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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 <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 The Union Declaration). 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.
While POSIX Yacc allows %type only for nonterminals, Bison accepts
that this directive be also applied to terminal symbols. To declare
exclusively nonterminal symbols, use the safer %nterm:
%nterm <type> nonterminal…