Separate compilation

Motivation

• separate "what" from "how"
  • abstraction
  • modularisation
• develop programs in small pieces

The first is the reason we focus on procedures as the blocks from which a program is constructed.

The second lets us develop and test a program a bit at a time. This makes testing and debugging simpler than if you test the whole program all at once.

For it to be possible to develop a program a bit at a time, there needs to be some help from the language. You need to be able to break programs into several files, each of which is compiled separately.

Ada library

To understand the mechanisms for separate compilation, we need to know how the compiler makes use of the Ada library

• when first created it contains standard information (eg TEXT_IO)
• result of every succesful compilation is placed there
• look there for needed items when compiling

A program can be compiled in pieces

• several separate compilations
• each adds one or more compilation units to library
• care: compile units in appropriate order
• bind to form executable once all are compiled

Units for separate compilation

Ada provides two mechanisms for separate compilation:

• subunits
• library units

Usually a library unit is a package.

When to use each sort of unit:

• if a procedure is defined as a sub-task, in the process of top-down decomposition, generally prefer a subunit
• if a procedure is a useful operation (eg bottom-up programming), generally prefer to make it a library unit (or an element of a package, in which case the package is the library unit).

A subunits is written to be part of a single specific program. A library unit may be used in many programs.