Re: subprogram must not be deeper than access type

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Mon, 26 Sep 2011 12:15:51 -0400, Robert A Duff wrote:

> 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.)

Yes, it would be much simpler to have subprogram types for downward
closures.

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

I'd like to have anonymous types in record components. It is annoying not
to be able to write:

   type Foo is record
       Nested : record
          end record;
       List : array (...) of ...;
   end record;

Even C can this!

> 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.

But structural type matching is more wrong than that.

>> 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?

E.g. things on the stack.

> 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.

Yes. There should be:

1. Strictly heap access types, manually allocated/deallocated, no
accessibility checks, no calls to Finalize outside Unchecked_Deallocation.
I would even provide a built-in operation Free for them.

2. References to aliased objects, shaped more like "X renames Y", fully
statically checked.

3. "Access all" for emergency cases.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de