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GNU Info (bison.info)Ltcalc RulesGrammar 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 %preg 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 (Note: Default Action for Locations.), and for very specific rules, `@$' can be computed by hand. automatically generated by info2www version 1.2.2.9 |