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From: Robert A Duff <bobduff@shell01.TheWorld.com>
Newsgroups: comp.lang.ada
Subject: Re: Generic procedures and their parameters
Date: 06 Sep 2006 08:56:35 -0400
Organization: The World Public Access UNIX, Brookline, MA
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Date: 2006-09-06T08:56:35-04:00
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Maciej Sobczak <no.spam@no.spam.com> writes:

> Hi,
> 
> I have found the following signature for the sorting procedure:
> 
> 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?

Well, if you simply erase the Array_Type formal, then the reference to
Array_Type in the declaration of To_Sort would be in error.
And you cannot say this:

generic
    type Index_Type   is (<>);
    type Element_Type is private;
    with function "<" (Left, Right : in Element_Type) return Boolean is <>;
procedure Sort(To_Sort : in out array (Index_Type range <>) of Element_Type); -- Illegal syntax!

because Ada uses "by name" type equivalence (mostly).  In this case,
structural equivalence might be more convenient.

You could do it this way:

generic
    type Index_Type   is (<>);
    type Element_Type is private;
    with function "<" (Left, Right : in Element_Type) return Boolean is <>;
package Sorting is

    type Array_Type   is array (Index_Type range <>) of Element_Type;
    procedure Sort(To_Sort : in out Array_Type);

end Sorting;

but that's less flexible for the client, because it can't use it's own
array type.

- Bob