info bison

4.4 Special Features for Use in Actions

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

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

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

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

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

Macro: YYABORT;: Return immediately from yyparse, indicating failure. See section The Parser Function yyparse.

Macro: YYACCEPT;: Return immediately from yyparse, indicating success. See section The Parser Function yyparse.

Macro: 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 is also disallowed in GLR parsers. 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.

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

Macro: YYEOF: Value stored in yychar when the look-ahead is the end of the input stream.

Macro: 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.

Macro: YYRECOVERING: The expression YYRECOVERING () yields 1 when the parser is recovering from a syntax error, and 0 otherwise. See section Error Recovery.

Variable: yychar: Variable containing either the look-ahead token, or YYEOF when the look-ahead is the end of the input stream, or YYEMPTY when no look-ahead has been performed so the next token is not yet known. Do not modify yychar in a deferred semantic action (see section GLR Semantic Actions). See section Look-Ahead Tokens.

Macro: yyclearin;: Discard the current look-ahead token. This is useful primarily in error rules. Do not invoke yyclearin in a deferred semantic action (see section GLR Semantic Actions). See section Error Recovery.

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

Variable: yylloc: Variable containing the look-ahead token location when yychar is not set to YYEMPTY or YYEOF. Do not modify yylloc in a deferred semantic action (see section GLR Semantic Actions). See section Actions and Locations.

Variable: yylval: Variable containing the look-ahead token semantic value when yychar is not set to YYEMPTY or YYEOF. Do not modify yylval in a deferred semantic action (see section GLR Semantic Actions). See section Actions.

Value: @$: Acts like a structure variable containing information on the textual location of the grouping made by the current rule. See section Tracking Locations.

Value: @n: Acts like a structure variable containing information on the textual location of the nth component of the current rule. See section Tracking Locations.

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