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From: "Alejandro R. Mosteo" <alejandro@mosteo.com>
Newsgroups: comp.lang.ada
Subject: Re: Traits and iterators
Date: Wed, 8 Jun 2016 13:18:41 +0200
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Date: 2016-06-08T13:18:41+02:00
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On 08/06/16 09:31, briot.emmanuel@gmail.com wrote:
>> generic
>>      with package Elements is new Elements_Policy (<>);
>>      type Container is private;
>>      type Cursor is private;
>>      with function First (Self : Container) return Cursor is <>;
>>      with function Has_Element
>>         (Self : Container; C : Cursor) return Boolean is <>;
>>      with function Element (C : Cursor) return Elements.Element is <>;
>>      with procedure Next (C : in out Cursor) is <>;
>> package Forward_Cursors_Policy is
>> end Forward_Cursors_Policy;
>>
>> That, in itself, I don't think will give the ability to use "of" in
>> iterations, although it enables one to iterate over an unknown type.
>
>
> Indeed, this signature package allows you to write an explicit iteration loop, where
> you manipulate cursors explicitly. I don't know, and I don't think it is doable, how to
> pass a formal type and mention that it should be iterable in the sense of the "for..of"
> loop.
> That means that any generic algorithm we write will take the above Policy package,
> but will not be able to use "for..of". That's a bit more work to write, but just as
> efficient in the end, but perhaps a limitation in the language.

Thanks for the confirmation. Sometimes I miss interfaces for these 
things in the language.

Álex.