Re: subprogram must not be deeper than access type

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> On Mon, 26 Sep 2011 09:43:07 -0400, Robert A Duff wrote:
>
>> Natasha Kerensikova <lithiumcat@gmail.com> writes:
>> 
>>> Having read the Rationale 2005, I understand now that naming an access
>>> type is far from being equivalent to text replacement.
>> 
>> Yeah.  That's a language design flaw.
>
> What is the flaw, naming or having it anonymous?

In my opinion, if anonymous types are allowed, they should behave
exactly like named types.  In the case of access types, I mean
"exactly like a named type declared immediately after the designated
[sub?]type".  (There's an obscure issue, in the case where the
designated subtype is more nested than the designated type,
and depends on nested data.  I'm not sure how to solve that;
perhaps just forbid it.)

The flaw I was referring to was that Ada doesn't have that equivalence.
Making an access type anonymous invokes all sorts of magical semantics,
which is confusing.  If such magical semantics are desirable, they
should be invoked by some other syntax.  (And by the way, I don't think
run-time accessibility checks are desirable.  That's another design flaw.)

Another flaw is that some types can be anonymous (task, protected,
array, access) and some can't.  It should be all or nothing.

And it's just weird that if you say "X, Y : array...;"
"X := Y;" and "if X = Y ..." are illegal.  Textual replacement is
a wrong way to define semantics.

> I think that the problem is that access types to automatically collected
> objects and access types to the objects allocated and freed dynamically
> should be different, if the former could not be eliminated at all.

Not sure what you mean by "automatically collected".  Local to a
procedure?  Or are you talking about garbage collection?  What
about library-package-body variables?  What about components
of variables?

Ada 83 required all access values to point to heap-allocated objects.
Ada 95 added "access all", which allows pointing at any aliased object.
That's sort of similar to the distinction you're making, I think.

- Bob