The default action

The default action associated with any rule can be written as:

$$ = $1

which means that the value of associated with $1 on the value stack is assigned $$ on the value stack when the rule is reduced. If, for example, $1 is an integer, then $$ is the same integer after the reduction occurs.

On the other hand, suppose that the recognition action associated with a rule explicitly states:

$$ = $1

This explicit assignment may not have the same effect as the implicit assignment. For example, suppose that you define:

%union {
    float floatval;
    int intval;
}

Also suppose that the type associated with $$ is floatval and the type associated with $1 is intval. Then the explicit statement:

$$ = $1

performs an integer to floating-point conversion when the value of $1 is assigned to $$, whereas the implicit statement did an integer to integer assignment and did not perform this conversion. You must therefore be careful and think about the effects of implicit versus explicit assignments.