Re: Array Help?

["Randy Brukardt" <randy@rrsoftware.com>]

<stefan-lucks@see-the.signature> wrote in message 
news:Pine.LNX.4.64.1203081150460.6571@medsec1.medien.uni-weimar.de...
> On Wed, 7 Mar 2012, Randy Brukardt wrote:
>
>> "Robert A Duff" <bobduff@shell01.TheWorld.com> wrote in message
>> news:wccy5rls0hl.fsf@shell01.TheWorld.com...
>> > stefan-lucks@see-the.signature writes:
>> >>   procedure Sort(Items: in out array(generic Positive range <>) of T) 
>> >> is
>> >
>> > Interesting idea.
>>
>> Can't you do this with a type conversion to an appropriate constrained 
>> array
>> subtype? (Such conversions slide.) I suppose it could be a pain to 
>> declare
>> such a subtype in front of every call, but it is a trick I've used a some
>> cases (usually in wrappers) to force sliding on arguments.
>
> Can you explain a bit more?

Bob and I both gave examples of it in other messages.

>> I suppose a more convinient way to force that would be helpful.
>
> Beyond the inconvenience, there are two further issues:
>
> 1. Testing
>
> When you declare
>
>  procedure Sort(Items: in out array(Positive range <>) of T)

Let's hope you never do that, because anonymous types are the bane of Ada. 
And you surely can't do it today!

> in a package specification, and you try to write a proper black-box test,
> you need *more* test cases then when you do the same with another popular
> language.

Right. Bob's suggestion for some semi-constrained array subtype would fix 
that. But note that it was killed in Ada 95 in part because the 
implementation needed was essentially a record type with some sort of magic 
indexing. Which was a mess.

Not sure if those issues can really go away, and I doubt there is much 
interest in new first-class kinds of constraints.

...
> 2. Generality
>
> Given the (Ada 2020?) specification
>
>  procedure Sort(Items: in out array(generic Positive range <>) of T);

Again, anonymous types are a disaster in Ada, and there is no way that we 
should have more of them. I would strongly hope that this never appears even 
in Ada 2525. ;-)

> Sort(X) should be callable if the index type of X is any discrete type,
> except when the index type is too large (i.e., X has more than
> Positive'Last elements).

If you want that, you need to declare that. Positive is a numeric type, and 
thus has "+" and "-" available; it is never going to be allowed to match 
"any discrete type" which includes enumeration types.

It's plenty easy to write a generic like this (see the predefined ones for 
examples), so I fail to see what possible need you are addressing here.

                                    Randy.