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From: geoff@boulder.UUCP (Geoffrey M. Clemm)
Newsgroups: net.lang.ada
Subject: Implementing mutually dependent access types in separate packages
Message-ID: <383@boulder.UUCP>
Date: Sun, 7-Jul-85 19:19:39 EDT
Article-I.D.: boulder.383
Posted: Sun Jul  7 19:19:39 1985
Date-Received: Thu, 11-Jul-85 04:53:05 EDT
Distribution: net
Organization: University of Colorado, Boulder
List-Id: <comp.lang.ada>

*

Problem :
   I have a large program with data structures whose implementations
   are all mutually dependent.  For example, a "FileHeader" is a structure
   containing a field that is a pointer to a "Parameter", a "Parameter" is a
   structure containing a field that is a pointer to an "Item", and an "Item"
   is a structure containing a field that is a pointer back to a "FileHeader".
   (For a simple example, see section 3.8.1 of the ADA standard).
   
   The desireable implementation would consist of one package for each data
   structure, where a package performs the usual data abstraction principle
   of providing accessing functions while hiding the physical implementation
   of the data structure.

   In the above example, the package for "FileHeader" must provide (amoung
   other things) the type "FileHeaderPtr" (ACCESS FileHeader) and an accessing
   function "FileHeader_Parameter" that takes a FileHeaderPtr as an input
   argument and returns a ParameterPtr.  Similarly for each of the other
   packages.

   Unfortunately, I can find no way of doing this in ADA.  The obvious
   implementation generates a "USE" cycle that is illegal according to
   the language standard (not to mention being rejected by our compiler).

   The only method I can discover for doing this in ADA is to take all
   of the data structure definitions out of their respective packages
   and then clump them together into a single "implementation package".
   All the individual packages then "USE" this implementation package.

   This is very unfortunate for a variety of reasons.  It's especially
   annoying since a variety of languages earlier than ADA allow this form
   of data abstraction.

Question :
   Can anyone come up with a way of leaving the implementation of each
   data structure in its own package, while providing the desired accessing
   functions and not violating the ADA standard ?

Note:
   If one was not required to provide accessing functions of the form
   "FileHeader_Parameter(FileHeaderPtr) RETURN ParameterPtr", there of
   course would be no problem, but these accessing functions are critical.

Geoffrey Clemm