Re: Ripple effect

["Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org>]

Robert A Duff wrote:
> 
> I think I want something like Ada's "use".  Pascal calls it "with" (for
> records, as opposed to packages).  It has nasty Beaujolais-like effects,
> but that's not necessary.  One of the brilliant things Ichbiah did was
> to eliminate Beaujolais effects.  Well, almost -- Ada 9X finished the
> job.  And Ichbiah didn't go far enough in that regard -- I would
> eliminate hiding entirely.

OK. I'm leaning the other way.

> But I think perhaps the "use"-like thing belongs on the package, rather
> than on the clients of the package, because the person who writes the
> package needs to use naming conventions that make sense for use or
> not-use.
> 
> I've considered calling it "open" -- it opens up the scope for
> visibility.  An "open" module provides direct visibility of its contents
> to clients.  And perhaps an "open" statement in addition, to be used by
> clients.

Thanks for the description. Are you saying that only an open module can 
be opened by a client, but clients can not open a module and treated 
like a closed module?

> My thinking is that if B uses A in a visible way, C pretty-much has to
> know about A.  E.g.:
> 
>     with A;
>     package B is
>         procedue Mumble(X: A.T);
>     end B;
> 
> If C uses B.Mumble, it most likely needs to declare objects of type A.T.
> C certainly knows about type A.T!  So non-transitive with's cause a lot
> of clutter -- C needs to 'with' everything it uses, plus everything
> those things are (visibly) based on, transitively.  So high-level
> packages tend to have so many with_clauses that nobody wants to read
> them, largely defeating the purpose.

True, but we're talking about the case where B doesn't actually 
reference A (the with of A is "otherwise unneeded").

> OTOH, if B uses A only for implementation, that's a different story.
> Then the import of A belongs on the body of B, and C shouldn't know
> about it.  (My language doesn't have private parts.)

Agreed.

> By the way, the issue of bogus with_clauses is a real maintenance
> problem -- nobody ever deletes a with_clause, so they tend to
> accumulate.  This problem is easily solved.  The compiler should give an
> error message.  An error that can be suppressed, so that you can
> temporarily have bogus with_clauses during development.  That's
> essentially what GNAT does, with appropriate warning switches.

My sediments exactly. In fact, I do delete unneeded withs when I find them.

-- 
Jeff Carter
"There's no messiah here. There's a mess all right, but no messiah."
Monty Python's Life of Brian
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