6.2 Making Ada ADTs More Abstract

Like many modern programming languages, Ada provides mechanisms for hiding information and structure. In particular, Ada provides a package 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[DOD83]), this aspect of Ada is certainly unpleasant.

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.