Re: Two ideas for the next Ada standard

[dewar@cs.nyu.edu (Robert Dewar)]

Ted Dennison said

You're right. I am confused. Let's try an example. Supose unit A withs spec
B. Spec B withs units C and D, and body B withs units E-K (7 extra units).

Now with private parts in the spec, the only time A has to be recompiled is
when A, the spec of B, C or D changes. But if B has a private part in the
BODY, it seems to me that A would have to be recompiled whenever A, the spec
of B, the body of B, or C, D, E, F, H, I, J, or K change. Thus this change
(assuming even distribution) makes A nearly 3 times more likely to be
obsoleted by a source change. If we start adding with's to units C-K, the
odds get even worse.

So what am I missing here?

  What you are missing is that the spec of A is not dependent on the body
  that is stored in the body file, so it does not transitively pick up
  the source dependencies of the body. Indeed the compiler won't even look
  at the body, it will look at the private part.

  It is just for packaging convenience that one would even consider having
  the private part in the body, simply to avoid yet another file, but there
  is no implication that the compiler would look at the body, just because
  it is in the same file!

  The upside is that you have one less file, and the private part is
  simply part of the implementation. Indeed think of the adb file now
  as an implementation file which contains two items, the private part
  and the body.

  The downside, not nearly so severe as you worry about above, is that
  if you modify the body, then you have to recompile specs, but you do
  not have to recompile if all you changed was a package on which the
  body depended (except in the inline case, where of course it is
  inevitable that this be the case).

Obviously this could only happen if a recompile of the entire system took
that long. Admittedly, the most extreme I have ever seen was 1 day. But
if they hadn't used a dedicated VAX with 2 processors, it could easily have
taken twice as long.

  Well what model of VAX? These are pretty slow machines by todays standards.
  After all the VAX/780 has a spec marc of 1 by definition, and you can't
  buy a PC with a spec marc less than about 50!