Re: "private with" proposal

[Vincent Marciante <INALID_EXCEPT_FOR_marciant@earthlink.net>]

Randy Brukardt wrote:
> 
> Vincent Marciante wrote in message <3DEC42E0.407B@earthlink.net>...
> >Stephen Leake wrote:
> >
> >> You are correct, the "extra syntax" of "private" in the context
> clause
> >> is not strictly necessary; the compiler and reader could figure out
> >> what's going on.
> >>
> >> But what is the downside of having the extra syntax?
> >
> >Discussing in the same AI as the one pertaining to
> >visability _within_ hierarchies confuses to issue.
> 
> The entire point is that visibility is the wrong model for this. The
> syntax is tied to a legality rule (not a visibility rule), in order that
> added or removing 'private' or moving a declaration from the private
> part to the visible part does not silently change the meaning of a
> program. (This is covered in the AI).

Okay, my phraseology reflected my inadaqate apreciation for 
the distinction between visability rules and legality rules.
If the rule would be a legality rule, does that imply that
new syntax is strictly necessary?
 
> A secondary issue is the desire to think of the private part as a
> separate part. We discussed allowing separate compilation of private
> parts; in that case, 'private with' is just the context clause of the
> private part.
> 
> The visibility rules are already way too complex in Ada, and your
> suggestion would make them even more complex. Moreover, changing a unit
> from private to non-private (or vice-versa) could silently change the
> meaning of a program from one legal meaning to a different legal
> meaning. (Currently, any such program is illegal, as the with would be
> illegal when the unit is private; with 'private with' as currently
> described, the use that causes the problem would be illegal.)

Okay, suggesting specifically a visibility rule was unintended.
 
> In any case, 'private with' is easy to describe, easy to understand,
> easy to use, and easy to implement. A better solution (such as separate
> compilation of private parts) may exist, but waiting for the perfect
> solution to an important problem when we have an adequate solution that
> we can use now would be a mistake.
>                     Randy Brukardt

Thanks for the condensed explanation.

-- 
Vinny