Re: Lower bounds of Strings

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

On 2021-01-09 03:18, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:rtaeok$l8b$1@gioia.aioe.org...
>> On 2021-01-08 18:23, Shark8 wrote:
>>> On Thursday, January 7, 2021 at 3:03:54 PM UTC-7, Randy Brukardt wrote:
>>
>>>> Any sort of multiple inheritance (not to mention multiple dispatch)
>>>> requires
>>>> searching a lookup table for the appropriate interface.
>>> Ah.
>>> It appears you're confusing the Ada concept underlying keyword INTERFACE
>>> with the general/abstract notion.
>>> It appears to me that Dmitry is referring to the latter, not the
>>> Ada-construct of INTERFACE, which requires a tagged type.
>>>
>>> I think what he's getting at is something that I considered/proposed here
>>> on C.L.A some years ago, adding the concept of an "ABSTRACT INTERFACE" to
>>> Ada. (IIRC, the proposal I had in mind was to be able to model the
>>> notional meta type-hierarchy; eg: Number ? Universal_Integer ?
>>> System.Integer.)
>>
>> Right, though I do not think that tags can inflict any cost. The situation
>> is same as with array bounds. You do keep bounds when the array is
>> statically constrained. Tag is just another constraint like bounds. It
>> must be handled just same way, removed when statically known. No penalty,
>> unless classes are actually used. I do not know why people always bring
>> dispatch into discussions about static cases.
> 
> The possibility of dynamic dispatch (in some code that doesn't exist yet,
> but *could*) is what is so expensive and is virtually impossible to remove
> after the fact (that is, in an optimizer).

The representation without the tag must be mandatory.

> If you wanted to include a
> declaration that you *never* were going to use any dynamic dispatch,

This declaration is already in the language. Only A'Class dispatches. 
Again, it is like with bounds, only array (... range <>) of ... has 
dynamic bounds. To enforce dispatch one would have to convert to A'Class 
first, that will add the tag to the array's dope vector.

> then
> you could talk try to ignore the possibility. But without some sort of
> dispatch, a defined interface buys you nothing other than complication. It
> doesn't simplify the description and would substantially complicate the
> implementation. What's the point??

Of course it will simplify everything. E.g. our beloved generics. Compare

    generic
       type I is (<>);
       type E is private;
       type A is array (I range <>) of E;

with

    generic
       type A is new Root_Array_Type with private;

or

    package Ada.Containers.Vectors is
       ...
       type Vector is tagged private
          with Constant_Indexing => Constant_Reference,
               Variable_Indexing => Reference,
               Default_Iterator  => Iterate,
               Iterator_Element  => Element_Type;
       -- hundreds of operation declarations

with

    type Vector is new array (Index_Type) of Element_Type with private;
       -- nothing else to declare

Everything in Ada could be formalized in interfaces:

   type Index is new Discrete_Type; -- Same as <>
   type Discrete_Type is abstract new Ordered_Type and Copyable_Type;

and so on.

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