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<TITLE>6.2 Making Ada ADTs More Abstract</TITLE>
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<P><FONT SIZE="5">6.2 Making Ada ADTs More Abstract</FONT><BR>
<P>Like many modern programming languages, Ada provides
mechanisms for hiding information and structure. In
particular, Ada provides a <STRONG>package</STRONG> facility
that allows the programmer to declare objects in a package
definition and define them in a corresponding package body.
This works well for functions and procedures. However, in
the case of types, implementation issues (in particular, the
need to know the size of exported types) have led the
designers of Ada to force the placement of private type
definitions in the definition package rather than the
implementation package. This means that some implementation
details are present in the package definition for all to
see. While not actually dangerous (the user of the package
cannot make use of the information without recourse to
"Chapter 13" of the Ada Language Reference
Manual<STRONG>[DOD83]</STRONG>), this aspect of Ada is certainly
unpleasant.
<P>During the development of some Ada programs, FunnelWeb
was used to solve this problem. Instead of creating a
separate file for the package specification and package
body, a single FunnelWeb file was created containing two
sections, one for the each package part. The
"private" part of the package specification was then
moved (using a FunnelWeb macro definition) to the section
describing the package body. Readers who wished only to read
the package specification could read only the first part,
which contained a fully documented description not
containing the private definition.
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