Re: feature tests and %before-header

[Joel E. Denny]

On Mon, 16 Oct 2006, Joel E. Denny wrote:

> +     Clean up and extend documentation for the prologue alternatives.
> +     * NEWS (2.3a+): Describe prologue alternatives.
> +     * doc/bison.texinfo (Prologue): Move discussion of prologue
> +     alternatives to...
> +     (Prologue Alternatives): ... this new section, and extend it to discuss
> +     all 4 directives in detail.
> +     (Bison Symbols): Clean up discussion of prologue alternatives and add
> +     %code-top.

I committed the following to revise the above.

Index: ChangeLog
===================================================================
RCS file: /sources/bison/bison/ChangeLog,v
retrieving revision 1.1590
diff -p -u -r1.1590 ChangeLog
--- ChangeLog   16 Oct 2006 05:25:36 -0000      1.1590
+++ ChangeLog   20 Oct 2006 22:06:42 -0000
@@ -1,3 +1,12 @@
+2006-10-20  Joel E. Denny  <address@hidden>
+
+       More improvements to the documentation of the prologue alternatives:
+       * NEWS (2.3a+): Mention the new `Prologue Alternatives' section in the
+       Bison manual.
+       * doc/bison.texinfo (Prologue Alternatives): Correct some errors.  Add
+       some text to clarify the relative importance of the new directives and
+       to show how these directives may be viewed as code labels.
+
 2006-10-16  Joel E. Denny  <address@hidden>
 
        Similar to the recently removed %before-header, add %code-top as the
Index: NEWS
===================================================================
RCS file: /sources/bison/bison/NEWS,v
retrieving revision 1.162
diff -p -u -r1.162 NEWS
--- NEWS        16 Oct 2006 05:25:36 -0000      1.162
+++ NEWS        20 Oct 2006 22:06:42 -0000
@@ -74,6 +74,8 @@ Changes in version 2.3a+ (????-??-??):
   Bison will concatenate the contents in the order they appear in the grammar
   file.
 
+  Also see the new section `Prologue Alternatives' in the Bison manual.
+
 Changes in version 2.3a, 2006-09-13:
 
 * Instead of %union, you can define and use your own union type
Index: doc/bison.texinfo
===================================================================
RCS file: /sources/bison/bison/doc/bison.texinfo,v
retrieving revision 1.209
diff -p -u -r1.209 bison.texinfo
--- doc/bison.texinfo   16 Oct 2006 05:48:34 -0000      1.209
+++ doc/bison.texinfo   20 Oct 2006 22:06:46 -0000
@@ -2687,8 +2687,8 @@ feature test macros can affect the behav
 The functionality of @var{Prologue} sections can often be subtle and
 inflexible.
 As an alternative, Bison provides a set of more explicit directives:
address@hidden, @code{%requires}, @code{%provides}, and @code{%code}.
address@hidden of Symbols}.
address@hidden, @code{%requires}, @code{%provides}, and @code{%code-top}.
address@hidden of Symbols,,Bison Symbols}.
 
 Look again at the example of the previous section:
 
@@ -2728,7 +2728,7 @@ You should prototype it in the second si
 
 This distinction in functionality between the two @var{Prologue} sections is
 established by the appearance of the @code{%union} between them.
-This behavior raises several questions.
+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}?
@@ -2777,7 +2777,7 @@ as the two kinds of @var{Prologue} secti
 kind you intend.
 Moreover, both kinds are always available even in the absence of @code{%union}.
 
-The first @var{Prologue} section above now logically contains two parts.
+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.
@@ -2787,7 +2787,7 @@ appear before the @code{YYSTYPE} definit
 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 first @var{Prologue} section, all but the first two
+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}:
@@ -2834,6 +2834,18 @@ definitions in both the parser code 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
@@ -2891,11 +2903,11 @@ file and the parser code file after the 
 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 will generally be easier to read if you also follow
+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.
 
-Any of these directives may be declared multiple times in the grammar file.
+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.
@@ -2926,6 +2938,15 @@ definitions section is going to adversel
 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
address@hidden 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.
address@hidden 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)