Outline of a Bison Grammar
* Prologue:: Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations:: Syntax and usage of the Bison declarations section.
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
@menu
* Prologue:: Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations:: Syntax and usage of the Bison declarations section.
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
feature test macros can affect the behavior of Bison-generated
@code{#include} directives.
-@findex %requires
+@node Prologue Alternatives
+@subsection Prologue Alternatives
+@cindex Prologue Alternatives
+
@findex %code
-If you've instructed Bison to generate a header file (@pxref{Table of Symbols,
-,%defines}), you probably want @code{#include "ptypes.h"} to appear
-in that header file as well.
-In that case, use @code{%requires}, @code{%provides}, and
-@code{%code} instead of @var{Prologue} sections
-(@pxref{Table of Symbols, ,%requires}):
+@findex %requires
+@findex %provides
+@findex %code-top
+The functionality of @var{Prologue} sections can often be subtle and
+inflexible.
+As an alternative, Bison provides a set of more explicit directives:
+@code{%code}, @code{%requires}, @code{%provides}, and @code{%code-top}.
+@xref{Table of Symbols,,Bison Symbols}.
+
+Look again at the example of the previous section:
@smallexample
+%@{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+ #include "ptypes.h"
+%@}
+
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%@{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+%@}
+
+@dots{}
+@end smallexample
+
+@noindent
+Notice that there are two @var{Prologue} sections here, but there's a subtle
+distinction between their functionality.
+For example, if you decide to override Bison's default definition for
+@code{YYLTYPE}, in which @var{Prologue} section should you write your new
+definition?
+You should write it in the first since Bison will insert that code into the
+parser code file @emph{before} the default @code{YYLTYPE} definition.
+In which @var{Prologue} section should you prototype an internal function,
+@code{trace_token}, that accepts @code{YYLTYPE} and @code{yytokentype} as
+arguments?
+You should prototype it in the second since Bison will insert that code
+@emph{after} the @code{YYLTYPE} and @code{yytokentype} definitions.
+
+This distinction in functionality between the two @var{Prologue} sections is
+established by the appearance of the @code{%union} between them.
+This behavior raises a few questions.
+First, why should the position of a @code{%union} affect definitions related to
+@code{YYLTYPE} and @code{yytokentype}?
+Second, what if there is no @code{%union}?
+In that case, the second kind of @var{Prologue} section is not available.
+This behavior is not intuitive.
+
+To avoid this subtle @code{%union} dependency, rewrite the example using
+@code{%code-top} and @code{%code}.
+Let's go ahead and add the new @code{YYLTYPE} definition and the
+@code{trace_token} prototype at the same time:
+
+@smallexample
+%code-top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+ /* The following code really belongs in a %requires; see below. */
+ #include "ptypes.h"
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+ static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+In this way, @code{%code-top} and @code{%code} achieve the same functionality
+as the two kinds of @var{Prologue} sections, but it's always explicit which
+kind you intend.
+Moreover, both kinds are always available even in the absence of @code{%union}.
+
+The @code{%code-top} block above logically contains two parts.
+The first two lines need to appear in the parser code file.
+The fourth line is required by @code{YYSTYPE} and thus also needs to appear in
+the parser code file.
+However, if you've instructed Bison to generate a parser header file
+(@pxref{Table of Symbols, ,%defines}), you probably want the fourth line to
+appear before the @code{YYSTYPE} definition in that header file as well.
+Also, the @code{YYLTYPE} definition should appear in the parser header file to
+override the default @code{YYLTYPE} definition there.
+
+In other words, in the @code{%code-top} block above, all but the first two
+lines are dependency code for externally exposed definitions (@code{YYSTYPE}
+and @code{YYLTYPE}) required by Bison.
+Thus, they belong in one or more @code{%requires}:
+
+@smallexample
+%code-top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+
%requires @{
#include "ptypes.h"
@}
tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
@}
+%requires @{
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+ static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+Now Bison will insert @code{#include "ptypes.h"} and the new @code{YYLTYPE}
+definition before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
+definitions in both the parser code file and the parser header file.
+(By the same reasoning, @code{%requires} would also be the appropriate place to
+write your own definition for @code{YYSTYPE}.)
+
+When you are writing dependency code for @code{YYSTYPE} and @code{YYLTYPE}, you
+should prefer @code{%requires} over @code{%code-top} regardless of whether you
+instruct Bison to generate a parser header file.
+When you are writing code that you need Bison to insert only into the parser
+code file and that has no special need to appear at the top of the code file,
+you should prefer @code{%code} over @code{%code-top}.
+These practices will make the purpose of each block of your code explicit to
+Bison and to other developers reading your grammar file.
