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The Collection of Value Types
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The `%union' declaration specifies the entire collection of possible
data types for semantic values. The keyword `%union' is followed by a
pair of braces 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 (Type Decl).
Note that, unlike making a `union' declaration in C, you do not write
a semicolon after the closing brace.