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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Generic procedures and their parameters
Newsgroups: comp.lang.ada
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Date: Wed, 6 Sep 2006 11:43:15 +0200
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Date: 2006-09-06T11:43:15+02:00
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On Wed, 06 Sep 2006 10:51:48 +0200, Maciej Sobczak wrote:

> eneric
>     type Index_Type   is (<>);
>     type Element_Type is private;
>     type Array_Type   is array (Index_Type range <>) of Element_Type;
>     with function "<" (Left, Right : in Element_Type) return Boolean is <>;
> procedure Sort(To_Sort : in out Array_Type);
> 
> My question is: what's the purpose of the third parameter (Array_Type)? 
> Isn't it implied by the first two and therefore just redundant?

Reverse. Actually Index_Type and Element_Type are redundant. In a better
Ada it should be:

   generic
      type Container_Type is array (<>) of <>;
             -- or "(<>) is limited private array"
      with function "<" (Left, Right : in Container_Type'Element)
         return Boolean is <>;
   procedure Sort (To_Sort : in out Container_Type);

[ Even better it be non-generic:

   procedure Sort (To_Sort : in out Container_Of_Ordered'Class);

where each container type (like an array) be a member of the class, if its
elements are in Ordered'Class. ]

Regarding Array_Type being implied, it is not. You can have any number of
array types based on exactly same combination of index and element types.
These types will be different types:

type I is ...;
type E is ...;
type Array1 is array (I range <>) of E;
type Array2 is array (I range <>) of E;

The types Array1 and Array2 are distinct types. Sort must be instantiated
for each of them (or else you have to convert types).

The reverse is a different story. It is a pity that Ada does not have
attributes to get the index and element types from the array type
(container type). The next step would be getting "<" from the element type,
which would eliminate a need to have Sort generic.

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