+@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 source 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 a
+@code{%code top} and an unqualified @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>
+
+ /* WARNING: The following code really belongs
+ * in a `%code 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 the unqualified @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 before the warning need to appear near the top of the
+parser source code file.
+The first line after the warning is required by @code{YYSTYPE} and thus also
+needs to appear in the parser source code file.
+However, if you've instructed Bison to generate a parser header file
+(@pxref{Table of Symbols, ,%defines}), you probably want that line to appear
+before the @code{YYSTYPE} definition in that header file as well.
+The @code{YYLTYPE} definition should also 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 required by the @code{YYSTYPE} and @code{YYLTYPE}
+definitions.
+Thus, they belong in one or more @code{%code requires}:
+
+@smallexample
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+
+%code requires @{
+ #include "ptypes.h"
+@}
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code 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 source code file and the parser header file.
+(By the same reasoning, @code{%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{%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
+source code file and that has no special need to appear at the top of that
+file, you should prefer the unqualified @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 the unqualified @code{%code} and
+@code{%code requires} to be the most important of the four @var{Prologue}
+alternatives.
+
+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 source code file.
+Since this function is not a dependency required by @code{YYSTYPE} or
+@code{YYLTYPE}, it doesn't make sense to move its prototype to a
+@code{%code requires}.
+More importantly, since it depends upon @code{YYLTYPE} and @code{yytokentype},
+@code{%code requires} is not sufficient.
+Instead, move its prototype from the unqualified @code{%code} to a
+@code{%code provides}:
+
+@smallexample
+%code top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+
+%code requires @{
+ #include "ptypes.h"
+@}
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code requires @{
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
+%code 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 source 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 source code and header files:
+@code{%code top}, @code{%code requires}, @code{%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
+%code requires @{ #include "type1.h" @}
+%union @{ type1 field1; @}
+%destructor @{ type1_free ($$); @} <field1>
+%printer @{ type1_print ($$); @} <field1>
+
+%code 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} alternatives 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{%code requires}, @code{%code provides}, or @code{%code top}
+as needed.
+
projects / bison.git / blobdiff