guideline

Consider using hierarchies of tagged types to promote generalization of software for reuse.

Consider using a tagged type hierarchy to decouple a generalized algorithm from the details of dependency on specific types.

with Wage_Info;
package Personnel is
   type Employee is abstract tagged limited private;
   type Employee_Ptr is access all Employee'Class;
   ...
   procedure Compute_Wage (E : Employee) is abstract;
private
   type Employee is tagged limited record
      Name  : ...;
      SSN   : ... ;
      Rates : Wage_Info.Tax_Info;
      ...
   end record;
end Personnel;
package Personnel.Part_Time is
   type Part_Timer is new Employee with private;
   ...
   procedure Compute_Wage (E : Part_Timer);
private
   ...
end Personnel.Part_Time;
package Personnel.Full_Time is
   type Full_Timer is new Employee with private;
   ...
   procedure Compute_Wage (E : Full_Timer);
private
   ...
end Personnel.Full_Time;

Given the following array declaration:

type Employee_List is array (Positive range <>) of Personnel.Employee_Ptr;

you can write a procedure that computes the wage of each employee, regardless of the different types of employees that you create. The Employee_List consists of an array of pointers to the various kinds of employees, each of which has an individual Compute_Wage procedure. (The primitive Compute_Wage is declared as an abstract procedure and, therefore, must be overridden by all descendants.) You will not need to modify the payroll code as you specialize the kinds of employees:

procedure Compute_Payroll (Who : Employee_List) is
begin -- Compute_Payroll
   for E in Who'Range loop
      Compute_Wage (Who(E).all);
   end loop;
end Compute_Payroll;
      

rationale