Re: Ripple effect

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Jeffrey R. Carter" <spam.not.jrcarter@acm.not.spam.org> writes:

> 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.

I suppose there are many other ways; I'm not sure which one you're
leaning toward.  ;-)  Surely not to increase Beaujolais-like effects!?

> 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?

I'm not sure.  The user can declare a module "open".  If you say
"use X", you get visibility on X (remember this is not Ada's "use").  If
X is open, you also get (direct) visibility on the contents of X.  The
part I'm not sure about is whether to _also_ provide something like
Ada's "use" clause, which allows clients of a non-open module to open
it.  It could work for records, too, like Pascal's "with" statement --
except that I would eliminate the nasty Beaujolais-like effects of
Pascal.

> > 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").

I guess I'm confused as to what the issue is, then.  If B doesn't
reference A, then it shouldn't import it -- and the compiler should
give a (suppressable) error.  Suppressable because you might want unused
imports temporarily during construction.  Also, there are some cases
where unused imports are needed.

> > 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.

Sure, so do I, but they're hard to find by hand.  Once when I turned on
the GNAT warning for unused with's on a program of about 200,000 lines,
I found hundreds and hundreds of them.

- Bob