bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces

[Augusto Stoffel <arstoffel@gmail.com>]

On Thu, 25 Apr 2024 at 14:03, Augusto Stoffel wrote:

> On Tue, 23 Apr 2024 at 02:10, Okamsn via "Bug reports for GNU Emacs, the Swiss army knife of text editors" wrote:
>
>> Hello,
>>
>> Currently, the use
>>
>>      (let ((arr (vector 0 1 2 3 4 5 6)))
>>        (setf (map-elt (cl-subseq arr 3) 0)
>>              27)
>>        arr)
>>
>> expands to
>>
>>      (let ((arr (vector 0 1 2 3 4 5 6)))
>>        (let* ((v arr))
>>          (condition-case nil
>>              (with-no-warnings
>>                (map-put! (cl-subseq v 3) 0 27 nil))
>>            (map-not-inplace
>>             (let* ((new (map-insert (cl-subseq v 3) 0 27)))
>>               (progn
>>                 (cl-replace v new :start1 3 :end1 nil)
>>                 new))
>>             27)))
>>        arr)
>
> Since map-put! may raise a not-in-place signal, and I doubt the macro
> expansion checks for whatever condition it is that leads to that, I
> would say this use-case is essentially broken.

Sorry, just ignore that :-).

What I actually wanted to say is that IMO there's a general conceptual
problem to consider map-like values as places.  There's no reasonable
way (cl-subseq arr 3) can be seen as a place AFAICT.

But even in a language like Python (where map-like things can be seen as
places, since there are no liked lists to ruin the party), I don't
understand what the above code is supposed to do.  This code

  arr = [0, 1, 2, 3, 4, 5, 6]
  arr[3:][0] = 27
  arr

will just copy a portion of arr as new list, mutate its first element,
then throw array that copy.