bug#22703: 24.5; PROPOSAL: Is there a way to update available choices for defcustom variable?

[Lars Ingebrigtsen <larsi@gnus.org>]

Oleksandr Gavenko <gavenkoa@gmail.com> writes:

> Recently I ask question:
>
>   http://emacs.stackexchange.com/questions/20350/defcustom-values-for-selecting-from-function-call
>
> because I didn't understand show to build code from that I read in docs.
>
> While answer to question help me to build my first "customize" code it miss
> main point and I think because the way Emacs customize API works, so nobody
> even think about doing as I asked.
>
> I have values:
>
>   (defvar my/python-checker-alist
>     '((pylint . (my/python-pylint-command my/python-pylint-args))
>       (pep8 . (my/python-pep8-command my/python-pep8-args))
>       (pyflakes . (my/python-pyflakes-command my/python-pyflakes-args)))
>     "Known Python source code checkers.")
>
> and want to build `defcustom' definition based on that values:
>
>   (choice (const . pylint) (const . pep8) (const . pyflakes))
>
>   (defcustom my/python-default-checker 'pyflakes
>     "Default Python source code checker. See `my/python-checker-alist' for full alist."
>     :group 'my/python
>     :type (cons 'choice (mapcar (lambda (e) (cons 'const (car e))) my/python-checker-alist)) )
>
> I am worrying is there a mechanic that **automatically** update `:type' value when
> user visit:
>
>   M-x customize-group
>   M-x customize-variable
>
> with my variable?
>
> Official info states:
>
>   The argument of ‘:type’ is evaluated, but only once when the
>   ‘defcustom’ is executed, so it isn’t useful for the value to vary.
>
> If someone potentially extend my/python-checker-alist his addition would be
> invisible in my/python-default-checker.
>
> I thing it is wrong to ask someone not only to update possible values:
>
>   (with-eval-after-load 'my/python
>     (add-to-list 'my/python-checker-alist ...))
>
> but also somehow to update default values (for my/python-default-checker)
> synchronously.
>
> Are there standard convention for such situation?

(I'm going through old bug reports that have unfortunately not gotten
any responses.)

Yes, the person extending the defcustom can call
`custom-declare-variable' with the new :type, too.  This should be
mentioned in the doc string for `defcustom', I think, and I'm making
that change on the Emacs trunk now.

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no