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From: Jeffrey Carter <jeffrey.carter@boeing.com>
Subject: Re: List container strawman
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Date: 2001-11-14T16:53:26+00:00
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Marin David Condic wrote:
> 
> I don't think I'd characterize it as "complicated" - more like "awkward". In
> the past, I've built data structures that would have a "private" flavor and
> a "limited private" flavor so that when I had the simple case (assignment
> and equality inherently available) I didn't have to go to the effort of
> inventing small subprograms or otherwise jockying around the fact that the
> limited case was going to insist I provide something that was already there.
> 
> I think the reason for "private" being preferable over "limited private" is
> that now we can build things that inherit from Controlled, so if you need to
> do something to control behavior on assignment, you can, but you don't have
> to for the simple case. Hence, you can save yourself some generic parameters
> and make instantiation a simpler matter. (Do you really want to have to
> define an "Assign" procedure every time you want to stack up some integers?
> Its not complicated - just a pain in the posterior.)
> 
> "Jeffrey Carter" <jeffrey.carter@boeing.com> wrote in message
> news:3BF1A33D.73DE084F@boeing.com...
> >
> > If the client has to provide an Assign procedure for the Element type,
> > then the Element type should be limited private. While I have no problem
> > with this, I think most of the participants in this discussion have
> > already decided that's too complicated for them.
> >

Yes, but I was referring to a comment that suggested an Assign procedure
needs to be provided even though Element is private. In that case you
might as well make Element limited private since it buys you additional
applicability without changing how the component is instantiated.

Of course, the real reason not to import Element as private is that it
is an incorrect specification, and precise specification is essential to
design by contract. A generic package has 2 specifications: the generic
formal part, which should specify precisely what the client must provide
to use the package, and the package specification, which should specify
precisely what services the package provides to its clients.

A formal private type specifies that the package requires "=" for type,
when in fact a list does not use "=" for its Element type.

-- 
Jeffrey Carter