`define-derived-mode' and `define-minor-mode' each define their mode
hook variable, including a standard docstring, using:
(defvar ,hook nil ".....")

In the uncommon (but not unheard of) scenario whereby a library
defines a mode and also explicitly defines that mode's hook variable
with defvar or defcustom, the library author will encounter problems
no matter which way around they define them.


1. If the mode is defined first:

(define-derived-mode foo-mode ...)
(defcustom foo-mode-hook '(default value) "docstring")

Then `foo-mode-hook' has already been defvar'd with a value of nil,
and so the defcustom default value never gets set, so that sequence
is no good.


2. If the variable is defined first:

(defcustom foo-mode-hook '(default value) "docstring")
(define-derived-mode foo-mode ...)

Then `foo-mode-hook' gets its default value, but its original docstring
is clobbered by the one provided by `define-derived-mode' -- because
while the value of a bound variable is not affected by evaling its
definition again, the docstring does get updated.


3. We can work around that with a sequence such as:

(defcustom foo-mode-hook '(default value) "" ...)
(define-derived-mode foo-mode ...)
(put 'foo-mode-hook 'variable-documentation "docstring")

which works, but is cumbersome, as the hook variable's docstring is
now separated from the hook variable definition by the entirety of
the mode definition.


My suggestion is that `define-derived-mode' and `define-minor-mode'
should only add the standard docstring to the mode hook variable
conditional on that variable not *already* having a docstring.

That would then allow approach (2) to have the same effect as (3)
but in a much tidier manner.

Patch attached for consideration.


-Phil





In GNU Emacs 26.0.90 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d
scroll bars)
 of 2017-10-17 built on shodan
Repository revision: ead257cbfc2c7e9196210b899f6d4e6496c0a42b
Windowing system distributor 'The X.Org Foundation', version 11.0.11804000
System Description:	Ubuntu 16.04.3 LTS

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --prefix=/home/phil/emacs/trunk/usr/local
 --with-x-toolkit=lucid --without-sound'

Configured features:
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