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More improvements to the documentation of the prologue alternatives:

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* 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.
This commit is contained in:
Joel E. Denny
2006-10-20 22:10:50 +00:00
parent 67a9768ed4
commit a501eca911
3 changed files with 39 additions and 7 deletions
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35
doc/bison.texinfo
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@@ -2687,8 +2687,8 @@ feature test macros can affect the behavior of Bison-generated
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-top}, @code{%requires}, @code{%provides}, and @code{%code}.
@xref{Table of Symbols}.
@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:
@@ -2728,7 +2728,7 @@ You should prototype it in the second since Bison will insert that code
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} sections, but it's always explicit which
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} 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 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 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
@@ -2891,11 +2903,11 @@ file and the parser code file after the definitions for @code{yytokentype},
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 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)