provides some extensions for
compatibility with bison and other implementations of yacc.
It accepts several
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%code keyword { code }
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Adds the indicated source code at a given point in the output file.
The optional keyword tells yacc where to insert the code:
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top
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just after the version-definition in the generated code-file.
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requires
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just after the declaration of public parser variables.
If the -d option is given, the code is inserted at the
beginning of the defines-file.
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provides
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just after the declaration of private parser variables.
If the -d option is given, the code is inserted at the
end of the defines-file.
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If no keyword is given, the code is inserted at the
beginning of the section of code copied verbatim from the source file.
Multiple %code directives may be given;
yacc inserts those into the corresponding code- or defines-file
in the order that they appear in the source file.
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%debug
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This has the same effect as the ``-t'' command-line option.
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%destructor { code } symbol+
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defines code that is invoked when a symbol is automatically
discarded during error recovery.
This code can be used to
reclaim dynamically allocated memory associated with the corresponding
semantic value for cases where user actions cannot manage the memory
explicitly.
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On encountering a parse error, the generated parser
discards symbols on the stack and input tokens until it reaches a state
that will allow parsing to continue.
This error recovery approach results in a memory leak
if the YYSTYPE value is, or contains,
pointers to dynamically allocated memory.
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The bracketed code is invoked whenever the parser discards one of
the symbols.
Within code, ``$$'' or
``$<tag>$'' designates the semantic value associated with the
discarded symbol, and ``@$'' designates its location (see
%locations directive).
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A per-symbol destructor is defined by listing a grammar symbol
in symbol+. A per-type destructor is defined by listing
a semantic type tag (e.g., ``<some_tag>'') in symbol+; in this
case, the parser will invoke code whenever it discards any grammar
symbol that has that semantic type tag, unless that symbol has its own
per-symbol destructor.
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Two categories of default destructor are supported that are
invoked when discarding any grammar symbol that has no per-symbol and no
per-type destructor:
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&#187;
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the code for ``<*>'' is used
for grammar symbols that have an explicitly declared semantic type tag
(via ``%type'');
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&#187;
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the code for ``<>'' is used
for grammar symbols that have no declared semantic type tag.
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%empty
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ignored by yacc.
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%expect number
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tells yacc the expected number of shift/reduce conflicts.
That makes it only report the number if it differs.
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%expect-rr number
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tell yacc the expected number of reduce/reduce conflicts.
That makes it only report the number if it differs.
This is (unlike bison) allowable in LALR parsers.
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%locations
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tells yacc to enable management of position information associated
with each token, provided by the lexer in the global variable yylloc,
similar to management of semantic value information provided in yylval.
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As for semantic values, locations can be referenced within actions using
@$ to refer to the location of the left hand side symbol, and @N
(N an integer) to refer to the location of one of the right hand side
symbols.
Also as for semantic values, when a rule is matched, a default
action is used the compute the location represented by @$ as the
beginning of the first symbol and the end of the last symbol in the right
hand side of the rule.
This default computation can be overridden by
explicit assignment to @$ in a rule action.
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The type of yylloc is YYLTYPE, which is defined by default as:
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typedef struct YYLTYPE {
int first_line;
int first_column;
int last_line;
int last_column;
} YYLTYPE;
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YYLTYPE can be redefined by the user
(YYLTYPE_IS_DEFINED must be defined, to inhibit the default)
in the declarations section of the specification file.
As in bison, the macro YYLLOC_DEFAULT is invoked
each time a rule is matched to calculate a position for the left hand side of
the rule, before the associated action is executed; this macro can be
redefined by the user.
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This directive adds a YYLTYPE parameter to yyerror().
If the %pure-parser directive is present,
a YYLTYPE parameter is added to yylex() calls.
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%lex-param { argument-declaration }
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By default, the lexer accepts no parameters, e.g., yylex().
Use this directive to add parameter declarations for your customized lexer.
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%parse-param { argument-declaration }
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By default, the parser accepts no parameters, e.g., yyparse().
Use this directive to add parameter declarations for your customized parser.
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%pure-parser
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Most variables (other than yydebug and yynerrs) are
allocated on the stack within yyparse, making the parser reasonably
reentrant.
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%token-table
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Make the parser's names for tokens available in the yytname array.
However,
yacc does not predefine ``$end'', ``$error''
or ``$undefined'' in this array.
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