+Following these practices, we expect @code{%code} and @code{%requires} to be
+the most important of the four @var{Prologue} alternative directives discussed
+in this section.
+
+At some point while developing your parser, you might decide to provide
+@code{trace_token} to modules that are external to your parser.
+Thus, you might wish for Bison to insert the prototype into both the parser
+header file and the parser code file.
+Since this function is not a dependency of any Bison-required definition (such
+as @code{YYSTYPE}), it doesn't make sense to move its prototype to a
+@code{%requires}.
+More importantly, since it depends upon @code{YYLTYPE} and @code{yytokentype},
+@code{%requires} is not sufficient.
+Instead, move its prototype from the @code{%code} to a @code{%provides}:
+
+@smallexample
+%code-top @{
+ #define _GNU_SOURCE
#include <stdio.h>
+@}
+
+%requires @{
+ #include "ptypes.h"
+@}
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+%requires @{
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
+%provides @{
+ void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+%code @{
static void print_token_value (FILE *, int, YYSTYPE);
#define YYPRINT(F, N, L) print_token_value (F, N, L)
@}
@dots{}
@end smallexample
+@noindent
+Bison will insert the @code{trace_token} prototype into both the parser header
+file and the parser code file after the definitions for @code{yytokentype},
+@code{YYLTYPE}, and @code{YYSTYPE}.
+
+The above examples are careful to write directives in an order that reflects
+the layout of the generated parser code and header files:
+@code{%code-top}, @code{%requires}, @code{%provides}, and then @code{%code}.
+While your grammar files may generally be easier to read if you also follow
+this order, Bison does not require it.
+Instead, Bison lets you choose an organization that makes sense to you.
+
+You may declare any of these directives multiple times in the grammar file.
+In that case, Bison concatenates the contained code in declaration order.
+This is the only way in which the position of one of these directives within
+the grammar file affects its functionality.
+
+The result of the previous two properties is greater flexibility in how you may
+organize your grammar file.
+For example, you may organize semantic-type-related directives by semantic
+type:
+
+@smallexample
+%requires @{ #include "type1.h" @}
+%union @{ type1 field1; @}
+%destructor @{ type1_free ($$); @} <field1>
+%printer @{ type1_print ($$); @} <field1>
+
+%requires @{ #include "type2.h" @}
+%union @{ type2 field2; @}
+%destructor @{ type2_free ($$); @} <field2>
+%printer @{ type2_print ($$); @} <field2>
+@end smallexample
+
+@noindent
+You could even place each of the above directive groups in the rules section of
+the grammar file next to the set of rules that uses the associated semantic
+type.
+And you don't have to worry that some directive (like a @code{%union}) in the
+definitions section is going to adversely affect their functionality in some
+counter-intuitive manner just because it comes first.
+Such an organization is not possible using @var{Prologue} sections.
+
+This section has been concerned with explaining the advantages of the four
+@var{Prologue} alternative directives over the original Yacc @var{Prologue}.
+However, in most cases when using these directives, you shouldn't need to
+think about all the low-level ordering issues discussed here.
+Instead, you should simply use these directives to label each block of your
+code according to its purpose and let Bison handle the ordering.
+@code{%code} is the most generic label.
+Move code to @code{%requires}, @code{%provides}, or @code{%code-top} as needed.
+
@node Bison Declarations
@subsection The Bison Declarations Section
@cindex Bison declarations (introduction)
@subsection Freeing Discarded Symbols
@cindex freeing discarded symbols
@findex %destructor
-@findex %symbol-default
-
+@findex <*>
+@findex <!>
During error recovery (@pxref{Error Recovery}), symbols already pushed
on the stack and tokens coming from the rest of the file are discarded
until the parser falls on its feet. If the parser runs out of memory,
When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
per-symbol @code{%destructor}.
You may also define a per-type @code{%destructor} by listing a semantic type
-among @var{symbols}.
+tag among @var{symbols}.
In that case, the parser will invoke this @var{code} whenever it discards any
-grammar symbol that has that semantic type unless that symbol has its own
+grammar symbol that has that semantic type tag unless that symbol has its own
per-symbol @code{%destructor}.
-Finally, you may define a default @code{%destructor} by placing
-@code{%symbol-default} in the @var{symbols} list of exactly one
-@code{%destructor} declaration in your grammar file.
-In that case, the parser will invoke the associated @var{code} whenever it
-discards any user-defined grammar symbol for which there is no per-type or
-per-symbol @code{%destructor}.
+Finally, you can define two different kinds of default @code{%destructor}s.
+You can place each of @code{<*>} and @code{<!>} in the @var{symbols} list of
+exactly one @code{%destructor} declaration in your grammar file.
