Re: syntaxic exploration

[Niklas Holsti <niklas.holsti@tidorum.invalid>]

On 17-12-22 23:15 , Simon Clubley wrote:
> On 2017-12-22, Robert Eachus <rieachus@comcast.net> wrote:
>> On Thursday, December 21, 2017 at 11:24:54 AM UTC-5, Jeffrey R. Carter wrote:
>>> On 12/21/2017 08:21 AM, Randy Brukardt wrote:
>>>>
>>>> Ada has no good way to write an empty array. I usually use (1 .. 0 => <>)
>>>> for that, but nothing is really satisfying.
>>>
>>> Except perhaps the empty array aggregate
>>>
>>> ""
>>>
>>> (only for string types, alas).
>>
>> Is it a reasonable language improvement request to allow "" for other
>> empty arrays?
>
>
> No. That would be too confusing IMHO because it would initially appear
> to be a string type to anyone unfamiliar with that part of the code
> and that just feels _way_ wrong.

I agree.

>>  Another possibility is the reserved word null.

    [snip]

> null is an interesting option for the empty array. However, for your
> strings idea, would people unfamiliar with the code and this new usage
> of null read this new usage as a null pointer instead of an empty string ?
> That could be confusing.

I agree, again.

I think the aggregate form (null array) was suggested some time ago, in 
analogy with the existing (null record). It looks good to me, but as I 
remember, there was some objection. One problem is that if the index 
type has exactly one value, then a null array with that index type 
cannot exist (because then A'First = A'Last for any such array A).

> The 1..0 notation is ugly but anyone who sees it knows instantly
> exactly what the original programmer meant.

Yes, but it becomes rather more ugly if the index type is an 
enumeration, or a generic formal discrete type, something like

    Thingummybob'Succ (Thingummybob'First) .. Thingummybob'First

Note that Thingummybob'Last .. Thingummybob'First won't be a null range 
if Thingummybob has exactly one value.

> Any cleaner replacement
> should instantly come across as the empty array without the
> possibility of confusion because someone reused Ada syntax
> for this in an ambiguous way.

I think (null array) satisfies that. I wish it could be introduced.

-- 
Niklas Holsti
Tidorum Ltd
niklas holsti tidorum fi
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