+@node Type Generation
+@subsection Generating the Semantic Value Type
+@cindex declaring value types
+@cindex value types, declaring
+@findex %define api.value.type union
+
+The special value @code{union} of the @code{%define} variable
+@code{api.value.type} instructs Bison that the tags used with the
+@code{%token} and @code{%type} directives are genuine types, not names of
+members of @code{YYSTYPE}.
+
+For example:
+
+@example
+%define api.value.type union
+%token <int> INT "integer"
+%token <int> 'n'
+%type <int> expr
+%token <char const *> ID "identifier"
+@end example
+
+@noindent
+generates an appropriate value of @code{YYSTYPE} to support each symbol
+type. The name of the member of @code{YYSTYPE} for tokens than have a
+declared identifier @var{id} (such as @code{INT} and @code{ID} above, but
+not @code{'n'}) is @code{@var{id}}. The other symbols have unspecified
+names on which you should not depend; instead, relying on C casts to access
+the semantic value with the appropriate type:
+
+@example
+/* For an "integer". */
+yylval.INT = 42;
+return INT;
+
+/* For an 'n', also declared as int. */
+*((int*)&yylval) = 42;
+return 'n';
+
+/* For an "identifier". */
+yylval.ID = "42";
+return ID;
+@end example
+
+If the @code{%define} variable @code{api.token.prefix} is defined
+(@pxref{%define Summary,,api.token.prefix}), then it is also used to prefix
+the union member names. For instance, with @samp{%define api.token.prefix
+@{TOK_@}}:
+
+@example
+/* For an "integer". */
+yylval.TOK_INT = 42;
+return TOK_INT;
+@end example
+
+This Bison extension cannot work if @code{%yacc} (or
+@option{-y}/@option{--yacc}) is enabled, as POSIX mandates that Yacc
+generate tokens as macros (e.g., @samp{#define INT 258}, or @samp{#define
+TOK_INT 258}).
+
+This feature is new, and user feedback would be most welcome.
+
+A similar feature is provided for C++ that in addition overcomes C++
+limitations (that forbid non-trivial objects to be part of a @code{union}):
+@samp{%define api.value.type variant}, see @ref{C++ Variants}.
+
+@node Union Decl
+@subsection The Union Declaration
+@cindex declaring value types
+@cindex value types, declaring
+@findex %union
+
+The @code{%union} declaration specifies the entire collection of possible
+data types for semantic values. The keyword @code{%union} is followed by
+braced code containing the same thing that goes inside a @code{union} in C@.
+
+For example:
+
+@example
+@group
+%union @{
+ double val;
+ symrec *tptr;
+@}
+@end group
+@end example
+
+@noindent
+This says that the two alternative types are @code{double} and @code{symrec
+*}. They are given names @code{val} and @code{tptr}; these names are used
+in the @code{%token} and @code{%type} declarations to pick one of the types
+for a terminal or nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}).
+
+As an extension to POSIX, a tag is allowed after the @code{%union}. For
+example:
+
+@example
+@group
+%union value @{
+ double val;
+ symrec *tptr;
+@}
+@end group
+@end example
+
+@noindent
+specifies the union tag @code{value}, so the corresponding C type is
+@code{union value}. If you do not specify a tag, it defaults to
+@code{YYSTYPE}.
+
+As another extension to POSIX, you may specify multiple @code{%union}
+declarations; their contents are concatenated. However, only the first
+@code{%union} declaration can specify a tag.
+
+Note that, unlike making a @code{union} declaration in C, you need not write
+a semicolon after the closing brace.
+
+@node Structured Value Type
+@subsection Providing a Structured Semantic Value Type
+@cindex declaring value types
+@cindex value types, declaring
+@findex %union
+
+Instead of @code{%union}, you can define and use your own union type
+@code{YYSTYPE} if your grammar contains at least one @samp{<@var{type}>}
+tag. For example, you can put the following into a header file
+@file{parser.h}:
+
+@example
+@group
+union YYSTYPE @{
+ double val;
+ symrec *tptr;
+@};
+@end group
+@end example
+
+@noindent
+and then your grammar can use the following instead of @code{%union}:
+
+@example
+@group
+%@{
+#include "parser.h"
+%@}
+%define api.value.type @{union YYSTYPE@}
+%type <val> expr
+%token <tptr> ID
+@end group
+@end example
+
+Actually, you may also provide a @code{struct} rather that a @code{union},
+which may be handy if you want to track information for every symbol (such
+as preceding comments).
+
+The type you provide may even be structured and include pointers, in which
+case the type tags you provide may be composite, with @samp{.} and @samp{->}
+operators.
+
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