services: xorg: Add 'set-xorg-configuration'.

* gnu/services/xorg.scm (gdm-service-type)[compose, extend]: New fields.
(set-xorg-configuration): New procedure.
* doc/guix.texi (Keyboard Layout): Use it.
(X Window): Document it.
* gnu/system/examples/desktop.tmpl: Add 'keyboard-layout' fields.

doc/guix.texi

@@ -11077,6 +11077,7 @@ Let's say you want your system to use the Turkish keyboard layout throughout
 your system---bootloader, console, and Xorg.  Here's what your system
 configuration would look like:
 
+@findex set-xorg-configuration
 @lisp
 ;; Using the Turkish layout for the bootloader, the console,
 ;; and for Xorg.
@@ -11088,18 +11089,17 @@ configuration would look like:
                 (bootloader grub-efi-bootloader)
                 (target "/boot/efi")
                 (keyboard-layout keyboard-layout))) ;for GRUB
-  (services (modify-services %desktop-services
+  (services (cons (set-xorg-configuration
-              (gdm-service-type config =>
-                (gdm-configuration
-                  (inherit config)
-                  (xorg-configuration
                     (xorg-configuration             ;for Xorg
-                      (keyboard-layout keyboard-layout))))))))
+                      (keyboard-layout keyboard-layout)))
+                  %desktop-services)))
 @end lisp
 
 In the example above, for GRUB and for Xorg, we just refer to the
 @code{keyboard-layout} field defined above, but we could just as well refer to
-a different layout.
+a different layout.  The @code{set-xorg-configuration} procedure communicates
+the desired Xorg configuration to the graphical log-in manager, by default
+GDM.
 
 @node Locales
 @section Locales
@@ -13566,6 +13566,16 @@ default is @code{-nolisten tcp}.
 @end table
 @end deftp
 
+@deffn {Scheme Procedure} set-xorg-configuration @var{config} @
+  [@var{login-manager-service-type}]
+Tell the log-in manager (of type @var{login-manager-service-type}) to use
+@var{config}, an <xorg-configuration> record.
+
+Since the Xorg configuration is embedded in the log-in manager's
+configuration---e.g., @code{gdm-configuration}---this procedure provides a
+shorthand to set the Xorg configuration.
+@end deffn
+
 @deffn {Scheme Procedure} xorg-start-command [@var{config}]
 Return a @code{startx} script in which the modules, fonts, etc. specified
 in @var{config}, are available.  The result should be used in place of

gnu/services/xorg.scm

@@ -99,7 +99,8 @@
 
             gdm-configuration
             gdm-service-type
-            gdm-service))
+            gdm-service
+            set-xorg-configuration))
 
 ;;; Commentary:
 ;;;
@@ -875,6 +876,15 @@ the GNOME desktop environment.")
                                            xorg-configuration-keyboard-layout
                                            gdm-configuration-xorg))))
 
+                ;; For convenience, this service can be extended with an
+                ;; <xorg-configuration> record.  Take the first one that
+                ;; comes.
+                (compose first)
+                (extend (lambda (config xorg-configuration)
+                          (gdm-configuration
+                           (inherit config)
+                           (xorg-configuration xorg-configuration))))
+
                 (default-value (gdm-configuration))
                 (description
                  "Run the GNOME Desktop Manager (GDM), a program that allows
@@ -908,4 +918,14 @@ password."
             (gdm gdm)
             (allow-empty-passwords? allow-empty-passwords?))))
 
+(define* (set-xorg-configuration config
+                                 #:optional
+                                 (login-manager-service-type
+                                  gdm-service-type))
+  "Tell the log-in manager (of type @var{login-manager-service-type}) to use
+@var{config}, an <xorg-configuration> record."
+  (simple-service 'set-xorg-configuration
+                  login-manager-service-type
+                  config))
+
 ;;; xorg.scm ends here

gnu/system/examples/desktop.tmpl

@@ -3,7 +3,7 @@
 ;; root partition is encrypted with LUKS.
 
 (use-modules (gnu) (gnu system nss))
-(use-service-modules desktop)
+(use-service-modules desktop xorg)
 (use-package-modules certs gnome)
 
 (operating-system
@@ -11,11 +11,16 @@
   (timezone "Europe/Paris")
   (locale "en_US.utf8")
 
+  ;; Choose US English keyboard layout.  The "altgr-intl"
+  ;; variant provides dead keys for accented characters.
+  (keyboard-layout (keyboard-layout "us" "altgr-intl"))
+
   ;; Use the UEFI variant of GRUB with the EFI System
   ;; Partition mounted on /boot/efi.
   (bootloader (bootloader-configuration
                 (bootloader grub-efi-bootloader)
-                (target "/boot/efi")))
+                (target "/boot/efi")
+                (keyboard-layout keyboard-layout)))
 
   ;; Specify a mapped device for the encrypted root partition.
   ;; The UUID is that returned by 'cryptsetup luksUUID'.
@@ -58,7 +63,10 @@
   ;; include the X11 log-in service, networking with
   ;; NetworkManager, and more.
   (services (append (list (service gnome-desktop-service-type)
-                          (service xfce-desktop-service-type))
+                          (service xfce-desktop-service-type)
+                          (set-xorg-configuration
+                           (xorg-configuration
+                            (keyboard-layout keyboard-layout))))
                     %desktop-services))
 
   ;; Allow resolution of '.local' host names with mDNS.