Re: Ada Distilled problem

[Brad Moore <bmoore.ada@gmail.com>]

On Friday, March 4, 2016 at 11:21:28 AM UTC-7, Simon Wright wrote:
> A Stack Overflow user has been having problems[1] trying to use the
> queue manager described at page 82 of Ada Distilled (2005 version)[2].
> 
> To see the problem in its essentials, the (inoperative!) spec could be
> 
>    generic
>       type Element is tagged private;
>    package Lists is
>       type Item is new Element with private;
>       type List is tagged private;
> 
>       procedure Insert (Self : in out List; I : Item'Class);
> 
>    private
>       type Item is new Element with null record;
>       type List is tagged null record;
>    end Lists;
> 
> with a calling program
> 
>    with Lists;
>    procedure Main is
>       type Temp is tagged null record;
>       package Temp_List is new Lists (Temp);
> 
>       FL : Temp_List.List;
> 
>       Instance : Temp := Temp'(null record);
> 
>    begin
>       Temp_List.Insert (Self => FL, I  => Instance);
>    end Main;
> 
> resulting in
> 
>    main.adb:11:40: expected type "Item'Class" defined at lists.ads:4,
>    instance at line 4
>    main.adb:11:40: found type "Temp" defined at line 3
> 
> and I can't see any way of twisting this so it can compile. And, in any
> case, what is the point of "type Item"?

I dont see any point. Looks to me like the example needs to be corrected.

I would change the 'I' parameter of the Insert to be of type Element'Class, then this should compile.

Brad

> 
> Has anyone managed to use the Ada Distilled design? (of course, nowadays
> one would implement either directly or over predesigned containers).
> 
> [1]
> http://stackoverflow.com/questions/35790004/ada-package-with-generic-tagged
> [2] http://www.adaic.org/learn/materials/#ada_2005_books - first entry