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Programs and subprograms

Programs are complex objects. Writing them is hard. Programs increase in complexity as they get larger, and people cannot cope with too much complexity.

To make program development and maintenance a manageable activity, the key is to control the complexity of the software.

The mechanisms we use to control complexity are:

Abstraction
- understand one section at a time
- related items together
Modularisation
- each abstraction a separate piece of code
Encapsulation
- internal details not visible from outside

Modularity

A module is a piece of the system that can be understood as a single unit

Partition system into modules:

reduced complexity; easier development and maintenance
parallel development; divide programming job for teams

Ada modules:

subprograms: procedures, functions
packages

Designing a system involves:

identifying the modules that will make up the system
working out how data will be communicated between them, by defining parameters and their modes

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