Functions

Produce a single return value
Called by expression
Can not modify the parameters passed to it
Requires a RETURN statement

FUNCTION add_bits (a, b : IN BIT) RETURN BIT IS
BEGIN -- functions can NOT return multiple values
        RETURN (a XOR b);
End add_bits;

FUNCTION add_bits2 (a, b : IN BIT) RETURN BIT IS
  VARIABLE result : BIT; -- variable is local to function
    BEGIN
      result := (a XOR b);
      RETURN result; -- the two functions are equivalent
END add_bits2;