Re: circular unit dependency

["John G. Volan" <johnvolan@sprintmail.com>]

W. Wesley Groleau (Wes) wrote:
> 
> John,
> Your example is reasonable, and a solution would be a good thing,
> but the problem is not a disaster.  I can only remember once in
> over ten years that I wished I could separately package two types
> but couldn't.

Of course it's not a disaster. There are, after all, a variety of
avoidance techniques, some of which I enumerate in section [3] of my
FAQ.  I think what happens is that most Ada programmers naturally fall
into the technique suggested by [3.2]: Arbitrarily choose one class to
be client-only and the other to be supplier-only, force the association
to be one-way.  I suspect many Ada programmers are not even conscious
that they are doing that. I exploit it whenever I can.  It can let you
go quite a long time without ever facing the Day of Mutual Reckoning.
:-)  However, this kind of one-way biasing is somewhat artificial and
stilted when, conceptually, the object classes are really peers.  The
alternative, mutually dependent packages, only _seems_ awkward because
of this quirk (or rather, this hole) in the language. If Ada already had
one of the enhancements that have been suggested (see section [5]), I
wager we wouldn't be having any of these religious debates about how
packages were "meant" to be used.

I just think, as we begin entering an era of increasingly large and
complex domains (e.g., distributed client/server object systems), these
kinds of situations will increase in frequency.  It would be a good idea
to have all our ducks in a row. I believe that separately encapsulated
yet mutually dependent object classes are a legitimate design pattern,
one that ought to be available as an option within every engineer's
repertoire.

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