Hello,

I'd like to share some code, thus prove a concept, ask for 
feedback, and ask if it could makes sense, under certain 
circumstances, to provide similar code in GNU Guix. The code can 
be used in a ~manifest.scm~ file (but probably could also be used 
with ~guix package -e~ or inside ~guix repl~) where it can be used 
to import all ~emacs-*~ packages that are loaded with 
~(use-package PACKAGE [...])~ inside an Elisp-file like ~init.el~:

#+begin_src scheme
;; ~/some/manifest.scm
(specifications->manifest
 (append
   (el/file->specifications "/home/user/.emacs.d/init.el")
   (list
     "emacs"
     "git"
     "gnupg"
     "mu"
     "syncthing")))
#+end_src

It has many severe caveats, including but not limited to:

1. The code uses Guile/Scheme's ~read~ function which interprets 
the Elisp-file as Scheme. Thus, the code does not really find out 
which packages are loaded, but rather it only looks for the 
~(use-package PACKAGE [...])~ pattern of an S-expression. E.g. 
something like ~(apply 'use-package foo)~ won't be caught. (I 
don't expect Guile's Emacs-Lisp language-feature to be mature 
enough to be usable for this.)

2. Not all external packages are loaded with ~use-package~. E.g. 
~(require 'foo)~ might be used directly. But it'd be possible to 
extend the code to allow several ways/patterns of importing 
packages, and maybe also to specify custom patterns.

3. Not all S-expressions that match against ~(use-package 
PACKAGE)~ are load external packages. E.g. ~(if nil (use-package 
foo))~ won't load anything at all. Also, neither ~(use-package 
emacs)~ nor ~(use-package savehist)~ will load anything, because 
they ~require~ features that ship with GNU Emacs by default. 
That's why the functions in the appended code offer an optional 
keyword argument, #:block, in order to block a list packages.

4. The ~manifest.scm~ is not the single source of truth anymore. 
I.e. this code compromises the purpose of a manifest. (It's 
possible to extend the code such that the Elisp-file's hash can 
optionally be verified with given hash.)

Nevertheless, for me, personally, it's pretty neat and handy to 
use, because I don't need to maintain the list of emacs-packages 
in two places. I also think that it could come pretty handy for 
many others, at least in order to initialize their user-profile, 
by running something like ~guix package -e '(some-magic 
"/home/user/.emacs.d/init.el")'~.

What do you think? Should this go into a separate, private 
channel? Into the Guix Cookbook? Into Guix, if so, then probably 
with lots of changes? Should it just stay here, in this mailing 
list thread? Or do you think this is just a bad idea in general?

I should mention that I go the idea for this code from the 
Nix-Community's Emacs-Overlay which offers a similar feature; as 
described in their README as follows:

#+begin_src nix
# 
   https://github.com/nix-community/emacs-overlay#extra-library-functionality
{
 environment.systemPackages = [
   (emacsWithPackagesFromUsePackage {
     # Your Emacs config file. Org mode babel files are also
     # supported.
     # NB: Config files cannot contain unicode characters, since
     #     they're being parsed in nix, which lacks unicode
     #     support.
     # config = ./emacs.org;
     config = ./emacs.el;
# ...
})]}
#+end_src

Here's the code. Please show mercy, it's my first real Scheme 
code: