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

[okamsn--- via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Stefan Monnier wrote:
>>> 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 [...]
>>
>> But... I must admit I'm not really convinced that this has to be
>> changed.
> 
> I'm also unconvinced.  AFAICT the same problem occurs with
> 
>      (setf (aref (cl-subseq arr 3) 0) 27)
> 
> and I can't think of a good reason why `map-elt` should behave differently.

I would have assumed that `aref` would work with subplaces. I am still 
thinking of `setf` like a more flexible version of Python allowing 
things like `my_list[0][0] = 27`.

> Furthermore, the change would also fundamentally change the way
> `map-elt` can be used as a gv-place, in the sense that
> 
>      (setf (map-elt (funcall foo bar) 0) 27)
> 
> would signal an error during macroexpansion because (funcall foo bar)
> is not a valid gv-place.


I am trying to come up with an example that triggers the second path 
using `map-insert` in the examples that I sent. In the second path, the 
current version of the `map-elt` setter will already correctly set the 
subplace using `cl-replace`. I am trying to find an example where the 
inconsistency makes a difference.

My purpose with this patch and for bug#68863 regarding `seq-subseq` 
(which does not currently support `setf`, and I think should allow 
subplaces like `substring` claims to) was for destructuring as 
`setf`-able places, like in cl-loop's `for VAR in-ref LIST`. I have 
implemented that for my Emacs Lisp package 
(https://github.com/okamsn/loopy), but not all of the `setf`-able 
destructuring constructs support sub-places in the expected way, due to 
how some of the GV expansions are defined. I did not consider calling 
`setf` on a function's output.

For example, with the two patches, one can do

     ;; => (1 2 [29 99] 29)
     (let ((arr (vector 1 2 88 99)))
       (loopy-ref (([a b &rest [&whole c &map (0 map-idx-0)]]
                    arr))
         (setf map-idx-0 29)
         (list a b c map-idx-0)))

and have the `setf`-able destructuring work as expected.

 >> But second - doesn't your patch lead to very inefficient code in this
 >> example, where nearly all elements of the original sequence get replaced
 >> by themselves in a loop (through the setter of `cl-subseq')?
 >
 > Yes, that's another issue.  I think if wvale want to support such 
`setf` in
 > a way that is good enough to that we can recommend its use, we'd need to
 > turn it into something that behaves more or less like:
 > val
 >      (map-set! arr (+ 3 0) 27)
 >
 > But I must admit that I don't know how to get there.

This is true.  I could have used `setf (map-elt (cl-subseq arr 3 4) 0) 
27)` to be more efficient.

Thank you.