gnu: home: Add Guix channels service.

* gnu/home/services/guix.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add guix.scm.
* doc/guix.texi: Add documentation for home-channels-service.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

gnu/home/services/guix.scm

@@ -0,0 +1,44 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2022 Reily Siegel <mail@reilysiegel.com>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu home services guix)
+  #:use-module (gnu home services)
+  #:use-module (guix channels)
+  #:use-module (guix gexp)
+  #:use-module (ice-9 pretty-print)
+  #:use-module (srfi srfi-1)
+  #:export (home-channels-service-type))
+
+(define (channels-xdg-files channels)
+  `(("guix/channels.scm"
+     ,(plain-file
+       "channels.scm"
+       (call-with-output-string
+         (lambda (port)
+           (pretty-print (cons 'list (map channel->code channels)) port)))))))
+
+(define home-channels-service-type
+  (service-type
+   (name 'home-channels)
+   (default-value %default-channels)
+   (compose concatenate)
+   (extend append)
+   (extensions
+    (list (service-extension home-xdg-configuration-files-service-type
+                             channels-xdg-files)))
+   (description "Manages the per-user Guix channels specification.")))