Re: Finding out when a token is consumed

[Hans Aberg]

At 01:38 +0200 2003/05/13, Frank Heckenbach wrote:
>> >> It would be nice with a feature that tells whether the lookahead is
>>needed
>> >> or not, so one can implement sure context switches. While at tweaking the
>> >> skeleton files, there is a segment:
>> >>
>> >> yybackup:
>> >>
>> >>   // Try to make a decision without lookahead:
>> >>   n_ = pact_[state_];
>> >>   if (n_ == pact_ninf_)
>> >>     goto yydefault;
>> >>
>> >>   // Read a lookahead token.
>> >>   if (lookahead_ == empty_) {
>> >>     YYCDEBUG << "Reading a token: ";
>> >>     lex_();
>> >>   }
...
>I don't know exactly where you copied that from (I didn't find
>`lookahead_ == empty_' in the source of 1.875 or 20030320), but both
>src/parse-gram.c and data/yacc.c contain:
>
>  /* YYCHAR is either YYEMPTY or YYEOF or a valid lookahead symbol.  */
>  if (yychar == YYEMPTY)
>    {
>      YYDPRINTF ((stderr, "Reading a token: "));
>      yychar = YYLEX;
>    }
>
>which is exactly what I've been saying all the time ...

The segment is from my C++ skeleton file, which then uses C++ names, clearly.

Therefore, one has to look for a more general way to describe the feature
one wants, and then let it be translated via the M4 approach into the
various skeleton files.

That is why I reluctant to just jump into speculations before having tried
it out in actual programming. The discussions here helped though to raise
the topic; I have had need for controlling the lookahead in context
switches in the past.

If you now have a good example how to use it, you might try it out
practically, and then report your findings back here.

  Hans Aberg