Manual

56 Bison 2.3
the types for a terminal or nonterminal symbol (see Section 3.7.5 [Nonterminal Symbols],
page 56).
As an extension to POSIX, a tag is allowed after the union. For example:
%union value {
double val;
symrec *tptr;
}
specifies the union tag value, so the corresponding C type is union value. If you do not
specify a tag, it defaults to YYSTYPE.
As another extension to POSIX, you may specify multiple %union declarations; their
contents are concatenated. However, only the first %union declaration can specify a tag.
Note that, unlike making a union declaration in C, you need not write a semicolon after
the closing brace.
3.7.5 Nonterminal Symbols
When you use %union to specify multiple value types, you must declare the value type of
each nonterminal symbol for which values are used. This is done with a %type declaration,
like this:
%type nonterminal...
Here nonterminal is the name of a nonterminal symbol, and type is the name given in the
%union to the alternative that you want (see Section 3.7.4 [The Collection of Value Types],
page 55). You can give any number of nonterminal symbols in the same %type declaration,
if they have the same value type. Use spaces to separate the symbol names.
You can also declare the value type of a terminal symbol. To do this, use the same
construction in a declaration for the terminal symbol. All kinds of token declarations allow
.
3.7.6 Performing Actions before Parsing
Sometimes your parser needs to perform some initializations before parsing. The %initialaction
directive allows for such arbitrary code.
%initial-action { code }
[Directive]
Declare that the braced code must be invoked before parsing each time yyparse is
called. The code may use $$ and @$ — initial value and location of the look-ahead
— and the %parse-param.
For instance, if your locations use a file name, you may use
%parse-param { char const *file_name };
%initial-action
{
@$.initialize (file_name);
};