Special Features for Use in Actions

Here is a table of Bison constructs, variables and macros that are useful in actions.

`$$'

Acts like a variable that contains the semantic value for the grouping made by the current rule. See section Actions.

`$n'

Acts like a variable that contains the semantic value for the nth component of the current rule. See section Actions.

`$<typealt>$'

Like $$ but specifies alternative typealt in the union specified by the %union declaration. See section Data Types of Values in Actions.

`$<typealt>n'

Like $n but specifies alternative typealt in the union specified by the %union declaration. See section Data Types of Values in Actions.

`YYABORT;'

Return immediately from yyparse, indicating failure. See section The Parser Function yyparse.

`YYACCEPT;'

Return immediately from yyparse, indicating success. See section The Parser Function yyparse.

`YYBACKUP (token, value);'

Unshift a token. This macro is allowed only for rules that reduce a single value, and only when there is no look-ahead token. It installs a look-ahead token with token type token and semantic value value; then it discards the value that was going to be reduced by this rule. If the macro is used when it is not valid, such as when there is a look-ahead token already, then it reports a syntax error with a message `cannot back up' and performs ordinary error recovery. In either case, the rest of the action is not executed.

`YYEMPTY'

Value stored in yychar when there is no look-ahead token.

`YYERROR;'

Cause an immediate syntax error. This statement initiates error recovery just as if the parser itself had detected an error; however, it does not call yyerror, and does not print any message. If you want to print an error message, call yyerror explicitly before the `YYERROR;' statement. See section Error Recovery.

`YYRECOVERING'

This macro stands for an expression that has the value 1 when the parser is recovering from a syntax error, and 0 the rest of the time. See section Error Recovery.

`yychar'

Variable containing the current look-ahead token. (In a pure parser, this is actually a local variable within yyparse.) When there is no look-ahead token, the value YYEMPTY is stored in the variable. See section Look-Ahead Tokens.

`yyclearin;'

Discard the current look-ahead token. This is useful primarily in error rules. See section Error Recovery.

`yyerrok;'

Resume generating error messages immediately for subsequent syntax errors. This is useful primarily in error rules. See section Error Recovery.

`@n'

Acts like a structure variable containing information on the line numbers and column numbers of the nth component of the current rule. The structure has four members, like this:

struct {
  int first_line, last_line;
  int first_column, last_column;
};

Thus, to get the starting line number of the third component, use `@3.first_line'. In order for the members of this structure to contain valid information, you must make yylex supply this information about each token. If you need only certain members, then yylex need only fill in those members. The use of this feature makes the parser noticeably slower.

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