Re: Generic procedures and their parameters

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Thu, 7 Sep 2006 22:35:44 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:w0vh1ugsvkj1$.2fnx5mt0tplm$.dlg@40tude.net...
> ...
>> 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.
> 
> That doesn't really work. "<" is what controls how the items are sorted, and
> it is common to want to sort the same type in multiple ways. If you have
> employee records, you might want to sort them both on Social Security
> numbers and on date of employment, for example. Two generic instantiations
> work fine for that, but there isn't any way to have two "<" with different
> behaviors.
>
> We had this discussion when looking at the possibility of having a sorting
> interface. It just doesn't work well, because there can only be a single
> implementation. (That is a problem with any scheme based on inheritance.)
> That's why I view interfaces less as types and more as properties that are
> either present or absent on any individual specific type.

Yes, that's when somebody wanted to have different orders. But that in my
view is a different case, in which "<" should be better passed to Sort as a
plain parameter, rather than generic or primitive subprogram. I think the
difference between two scenarios is whether the order is arbitrary, ad-hoc
or else involved in some loose set of operations. In the first case "<"
should be a parameter in the latter case a part of the interface (be it
generic or primitive operation).

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