Re: Subverting 'Access for Sub-programs

[Robert Dewar <robert_dewar@my-deja.com>]

In article <wcczp06jj39.fsf@world.std.com>,
  Robert A Duff <bobduff@world.std.com> wrote:
> The language ended up with call backs, but not downward
> closures.

Not inappropriate, because call backs were the new feature not
considered in the design of Ada 83. The downward closure issue
has not changed at all from the design of Ada 83, and in fact
it was not clear there was a sufficiently good reason to
embroider in this area.

> Your comment is, of course, also true of what we ended up with
> 'Unrestricted_Access, which is neither safe nor portable --
> it's even less checked than 'Unchecked_Access.

Well that's a bit confused. There is no Unchecked_Access for
subprograms, so I am not quite sure what the comparison would
mean there ...

For other than subprograms, Unrestricted_Access is equivalent to
(though preferable to, since more checked) than taking 'Address
followed by an unchecked conversion. It is to that construct
that Unrestricted_Access should be compared, not
Unchecked_Access.

The fact of the matter is that even with the design teams
proposals, there are still cases where Unrestricted_Access
would be useful for subprograms, since there are cases which
are safe, but the compiler cannot tell they are safe.

To me the dangling data references of Unchecked_Access are not
particularly better than the dangling subprogram pointers of
Unrestricted_Access. The consequence of either is possible
data corruption (in the former case, from a bad address, in
the latter case from a bad static link).

Note that Unrestricted_Access applied to an inner procedure with
no non-local references is in fact entirely safe (such nested
procedures are not at all uncommon, they are used in cases where
you want to restrict the visibility of a local nested
subprogram).


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