After a production, you can write an action that is performed when the parser does a reduce by that production. Write it in C, surrounded by braces. For example,
expression : expression '+' expression
{printf("Reducing by production E -> E + E\n");
}
Type YYSTYPE is a union type. Suppose that it is defined by
typedef union
{
int ival;
const char* strval;
} YYSTYPE;
You will want to tell which field of the union is used
for each token. Lines
%token <ival> TOK_NUMBER %token <strval> TOK_IDENTIFIERtell Bison that TOK_NUMBER uses field ival and TOK_IDENTIFIER uses field strval.
In an action, refer to a token by its position on the right-hand side of the production, numbering from 1. The attribute of the first thing is $1, of the second thing is $2, etc.
For example, in
expression : TOK_LET TOK_IDENTIFIER
'=' expression TOK_IN
expression TOK_END
{const char* id = $2;
printf("I see a let of %s\n", id);
}
the action shows the attribute of the TOK_IDENTIFIER token.
Notice that you don't explicitly write .strval. Bison adds that automatically.