Re: Lower bounds of Strings

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

"Shark8" <onewingedshark@gmail.com> wrote in message 
news:50f68100-8909-4fdb-ad26-14bcbc010775n@googlegroups.com...
> On Tuesday, January 12, 2021 at 1:12:35 AM UTC-7, Randy Brukardt wrote:
>> "Shark8" wrote in message
>> news:26cac901-b901-4c4f
>> On Friday, January 8, 2021 at 7:31:40 PM UTC-7, Randy Brukardt wrote:
>> ...
>> > >Can you give some examples here?
>> > All of the super-null nonsense occurs because of slices, which slows 
>> > down
>> > any code that uses them.
>> What do you mean "super-null" nonsense?
>> Any range that is null is treated as the same by Ada. Any null range 
>> where
>> the bounds are more than 1 apart is known as a "super-null" range. For
>> instance, one typically writes:
>>
>> N : String(1..0)
>>
>> to define a null string object. But you can write *any* null range here:
>>
>> N2 : String(314 .. 25);
>>
>> And it takes code to figure this out at runtime if either bound is 
>> nonstatic
>> (which is usually the case with slices). And you still have to store the
>> bounds (you can still ask for the bounds of N2, and one better get 314 
>> and
>> 25, but the length is still zero). And N = N2, so compares are 
>> complicated
>> (one has to check the length, but that's more expensive to figure out 
>> than
>> just a subtract). You end up generating a lot of code to deal with a 
>> rather
>> unlikely case.
> I understand now.
> You mention the case where the difference in the indices is 1 as being 
> separate; why?

Because the "natural" implementation of the length of an array works when 
the indices are one apart; you don't need extra code to deal with 
String(1..0) - subtracting the bounds and adding 1 (the usual length 
formula) works fine.

> Also, would the sometimes talked about idea of a "null range" have helped 
> the situation out?
> (I don't think so, since the one is the required implementation, and the 
> other is a syntax- and partially semantic-issue.)

Right, the problem is that the syntax allows too much, and any rule to avoid 
the problem necessarily will take code to make some sort of check.

...

                                       Randy.