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Question about defcustom and :set-after

From: Eric Abrahamsen
Subject: Question about defcustom and :set-after
Date: Mon, 30 Apr 2018 17:00:28 -0700
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

I've been trying to do some fairly deep refactoring to Gnus, and have
been frustrated by the fact that it's very difficult to set up a clean
test environment for Gnus that also has some data in it to work with.

So I started writing support for a Gnus mock environment: you call
`gnus-mock' and it starts up a Gnus session completely independent of
your own Gnus customizations, that has some dummy data in it that gets
flushed and restored each time you restart the mock session.

That involves saving all relevant customizations (and some defvars) and
putting them aside, then setting all customization options (and some
defvars) back to their default values, and starting Gnus.

It isn't working quite right for me, I think because some customization
options depend on other customization options, and if they're set in the
wrong order, things don't get overwritten cleanly.

In particular I'm looking at options like this one:

(defcustom gnus-directory (or (getenv "SAVEDIR")
                              (nnheader-concat gnus-home-directory "News/"))
  "Directory variable from which all other Gnus file variables are derived.

Note that Gnus is mostly loaded when the `.gnus.el' file is read.
This means that other directory variables that are initialized from
this variable won't be set properly if you set this variable in `.gnus.el'.
Set this variable in `.emacs' instead."
  :group 'gnus-files
  :type 'directory)

`gnus-home-directory' is also an option. Shouldn't this option declare a
:set-after dependency on `gnus-home-directory'? There have also been bug
reports over the years about weirdness in this area, and I wonder if it
isn't all related.

Is this what :set-after is for, and would it make sense to add that
property to all Gnus options that depend on other options?


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