Re: lalr1.cc: use %printer in the syntax error messages

[Akim Demaille]

Le 27 août 09 à 09:11, Joel E. Denny a écrit :

> On Wed, 26 Aug 2009, Akim Demaille wrote:
>
> > So I think that what we really need is to open yysyntax_error to  
> > the user.  We
> > should provide the user with the lookahead, the (full) list of  
> > expected
> > tokens, the location, and *she* should forge the error message she  
> > wants.
>
> That makes sense to me.  Some time ago, we discussed adding a
> %error-report declaration in order to avoid the fluctuating yyerror
> prototype.  The user could access all these elements from that.  For  
> C, it
> might be:
>
>  %error-report {
>    char const *loc = yylocation_to_string (@$);
>    fprintf (stderr, "%s: %s\n", loc, yymessage);
>    fprintf (stderr, "%s: unexpected %s", loc, yytname[yyunexpected]);
>    if (yyunexpected_is_typed)
>      {
>        fprintf (stderr, " with value ");
>        yyunexpected_value_printer (stderr);
>      }
>    fprintf (stderr, "\n%s: expected any of:\n", loc);
>    for (int i = 0; i < yynexpected; ++i)
>      fprintf (stderr, "%s:   %s\n, loc, yytname[yyexpected[i]]);
>  } <*> <>

Yes, indeed.  But note that you are using %printer conveniently since  
you assume that the message is to be sent to the console.  If we're to  
send it to a GUI, to syslog, whatever, we don't have a nice means to  
use %printer.  This is a pity.  I would have preferred to have this  
%error-report in charge of building the error message, and then  
another function, yyerror, to be in charge of reporting the message.   
As is the case today with yysyntax_error vs. yyerror.

No problem for C++, and I guess neither for Java.

Inappropriate design for C :(  Actually, the flaw is arguably in C  
itself, too bad there is no standard open_memstream, as provided by  
the GNU Libc :(

> The yyunexpected_value_printer keeps this code generic.  The user  
> could
> push more of this code into individual %printer's to tailor the  
> message
> more for each symbol or type, but that can't go too far if %printer is
> still going to be useful for parser tracing.

I don't understand how yyunexpected_value_printer would be different  
from merely using %printer as provided by the user.  So actually just  
yyvalue_printer.

> For the user who is fine with Bison's traditional verbose error  
> messages
> but still wants to avoid the yyerror prototype confusion, he could  
> write:
>
>  %error-report {
>    fprintf (stderr, "%s\n", yymessage_verbose);
>  } <*> <>

That's nice, indeed.

> We could probably just deprecate yyerror (not for Yacc of course) and
> %error-verbose and tell the users to just choose between yymessage and
> yymessage_verbose in their %error-report.

Agreed.