Proper structure improves program clarity. This is analogous to readability on lower levels and facilitates the use of the readability guidelines (Chapter 3). The various program structuring facilities provided by Ada were designed to enhance overall clarity of design. These guidelines show how to use these facilities for their intended purposes.
The concept of child packages supports the concept of subsystem, where a subsystem is represented in Ada as a hierarchy of library units. In general, a large system should be structured as a series of subsystems. Subsystems should be used to represent logically related library units, which together implement a single, high-level abstraction or framework.
Abstraction and encapsulation are supported by the package concept and by private types. Related data and subprograms can be grouped together and seen by a higher level as a single entity. Information hiding is enforced via strong typing and by the separation of package and subprogram specifications from their bodies. Exceptions and tasks are additional Ada language elements that impact program structure.