Modular home configuration

[Ian Eure]

Hi folks,

I’m trying to make my home configuration more modular, so I can 
better support system variances.  For example, I have a laptop I 
use interactively, and a headless machine that runs Cuirass.  It’s 
advantageous to share certain aspects of the home configuration 
between the two machines (shell prompt/environment, GPG agent, 
etc), but not others (anything X11/graphical stuff shouldn’t be on 
the build machine).  One approach to this is to define packages 
and services and reference them in the home configuration.  What I 
dislike about this is that many things require both packages and 
services, and I’d prefer to have a way to completely encapsulate 
that -- for example, the mpd-mpc package to control my music 
server, plus a home-environment-variables-service-type to set 
MPD_HOST.

I attempted to solve this by writing a procedure:

   (define (+mpd-client home-config)
     (home-environment
       (inherit home-config)
       (packages (cons mpd-mpc (home-environment-packages 
       home-config)))
       (services
         (cons
           (simple-service
             'mpd-environment-service
             home-environment-variables-service-type
             '(("MPD_HOST" . "audio.box")))
           (home-environment-services home-config)))))

Which I can then wrap around a home-environment to add the mpd-mpc 
package and environment variable it needs to work:

   (+mpc-client (home-environment ...))

Surprisingly, this doesn’t work -- it complains that there’s more 
than one "home" service type.  I’m not sure why that is, and I 
haven’t been able to see anything obviously wrong in the REPL -- 
though I haven’t been able to get my actual home configuration up 
in the Emacs-Guix REPL, due to #67290.

Does anyone have a suggestion for a workaround for this issue, 
explanation of how two home services are ending up in the config, 
or a better approach for building modular home configs?

Thanks,

 — Ian