Where does $4 come from here?

This is in the first rule of Perl:

grammar :   GRAMPROG
            {
              PL_parser->expect = XSTATE;
            }
        remember stmtseq
            {
              newPROG(block_end($3,$4));
              $$ = 0;
            }

How can $4 work when there're only 3 e开发者_运维百科lements on the right side?

---

An embedded action (the code { PL_parser->expect = XSTATE; } which occurs in the middle of the rule) counts as an element. So there are 4 elements. $1 is the terminal GRAMPROG, $2 is the embedded action, $3 is the nonterminal remember, and $4 is the nonterminal stmtseq. (The value of $2 is whatever value is assigned to $$ inside the embedded action. Currently it would be garbage.)

---

Under the covers, yacc only really supports actions at the end of a production. So when you interleave an action { PL_parser->expect = XSTATE; } in the middle of a production, yacc (or whatever descendent you're using) pulls out the action and sticks it at the end of an empty rule as such:

grammar: GRAMPROG $$1 remember stmtmseq
    {
        newPROG(block_end($3, $4));
        $$ = 0;
    }

$$1:
   {
       PL_parser->expect = XSTATE;
   }

(If your yacc variant support dumping the verbose grammar and you do that, you'll see a lot of $$1, $$2, etc. rules for actions.)

In this case the interleaved action doesn't actually assign anything to $$, but if it had, the grammar rule could have accessed the value as $2.