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More Than One Value Type
In most programs, you will need different data types for different
kinds of tokens and groupings. For example, a numeric constant may
need type `int' or `long', while a string constant needs type `char *',
and an identifier might need a pointer to an entry in the symbol table.
To use more than one data type for semantic values in one parser,
Bison requires you to do two things:
* Specify the entire collection of possible data types, with the
`%union' Bison declaration (*note The Collection of Value Types:
* Choose one of those types for each symbol (terminal or nonterminal)
for which semantic values are used. This is done for tokens with
the `%token' Bison declaration (Note: Token Type Names
Decl.) and for groupings with the `%type' Bison declaration (*note
Nonterminal Symbols: Type Decl.).
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