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bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces
From: |
okamsn |
Subject: |
bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces |
Date: |
Thu, 25 Apr 2024 01:59:36 +0000 |
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.
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Okamsn, 2024/04/22
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Michael Heerdegen, 2024/04/24
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Stefan Monnier, 2024/04/24
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces,
okamsn <=
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Augusto Stoffel, 2024/04/25
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Michael Heerdegen, 2024/04/26
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, okamsn, 2024/04/28
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Stefan Monnier, 2024/04/28
- bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Michael Heerdegen, 2024/04/30
bug#70524: [PATCH] Fix `map-elt` with `setf` for subplaces, Augusto Stoffel, 2024/04/25