guideline

Use named numbers and static expressions to allow multiple dependencies to be linked to a small number of symbols.

------------------------------------------------------------------------
procedure Disk_Driver is
   -- In this procedure, a number of important disk parameters are
   -- linked.
   Number_Of_Sectors  : constant :=     4;
   Number_Of_Tracks   : constant :=   200;
   Number_Of_Surfaces : constant :=    18;
   Sector_Capacity    : constant := 4_096;
   Track_Capacity   : constant := Number_Of_Sectors  * Sector_Capacity;
   Surface_Capacity : constant := Number_Of_Tracks   * Track_Capacity;
   Disk_Capacity    : constant := Number_Of_Surfaces * Surface_Capacity;
   type Sector_Range  is range 1 .. Number_Of_Sectors;
   type Track_Range   is range 1 .. Number_Of_Tracks;
   type Surface_Range is range 1 .. Number_Of_Surfaces;
   type Track_Map   is array (Sector_Range)  of ...;
   type Surface_Map is array (Track_Range)   of Track_Map;
   type Disk_Map    is array (Surface_Range) of Surface_Map;
begin  -- Disk_Driver
   ...
end Disk_Driver;
------------------------------------------------------------------------

rationale

To reuse software that uses named numbers and static expressions appropriately, just one or a small number of constants need to be reset, and all declarations and associated code are changed automatically. Apart from easing reuse, this reduces the number of opportunities for error and documents the meanings of the types and constants without using error-prone comments.