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From: Stephen Leake <stephen_leake@stephe-leake.org>
Newsgroups: comp.lang.ada
Subject: Re: Generalized Loop Iteration and User-Defined Indexing with more than two parameters.
Date: Tue, 03 Nov 2020 03:05:50 -0800
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Blady <p.p11@orange.fr> writes:

> Hello,
>
> Let's take the following container with constant indexing for iteration:
>
>      6.       type UXString is tagged private with
>      7.          Constant_Indexing => Get,
>      8.          Iterable          => (First => First, Next => Next,
> Has_Element => Has_Element, Element => Get);

>     11.       function Get (Self : UXString; Index : Positive;
> Substitute : in Character) return Character;

Hmm. Aspect "Iterable" is not defined by Ada. It is defined by GNAT. The
gnat reference manual says:

   * The value of `Element' is a primitive operation of the container
     type that takes both a container and a cursor and yields an
     `Element_Type', which must be a type declared in the container
     package or visible from it. For example:

    function Get_Element (Cont : Container; Position : Cursor) return Element_Type;

So it should have complained that your "Element => Get" is illegal.

> What is the correct usage?

An iterator 'Element' function can take only two parameters; the same is
true for standard Ada generalized iterators.

A Constant_Indexing function can take more than two parameters, but you
can only use it by indexing a container object directly (see LRM 4.1.6), 

A : UXString;

for Position in A.First .. A.Last loop

    Foo := A (Position, Substitute => 'C');
    ...
    
-- 
-- Stephe