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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: How to access Vector.Index_Type?
Date: Mon, 18 Mar 2019 09:31:52 +0100
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Xref: reader01.eternal-september.org comp.lang.ada:55888
Date: 2019-03-18T09:31:52+01:00
List-Id: <comp.lang.ada>

On 2019-03-18 06:27, J-P. Rosen wrote:
> Le 17/03/2019 à 21:43, jakub.dabek@gmail.com a écrit :
>> I have the following program
>>
>>      with Ada.Containers.Vectors;
>>
>>      procedure Test is
>>          package MyVec is new Ada.Containers.Vectors(Natural, Natural);
>>          Var : MyVec.Index_Type;
>>      begin
>>          null;
>>      end;
>>
>> I get the following error:
>> "Index_Type" is not a visible entity of "MyVec"
>>
>> How can I access the index type (`Element_Type` gives the same error 
>> message), without just writing `Natural`?
>>
> Why would you do that? If you instantiate Vectors on Natural, it's 
> because you want a vector of Natural! Therefore it makes sense to use 
> Natural. The name "Index_Type" is a formal, not an actual type, it's 
> just the package's business.

surely for maintenance and code quality reasons. In the example 
everything is clear, but in other (more realistic) scenarios the package 
MyVec might come from other packages only to be used in Test. The 
procedure Test must then use the least possible assumption about the 
index type and be ready to the cases when the type would change, e.g. 
become an enumeration type.

P.S. It is a longstanding issue with visibility of formal generic 
parameters. Anybody who used generics was hit by it more than once. I 
don't know if anything was attempted recently to fix that. A life-hack 
(that does not apply to existing packages) was/is to rename or subtype 
each formal parameter of the generic:

    generic
       type T is <>;
       with package Bar ...
    package Foo is
       subtype The_T is T;
       package The_Bar renames Bar;

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