Re: T'Interface attribute

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

On 2017-08-03 06:46, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:olskua$1bfd$1@gioia.aioe.org...
>> In order to reduce name space contamination I would propose attribute
>> T'Interface which for any tagged type T would denote an interface type
>> with all *new* primitive operations, e.g.
> 
> Past experience say that such attributes (that is, those that designate some
> sort of type or subtype) don't work out very well in Ada. For instance, we
> considered T'Access in Ada 2005, but gave that up because of various
> problems (and replaced with with anonymous access types, which are worse).
> The worst problem was that there was no limit to the attribute, you could
> write T'Access'Access'Access - and you can't prevent this since T could be a
> generic formal type that itself is some T'Access.

In this case generic would not be a problem because T must be tagged to 
have T'Interface.

As a side note, differently to 'Access, 'Length and actually 'Class, 
'Interface is idempotent:

    T'Interface ::= T'Interface'Interface

> Almost all Ada compilers materialize all Ada types in their symbol table, so
> the effect of having a T'Interface would be that it would be automatically
> declared for all tagged type declarations. Most Ada compilers also
> materialize all of the primitive operations in their symbol table, so there
> would be a large increase in memory usage and some time increase (all
> lookups would have twice as many primitives to look through).

This is an intended effect. Yes, there would likely be two tags for each 
declared type.

Which by the way will fix Ada.Streams and System.Storage_Pools design 
for free. Since we will finally have:

    Root_Stream_Type'Interface

and

    Root_Storage_Pool'Interface

E.g.

    type Serialized_Widget is
       new Gtk_Widget_Record and Root_Stream_Type'Interface with private;

> P.S. And as always, explain the problem to be solved as well as the possible
> solution. That is always necessary to understand the value of a proposal.

I have a design rule to have an explicit interface declared for each 
tagged type. It saves much of redesign later.

> P.P.S. I personally have never seen a real-world example where an interface
> actually helps. There's lots and lots of book examples and show examples and
> so on, but in practice you end up with only a single implementation so there
> really is nothing gained by involving a lot of dispatching calls. Ergo, I'm
> against anything (new) involving interfaces. YMMV.

I have no idea how you manage keep Claw out of interfaces, really. 
Multiple inheritance is a huge help and relief. I am using interfaces 
extensively unless backward Ada 95 compatibility hinders me. E.g. in 
AICWL all widget layers implement interfaces like Scalable_Layer, 
Gauge_Needle, Waveform_Amplifier etc. It would not work otherwise.

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