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>

doc/guix.texi

@@ -105,6 +105,7 @@ Copyright @copyright{} 2022 Aleksandr Vityazev@*
 Copyright @copyright{} 2022 Philip M@sup{c}Grath@*
 Copyright @copyright{} 2022 Karl Hallsby@*
 Copyright @copyright{} 2022 Justin Veilleux@*
+Copyright @copyright{} 2022 Reily Siegel@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -39433,6 +39434,7 @@ services)}.
 * Shepherd: Shepherd Home Service.       Managing User's Daemons.
 * SSH: Secure Shell.                     Setting up the secure shell client.
 * Desktop: Desktop Home Services.        Services for graphical environments.
+* Guix: Guix Home Services.              Services for Guix.
 @end menu
 @c In addition to that Home Services can provide
 
@@ -40073,6 +40075,37 @@ format.
 
 @end deftp
 
+@node Guix Home Services
+@subsection Guix Home Services
+
+The @code{(gnu home services guix)} module provides services for
+user-specific Guix configuration.
+
+@defvr {Scheme Variable} home-channels-service-type
+This is the service type for managing
+@file{$XDG_CONFIG_HOME/guix/channels.scm}, the file that controls the
+channels received on @command{guix pull} (@pxref{Channels}).  Its
+associated value is a list of @code{channel} records, defined in the
+@code{(guix channels)} module.
+
+Generally, it is better to extend this service than to directly
+configure it, as its default value is the default guix channel(s)
+defined by @code{%default-channels}.  If you configure this service
+directly, be sure to include a guix channel.  @xref{Specifying
+Additional Channels} and @ref{Using a Custom Guix Channel} for more
+details.
+
+A typical extension for adding a channel might look like this:
+
+@lisp
+(simple-service 'variant-packages-service
+                home-channels-service-type
+                (list
+                 (channel
+                  (name 'variant-packages)
+                  (url "https://example.org/variant-packages.git")))
+@end lisp
+@end defvr
 
 @node Invoking guix home
 @section Invoking @code{guix home}

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.")))

gnu/local.mk

@@ -83,6 +83,7 @@ GNU_SYSTEM_MODULES =				\
   %D%/home/services/desktop.scm			\
   %D%/home/services/symlink-manager.scm		\
   %D%/home/services/fontutils.scm		\
+  %D%/home/services/guix.scm			\
   %D%/home/services/shells.scm			\
   %D%/home/services/shepherd.scm		\
   %D%/home/services/ssh.scm			\