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 22:34:29 +0900 (JST)

On Thu, 2 Mar 2017, Nicolas Petton wrote:

> Nicolas Petton <nicolas <at> petton.fr> writes:
>
>> 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:
>
> Here's a fixed version:
>
>  (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
>      `(progn
>         (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)))
>
> And the associated regression test:
>
>  (ert-deftest test-map-delete-first-key-alist ()
>    (let ((alist '((a . 1) (b . 2) (c . 3))))
>      (map-delete alist 'a)
>      (should (null (map-elt alist 'a)))))

Sorry, i don't understand why following doesn't work after the patch:
;; I have byte-compiled map.el after applying the patch.
emacs -Q

(let ((alist '((a . 1) (b . 2) (c . 3))))
  (map-delete alist 'a)
  (map-elt alist 'a))

Debugger entered--Lisp error: (void-function map--dispatch)

We shouldn't ask the user to compile the file on each interactive session
in order to `map--dispatch' be defined.

We might also mention in the doc strings that MAP must be a generalized
variable or something.
To explain that I) below is allowed but II) is not:
I)
 (assq-delete-all 'foo '((foo . 1) (bar . 2)))

II)
 (map-delete '((foo . 1) (bar . 2)) 'foo)


Regards,
Tino

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