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:
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}?
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.
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}:
(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
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.
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
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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