3.5 System Structures: The Importance of Packages

Consider the input/output libraries we have been using in this book. Each of the various Get and Put statements in the earlier examples is really a procedure call statement. A procedure is a kind of system building block, a way of putting together a group of program statements and treating them as a unit, causing them to be executed by means of procedure calls. In this book you will learn how to write procedures; in this chapter you will continue just to use procedures written by others.

The Get and Put procedures we have been using were written by another programmer at another time; they were supplied to us as part of a package called Ada.Text_IO. Just as a procedure is a way of grouping statements, a package is a way of grouping procedures (and other program entities we will introduce later on). It is through the use of packages that procedures can be written and tested for general use (that is, by other programmers) and put in a form in which they can be supplied to others. Ada compilers come with several standard library packages. Ada.Text_IO is one of these; in the next section you will see another, called Ada.Calendar.

The package concept is one of the most important developments to be found in modern programming languages, such as Ada, Modula-2, Turbo Pascal, C++, and Eiffel. The designers of the different languages have not agreed on what terms to use for this concept: Package, module, unit, and class are commonly used. But it is generally agreed that the package--as it is called in Ada--is the essential programming tool to be used for going beyond the programming of very simple class exercises to what is generally called software engineering, or building real programs of real size for the real world. It is the package that allows us to develop a set of related operations and other entities, especially types, to test these thoroughly, and then to store them in an Ada program library for our future use or even to distribute them to others.

You will work with three kinds of packages in this book:

standard packages--such as Ada.Text_IO and Ada.Numerics, which you have seen already, and Ada.Calendar, introduced in Section 3.6--which are required by the Ada standard and supplied with all compilers;
packages supplied along with this book, such as the screen-control and spider packages introduced in Sections 3.7 and 3.8; and
packages written as part of your study of this book, such as the packages introduced starting in Chapter 4 and continuing throughout the book.