+%@{
+ #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"
+@}
+%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;
+@}
+
+%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
projects / bison.git / blobdiff