+The parser will invoke the @var{code} associated with one of these whenever it
+discards any user-defined grammar symbol that has no per-symbol and no per-type
+@code{%destructor}.
+The parser uses the @var{code} for @code{<*>} in the case of such a grammar
+symbol for which you have formally declared a semantic type tag (@code{%type}
+counts as such a declaration, but @code{$<tag>$} does not).
+The parser uses the @var{code} for @code{<!>} in the case of such a grammar
+symbol that has no declared semantic type tag.
@end deffn
@noindent
-For instance:
+For example:
@smallexample
%union @{ char *string; @}
%union @{ char character; @}
%token <character> CHR
%type <character> chr
-%destructor @{ free ($$); @} %symbol-default
-%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%token TAGLESS
+
%destructor @{ @} <character>
+%destructor @{ free ($$); @} <*>
+%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%destructor @{ printf ("Discarding tagless symbol.\n"); @} <!>
@end smallexample
@noindent
guarantees that, when the parser discards any user-defined symbol that has a
semantic type tag other than @code{<character>}, it passes its semantic value
-to @code{free}.
+to @code{free} by default.
However, when the parser discards a @code{STRING1} or a @code{string1}, it also
prints its line number to @code{stdout}.
It performs only the second @code{%destructor} in this case, so it invokes
@code{free} only once.
-
-Notice that a Bison-generated parser invokes the default @code{%destructor}
-only for user-defined as opposed to Bison-defined symbols.
-For example, the parser will not invoke it for the special Bison-defined
-symbols @code{$accept}, @code{$undefined}, or @code{$end} (@pxref{Table of
-Symbols, ,Bison Symbols}), none of which you can reference in your grammar.
-It also will not invoke it for the @code{error} token (@pxref{Table of Symbols,
-,error}), which is always defined by Bison regardless of whether you reference
-it in your grammar.
-However, it will invoke it for the end token (token 0) if you redefine it from
-@code{$end} to, for example, @code{END}:
+Finally, the parser merely prints a message whenever it discards any symbol,
+such as @code{TAGLESS}, that has no semantic type tag.
+
+A Bison-generated parser invokes the default @code{%destructor}s only for
+user-defined as opposed to Bison-defined symbols.
+For example, the parser will not invoke either kind of default
+@code{%destructor} for the special Bison-defined symbols @code{$accept},
+@code{$undefined}, or @code{$end} (@pxref{Table of Symbols, ,Bison Symbols}),
+none of which you can reference in your grammar.
+It also will not invoke either for the @code{error} token (@pxref{Table of
+Symbols, ,error}), which is always defined by Bison regardless of whether you
+reference it in your grammar.
+However, it may invoke one of them for the end token (token 0) if you
+redefine it from @code{$end} to, for example, @code{END}:
@smallexample
%token END 0
@end smallexample
+@cindex actions in mid-rule
+@cindex mid-rule actions
+Finally, Bison will never invoke a @code{%destructor} for an unreferenced
+mid-rule semantic value (@pxref{Mid-Rule Actions,,Actions in Mid-Rule}).
+That is, Bison does not consider a mid-rule to have a semantic value if you do
+not reference @code{$$} in the mid-rule's action or @code{$@var{n}} (where
+@var{n} is the RHS symbol position of the mid-rule) in any later action in that
+rule.
+However, if you do reference either, the Bison-generated parser will invoke the
+@code{<!>} @code{%destructor} whenever it discards the mid-rule symbol.
+
@ignore
@noindent
In the future, it may be possible to redefine the @code{error} token as a
@xref{Rules, ,Syntax of Grammar Rules}.
@end deffn
+@deffn {Directive} <*>
+Used to define a default tagged @code{%destructor} or default tagged
+@code{%printer}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} <!>
+Used to define a default tagless @code{%destructor} or default tagless
+@code{%printer}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
@deffn {Symbol} $accept
The predefined nonterminal whose only rule is @samp{$accept: @var{start}
$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
@end deffn
@deffn {Directive} %code @{@var{code}@}
-Specifies code to be inserted into the code file after the contents of the
-header file.
-@xref{Table of Symbols, ,%requires}.
-@end deffn
-
-@deffn {Directive} %provides @{@var{code}@}
-Specifies code to be inserted both into the header file (if generated;
-@pxref{Table of Symbols, ,%defines}) and into the code file after any
-Bison-generated definitions.
-@xref{Table of Symbols, ,%requires}.
-@end deffn
-
-@deffn {Directive} %requires @{@var{code}@}
-Specifies code to be inserted both into the header file (if generated;
-@pxref{Table of Symbols, ,%defines}) and into the code file before any
-Bison-generated definitions.
