Re: Generic procedures and their parameters

[Maciej Sobczak <no.spam@no.spam.com>]

Dmitry A. Kazakov 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);

OK, it makes perfect sense.

> [ 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. ]

But then, you would need to have even such fundamental things like 
Integer belong to Ordered'Class. It's a shaky ground.

> 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.

And now I see. I'm still having troubles with the fact that 
"granularity" of type system in Ada can be much smaller than what can be 
achieved with structural differences only.

> 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).

Yes.

> The next step would be getting "<" from the element type,
> which would eliminate a need to have Sort generic.

No, I disagree. There is a place for generic subprograms, otherwise we'd 
have everything in a single hierarchy with Object at the top. It's not a 
coincidence that "other languages" drift towards supporting generics, 
even if they already had such single-rooted hierarchies.


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/