Manual

Chapter 4: Parser C-Language Interface 69
YYBACKUP (token, value );
[Macro]
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.
YYEMPTY
Value stored in yychar when there is no look-ahead token.
YYEOF
Value stored in yychar when the look-ahead is the end of the input stream.
[Macro]
[Macro]
YYERROR;
[Macro]
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 Chapter 6 [Error Recovery], page 83.
YYRECOVERING
[Macro]
The expression YYRECOVERING () yields 1 when the parser is recovering from a syntax
error, and 0 otherwise. See Chapter 6 [Error Recovery], page 83.
yychar
[Variable]
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 1.5.3 [GLR Semantic Actions], page 19). See Section 5.1 [Look-Ahead
Tokens], page 71.
yyclearin;
[Macro]
Discard the current look-ahead token. This is useful primarily in error rules. Do not
invoke yyclearin in a deferred semantic action (see Section 1.5.3 [GLR Semantic
Actions], page 19). See Chapter 6 [Error Recovery], page 83.
yyerrok;
[Macro]
Resume generating error messages immediately for subsequent syntax errors. This is
useful primarily in error rules. See Chapter 6 [Error Recovery], page 83.
yylloc
[Variable]
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 1.5.3 [GLR
Semantic Actions], page 19). See Section 3.6.2 [Actions and Locations], page 52.
yylval
[Variable]
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 1.5.3 [GLR Semantic Actions], page 19). See Section 3.5.3 [Actions], page 47.