GNU bug report logs - #25929
25.2; map-delete doesn't delete permanently 1st alist elt

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From: Tino Calancha <tino.calancha <at> gmail.com>
To: Nicolas Petton <nicolas <at> petton.fr>
Cc: 25929 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>, Tino Calancha <tino.calancha <at> gmail.com>
Subject: bug#25929: 25.2; map-delete doesn't delete permanently 1st alist elt
Date: Thu, 2 Mar 2017 20:30:35 +0900 (JST)

[Message part 1 (text/plain, inline)]

On Thu, 2 Mar 2017, Nicolas Petton wrote:

> Tino Calancha <tino.calancha <at> gmail.com> writes:
>
>
>> (cl-loop for i below 3 collect
>>   (let ((map (list (cons 0 3)
>>                    (cons 1 4)
>>                    (cons 2 5))))
>>     (map-delete map i)
>>     (null (map-elt map i))))
>> => (nil t t) ; The first pair, i.e. '(0 . 3) is not permanently deleted.
>
> The alist is indeed modified within the `map-delete' function, but in an
> unexpected way: if the first key is deleted, the variable `map' is
> `setq'ed, which has no effect outside of the function.
>
> One fix would be to make `map-delete' a macro:
>
> (defmacro map-delete (map key)
>  "Delete KEY from MAP and return MAP.
> No error is signaled if KEY is not a key of MAP.  If MAP is an
> array, store nil at the index KEY.
>
>  MAP can be a list, hash-table or array."
>    `(macroexp-let2 nil key
>       (map--dispatch ,map
>         :list (setf (alist-get ,key ,map nil t) nil)
>         :hash-table (remhash ,key ,map)
>         :array (and (>= ,key 0)
>                     (<= ,key (seq-length ,map))
>                     (aset ,map ,key nil)))
>       ,map))
>

After this patch if i run the above recipe i get an error:
let*: Symbol’s function definition is void: map--dispatch

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