GNU bug report logs - #46407
27.1; Hooks with permanent-local-hook are not cleared of lambdas

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Subject: 27.1; Hooks with permanent-local-hook are not cleared of lambdas
Date: Tue, 09 Feb 2021 20:29:48 +0100
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When kill-all-local-variables encounters a hook variable with its
'permanent-local property set to 'permanent-local-hook, it removes from
its value every element except for t, functions with
'permanent-local-hook property and anything that isn't a symbol
(see the comment at src/buffer.c:1072).

This means that, for the following code

     (defvar   'some-hook nil)
     (add-hook 'some-hook #'some-fun         nil t)
     (add-hook 'some-hook (lambda () (test)) nil t)

whether some-fun is removed depends on some-fun's permanent-local-hook
property, which is expected. As for the anonymous lambda function, it is
not predictable, whether it will be kept or removed. In fact, it depends
on some-fun's permanent-local-hook property.

Perhaps it would make things more predictable to also remove non-symbol
functions such as lambda expressions.

See also info node (elisp) Creating Buffer-Local:
>  -- Function: kill-all-local-variables
>      This function eliminates all the buffer-local variable bindings
>      except for [...] and local hook functions that have a non-=E2=80=98n=
il=E2=80=99
>      =E2=80=98permanent-local-hook=E2=80=99 property.
Which suggests that functions with a permanent-local-hook property
should be the only exception.