Re: class wide iterable (and indexable)

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

On 2019-01-22 23:00, Randy Brukardt wrote:
> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message
> news:q26lsh$1ee3$1@gioia.aioe.org...
>> On 2019-01-22 00:15, Randy Brukardt wrote:
>>> For instance, in a Vector, the
>>> lower bound is fixed for all values of a particular type. That would
>>> eliminate a lot of programming errors that happen in Ada programs (many
>>> programs fail to deal with unusual lower bounds, because it's hard to get
>>> right).
>>
>> I disagree. It would make array slices more difficult to use.
> 
> Array slices as defined in Ada aren't worth the substantial implementation
> complexity. I would replace them by slicing functions, and the bounds are
> lost.

The reverse. A container without slices is half useless to me. BTW, nD 
arrays must support slicing, subarrays, subplanes etc.

>> Presently we can do things like
>>
>>     Read (Buffer (From..To), Last);
> 
> Surely. But "Last" is a weird Ada-ism that exists solely because array
> bounds are so generally accessible. It is much more natural (and sensible
> for most uses) to return the count of characters read.

No, because it is a break of the abstraction, unsafe and error-prone.

>> Foo can move the index Last and this is the same index as outside in the
>> caller because indices do not slide. If you make the lower bound fixed,
>> indices would slide and that would require a lot of index arithmetic upon
>> calling subprograms on slices. This would be far more error-prone than
>> occasionally forgetting to use Buffer'First instead of 1.
> 
> True, in the rare case where that matters. But most of the time, one has to
> do that math to convert "Last" into a number of character read. So this is a
> wash to me - some things are easier, som things are harder. (Again, "Last"
> is an odd Ada-ism, specs would return a character count, not some array
> index.)

You want to get rid of indices and have only positions. Positions is not 
a substitute to indices. With multiple interfaces there would be no 
problem to have a positional view on the array. Which is not an array 
interface anymore. It would be an interface of a tuple. Ada does not 
have builtoin tuples, but it could have them. Just do not force 
everything into it.

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