3.7.4 The Collection of Value Types

The %union declaration specifies the entire collection of possible data types for semantic values. The keyword %union is followed by braced code containing the same thing that goes inside a union in C.

For example:

%union {
  double val;
  symrec *tptr;
}

This says that the two alternative types are double and symrec *. They are given names val and tptr; these names are used in the %token and %type declarations to pick one of the types for a terminal or nonterminal symbol (see section Nonterminal Symbols).

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.

Instead of %union, you can define and use your own union type YYSTYPE if your grammar contains at least one ‘<type>’ tag. For example, you can put the following into a header file ‘parser.h’:

union YYSTYPE {
  double val;
  symrec *tptr;
};
typedef union YYSTYPE YYSTYPE;

and then your grammar can use the following instead of %union:

%{
#include "parser.h"
%}
%type <val> expr
%token <tptr> ID