:
> 
> 
> 
>> Le 10 mai 2019 à 16:28, Christian Schoenebeck <address@hidden> a écrit :
>> 
>> On Freitag, 10. Mai 2019 06:57:00 CEST Derek Clegg wrote:
>>> In fact, I do not think error messages in the generated parser should be
>>> provided by bison, except as a trivial default. Instead, I’d like to see
>>> bison call a function (which I provide) when a syntax error occurs, with a
>>> list of what was expected along with what was encountered. Obviously, they
>>> should be provided without translation. That would be the most flexible.
>> 
>> Yep, I can just agree.
> 
> Except that you don't.  The API that was proposed initially did not
> fit your need.  At least, that was my understanding of
> 
> https://lists.gnu.org/archive/html/help-bison/2019-02/msg00018.html
> 
> Listen guys (Derek, Christian): how about you submit a patch of what
> you think the feature should be? 

I believe that my suggestion in the above post would be a good starting point. 
While Christian’s concern is valid, I think my proposal would be an improvement 
over the current method of error reporting. Just to reiterate, I suggested this 
approach:

> ...replace the code starting with
> 
>    char const* yyformat = YY_NULLPTR;
>    switch (yycount)
>      {
> #define YYCASE_(N, S)                        \
>        case N:                               \
>          yyformat = S;                       \
>        break
> …
>       return yyres;
> 
> in yysyntax_error_ with a call like
> 
>    my_yysyntax_error(yyarg, yycount);
> 
> where “my_yysyntax_error” is a user-defined function which does the work of 
> handling syntax error reporting.
> 
> In C++, we could be fancier: have two functions, one for a “pure” syntax 
> error:
> 
>    void my_yysyntax_error();
> 
> and one for syntax errors involving unexpected tokens:
> 
>     void my_yysyntax_error(std::string unexpected_token, 
> std::vector<std::string> expected_tokens);
> 
> The call would be something like this:
> 
>    if (yycount == 0) {
>      my_yysyntax_error();
>    } else {
>      my_yysyntax_error(yyarg[0], std::vector<std::string>{yyarg + 1, yyarg + 
> yycount});
>    }