Re: Array slices and types

"Adam Beneschan" <adam@xxxxxxxxxx> wrote in message
news:fc112b42-285c-4ed5-ae78-34be4e60f6df@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Aug 21, 1:53 am, "Dmitry A. Kazakov" <mail...@xxxxxxxxxxxxxxxxx>
wrote:
On Thu, 21 Aug 2008 08:26:26 +0300, Niklas Holsti wrote:
But doesn't this example show that it would be useful to have 'Base
also for array types?

For any type, actually.

Consider this language design fault:

procedure Initialize (X : in out S) is
begin
Initialize (T (X));
... -- My stuff
end Initialize;

A call to Initialize should be done automatically, but it is not. So the
parent of S must be explicitly specified and known to all descendant.
This
is a really *bad* thing:

package Foo is
type S is new T with private;
private
type S is new Private_Decendant_Of_T with ...;
end Foo;

What would happen if Private_Decendant_Of_T overrode Initialize of T? The
result would be an inability to publicly derive from S any new types if
Initialize should be extended!

S'Base could mend it:

procedure Initialize (X : in out S) is
begin
(S"Base (X)).Initialize; -- Call to parent whatever it be
... -- My stuff
end Initialize;

Sorry, Dmitry, but if you'll pardon the expression, you're a little
off-base here. S'Base and S are subtypes of the same *type*. They
may be different subtypes (have different constraints), but they're
the same type. You're asking for an attribute that would give you a
different type. There may be merit in having such an attribute (I
haven't looked into it closely), but calling it 'Base would be a bad
idea. S'Parent might be better.

I tried to propose such a thing for Ada 2005, but it didn't work out. There
isn't necessarily a single type that is the parent, unless you break
privacy. Specifically:

package P is
type New_Type is new Some_Base with private;
procedure Some_Operation (Obj : in out New_Type; Parent : in
New_Type'Parent);
private
type New_Type is new Some_Derived with record ...
end P;

In this case, what does New_Type'Parent give you in the body? We'd want it
to resolve to Some_Derived. But a client of P can't see the full declaration
(and shouldn't be able to depend on it), and thus P.New_Type'Parent would
have to resolve to Some_Base.

So far, we can live with this. But now consider the completion to
Some_Operation: the body would use the same parameter types in order to
conform, but the *meaning* of the second parameter would be different to
clients and the body. Oops. One could ban using this attribute in such
specs, but that looks like a nasty wart (as bad as the original one). Thus,
we didn't get a 'Parent attribute.

Niklas is asking for S'Base to be the unconstrained array subtype of
the (possibly constrained) array subtype S. It would still fit into
the definition of 'Base (3.5(15)). I'm not sure why this wasn't
defined for array subtypes, or for any other type that could have
discriminants; offhand I don't see how this would cause any problems.

It does cause problems, and they're pretty severe as I recall. After all,
Ada 83 allowed 'Base on everything (but only as a prefix of another
attribute). That was removed in Ada 95 because of problems. But I can't
remember what they were precisely. Sorry about that.

Randy.


.

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