+Other than semantic actions, this is probably the most common place you should
+write verbatim code for the parser implementation.
+For C/C++, it replaces the traditional Yacc prologue,
+@code{%@{@var{code}%@}}, for most purposes.
+For Java, it inserts code into the parser class.
@cindex Prologue
@findex %union
-@findex %provides
-@findex %code
-For example, the following declaration order in the grammar file reflects the
-order in which Bison will output these code blocks. However, you are free to
-declare these code blocks in your grammar file in whatever order is most
-convenient for you:
+Compare with @code{%@{@var{code}%@}} (@pxref{Prologue, ,The Prologue})
+appearing after the first @code{%union @{@var{code}@}} in a C/C++ based grammar
+file.
+While Bison will continue to support @code{%@{@var{code}%@}} for backward
+compatibility, @code{%code @{@var{code}@}} is cleaner as its functionality does
+not depend on its position in the grammar file relative to any
+@code{%union @{@var{code}@}}.
+Specifically, @code{%code @{@var{code}@}} always inserts your @var{code} into
+the parser code file after the usual contents of the parser header file.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
+@deffn {Directive} %code-top @{@var{code}@}
+Occasionally for C/C++ it is desirable to insert code near the top of the
+parser code file.
+For example:
@smallexample
-%requires @{
- /* Bison inserts this block into both the header file and the code
- * file. In both files, the point of insertion is before any
- * Bison-generated token, semantic type, location type, and class
- * definitions. This is a good place to define %union
- * dependencies, for example. */
-@}
-%union @{
- /* Unlike the traditional Yacc prologue blocks, the output order
- * for %requires, %provides or %code blocks is not affected by their
- * declaration position relative to any %union in the grammar file. */
-@}
-%provides @{
- /* Bison inserts this block into both the header file and the code
- * file. In both files, the point of insertion is after the
- * Bison-generated definitions. This is a good place to declare or
- * define public functions or data structures that depend on the
- * Bison-generated definitions. */
-@}
-%code @{
- /* Bison treats this block like a post-prologue block: it inserts
- * it into the code file after the contents of the header file. It
- * does *not* insert it into the header file. This is a good place
- * to declare or define internal functions or data structures that
- * depend on the Bison-generated definitions. */
+%code-top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
@}
@end smallexample
-If you have multiple occurrences of any one of the above declarations, Bison
-will concatenate the contents in declaration order.
+@noindent
+For Java, @code{%code-top @{@var{code}@}} is currently unused.
-@xref{Prologue, ,The Prologue}.
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} appearing before the first
+@code{%union @{@var{code}@}} in a C/C++ based grammar file.
+@code{%code-top @{@var{code}@}} is cleaner as its functionality does not depend
+on its position in the grammar file relative to any
+@code{%union @{@var{code}@}}.
+
+@xref{Prologue Alternatives}.
@end deffn
@deffn {Directive} %debug
@xref{Contextual Precedence, ,Context-Dependent Precedence}.
@end deffn
+@deffn {Directive} %provides @{@var{code}@}
+This is the right place to write additional definitions you would like Bison to
+expose externally.
+For C/C++, this directive inserts your @var{code} both into the parser header
+file (if generated; @pxref{Table of Symbols, ,%defines}) and into the parser
+code file after Bison's required definitions.
+For Java, it inserts your @var{code} into the parser java file after the parser
+class.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
@deffn {Directive} %pure-parser
Bison declaration to request a pure (reentrant) parser.
@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
Require a Version of Bison}.
@end deffn
+@deffn {Directive} %requires @{@var{code}@}
+This is the right place to write dependency code for externally exposed
+definitions required by Bison.
+For C/C++, such exposed definitions are those usually appearing in the parser
+header file.
+Thus, this is the right place to define types referenced in
+@code{%union @{@var{code}@}} directives, and it is the right place to override
+Bison's default @code{YYSTYPE} and @code{YYLTYPE} definitions.
+For Java, this is the right place to write import directives.
+
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} (@pxref{Prologue, ,The Prologue})
+appearing before the first @code{%union @{@var{code}@}} in a C/C++ based
+grammar file.
+Unlike @code{%@{@var{code}%@}}, @code{%requires @{@var{code}@}} inserts your
+@var{code} both into the parser code file and into the parser header file (if
+generated; @pxref{Table of Symbols, ,%defines}) since Bison's required
+definitions should depend on it in both places.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
@deffn {Directive} %right
Bison declaration to assign right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
Start-Symbol}.
@end deffn
-@deffn {Directive} %symbol-default
-Used to declare a default @code{%destructor} or default @code{%printer}.
-@xref{Destructor Decl, , Freeing Discarded Symbols}.
-@end deffn
-
@deffn {Directive} %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
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