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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Generators/coroutines in future Ada?
Date: Wed, 12 Jul 2017 15:38:54 +0200
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Xref: news.eternal-september.org comp.lang.ada:47381
Date: 2017-07-12T15:38:54+02:00
List-Id: <comp.lang.ada>

On 12/07/2017 15:07, Victor Porton wrote:
> J-P. Rosen wrote:
> 
>> Le 12/07/2017 à 01:19, Victor Porton a écrit :
>>> Dmitry A. Kazakov wrote:
>>>> I don't understand what is wrong with:
>>>>
>>>>      type Stateful is limited private;
>>>>      function Generate (X : not null access Stateful) return Things;
>>>
>>> The thing "wrong" with this, is that it is sometimes hard to implement
>>> Generate(). In Python this can be done in an easier way by using
>>> generators (Python's "yield" keywords).
>>>
>> Please provide a concrete example, so we can compare solutions!
> 
> I don't have at hand an Ada example. So instead I will give two examples in
> Python with and without explicit generators. Which of the two is simpler?
> 
>     1 # Using an explicit generator function
>     2 def firstn(n):
>     3     num, nums = 0, []
>     4     while num < n:
>     5         yield num
>     6         num += 1


    type First (N : Positive) is tagged limited record
       Self    : not null access First'Class := First'Unchecked_Access;
       Current : Positive := 1;
    end record;
    function Value (Generator : First) return Integer;

    function Value (Generator : First) return Integer is
    begin
       if Generator.Current > Generator.N then
          raise End_Error;
       else
          return Result : Integer := Generator.Current do
             Generator.Self.Current := Generator.Current + 1;
          end return;
       end if;
    end Value;

and then

    Dozen : First (12);
begin
    loop
       Put_Line (Integer'Image (Dozen.Value));
    end loop;
exception
    when End_Error =>
       null;

This should print
  1
  2
...
  12

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