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2.4.2 Grammar Rules for ltcalc

Whether handling locations or not has no effect on the syntax of your language. Therefore, grammar rules for this example will be very close to those of the previous example: we will only modify them to benefit from the new information.

Here, we will use locations to report divisions by zero, and locate the wrong expressions or subexpressions.

input:
  %empty
| input line
;
line:
  '\n'
| exp '\n' { printf ("%d\n", $1); }
;
exp:
  NUM           { $$ = $1; }
| exp '+' exp   { $$ = $1 + $3; }
| exp '-' exp   { $$ = $1 - $3; }
| exp '*' exp   { $$ = $1 * $3; }
| exp '/' exp
    {
      if ($3)
        $$ = $1 / $3;
      else
        {
          $$ = 1;
          fprintf (stderr, "%d.%d-%d.%d: division by zero",
                   @3.first_line, @3.first_column,
                   @3.last_line, @3.last_column);
        }
    }
| '-' exp %prec NEG     { $$ = -$2; }
| exp '^' exp           { $$ = pow ($1, $3); }
| '(' exp ')'           { $$ = $2; }

This code shows how to reach locations inside of semantic actions, by using the pseudo-variables @n for rule components, and the pseudo-variable @$ for groupings.

We don’t need to assign a value to @$: the output parser does it automatically. By default, before executing the C code of each action, @$ is set to range from the beginning of @1 to the end of @n, for a rule with n components. This behavior can be redefined (see Default Action for Locations), and for very specific rules, @$ can be computed by hand.