Re: Q: access to subprogram

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

Jon said

"irrelevant nonsense (that would just be plain stupid), I said that
most programs don't have much nesting and that without nesting no cost
is incurred and therefore any argument about distributed cost in the
user model as a reason for not having them was nonsense.  And that is
still just plain true."

Just to be absolutely clear on this, since it seems to have caused a lot
of confusion :-)

The Steelman (and hence Ada 83) reasoning for not allowing procedure
parameters was indeed influenced by concerns about distributed cost
of uplevel reference mechanisms (if you read Steelman, you will see that
concern expressed in black-and-white).

Whether this concern is legitimate or not probably can't be settled without
some hard data on existing Ada programs (what you need is dynamic traces
that show how many calls are to outer level procedures, and how many
calls are to procedures that contain no nested procedures). At least that
will allow you to track the cost of display/static-chain handling. If you
think uplevel references themselves are a significant factor, as some do
in this thread, then you also need dynamic data on the number of such
references and how deep they go (by procedure invocation, so that you
can factor out optimization effects). As is clear from the thread, Jon
and I simply disagree on the bottom line here, but I don't think any
more arguments without data can illuminate this discussion further (it
is by the way a *very* old discussion).

The Ada 95 reasoning for not allowing completely general procedure
parameters (that would handle the case where nested procedures are
involved in all cases), has NOTHING to do with distributed cost. It
had solely to do with concerns about the cost of implementation, and
if you want to track Norman's very nice precis of the written discussion
on this issue, you will see how the back-and-forth went on this issue.

Again, the question of whether the trade-off was made correctly can
be discussed. That's more of a subjective issue, but it would be
influenced in retrospect by nice examples of things you can't do but
would be nice to be able to do, but you can't do in any other convenient
way, and one would think that if there are such, some of them might have
surfaced by now.