Re: Generic procedures and their parameters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> writes:

> 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. ]

I think you mean Sequence_Of_Ordered -- not all containers can be
sorted.

But I agree that it would be nice to have such a feature.  It works for
the built-in array types -- you magically get "and" if the component is
Boolean, "<" if the component is discrete, ":=" if the component is
non-limited.  I don't know of a good way to do that sort of thing for
user-defined containers.  If generics could do that, then "array" could
just be a generic, rather than having built-in syntax and semantics.
Eiffel does it that way.

- Bob