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			13.1 THE FORM OF ADA PACKAGES

A package forms a collection of logically related entities or computational
resources.  Formally, we say that a package encapsulates (puts a wall
around) these resources.  In Chapter 6, we introduced the symbol for an Ada
package, repeated herein Figure 13-1.  From this figure we see that a
package consists of two parts, the specification and the body.  The
specification identifies and describes the visible parts of the package;
formally, the package exports these entities.  We have used two different
symbols in the package specification to distinguish between exporting
objects and type (the rounded rectangle)and exporting operations (the
regular rectangle).

	In a sense, this package specification is the contract between the
implementer of the package and the user.  This interface specifies which
parts of the package may be used and, furthermore, how they may be used. 
It is not important for a user to understand how these operations are
actually implemented.  In addition, a package user may refer only to these
visible entities.

	For example, the human interface to a car includes the steering
wheel, brakes, and accelerator; these are the resources that are visible. 
The driver doesn't need to know how these resources work-that's an
implementation detail.  In Ada, similar details can be hidden in the
package body.  This structure directly supports the principles of
modularity, abstraction, localization, and information hiding.  Of course,
good programmers can apply these principles in other languages, including
FORTRAN or even assembly languages.  The difference between Ada and most
other programming languages is is that Ada packages enforce and encourage
these principles.  The language rules do not permit a package user to do
anything more than what the package specification allows.  If the user
tries, he or she will be trapped at compilation time with syntax errors.

	Since the specification and body may be separately compiled, it is
a simple matter to create the specification early in the software design
and then add the body later.  Furthermore, it is good practice to separate
the two parts physically-supporting information hiding literally.  Perhaps
most importantly, though, Ada packages help the programmer control the
complexity of software solutions by giving him or her a mechanism with
which to physically group related entities into a logical chunk.