Re: Subverting 'Access for Sub-programs

[Robert A Duff <bobduff@world.std.com>]

Robert Dewar <robert_dewar@my-deja.com> writes:

> Yes, this would be annoying for the display folks, because, just
> as is necessary in Pascal if you use displays, you need to save
> the display as part of a subprogram access value. THis is not
                                                            ^^^
> possible, but it is probably not the right approach.
  ^^^^^^^^
I think that's a typo -- you meant "not IMpossible" = "possible"?

I know of one Pascal compiler that used displays and implemented
downward closures, which proves that it's not only possible, but
feasible.  But I agree that it's not the right aproach.

> Yes, we probably want the limited-access types that are
> completely safe, but I would also put in the Unchecked_Access
> possibility, since it will cover additional useful cases.

You've convinced *me* that the Unch_Acc on subprogs has its uses.
Tucker, on the other hand, has a different point of view toward
nesting in general ("for the birds").

>... I
> would not go as far as the Unrestricted_Access semantics of
> GNAT for objects in any case as part of a semi-standard proposal
> since aspects of this (accesses-to-slices for example) are
> highly implementation dependent.

One thing that bothers me about the object'un_acc thing, is that if the
object is not addressable (component of something packed), then it just
gives a "wrong" answer -- wrong in the sense that the result doesn't
point at the object.  At least that's my understanding, since you've
defined 'unr_acc in terms of 'address, which also has this
unfortunate behavior.

- Bob
-- 
Change robert to bob to get my real email address.  Sorry.