home: services: Add home-xmodmap-service-type.

* gnu/home/services/desktop.scm (home-xmodmap-service-type)
(home-xmodmap-configuration): New variables;
(serialize-xmodmap-configuration)
(xmodmap-shepherd-service): New procedures;
* doc/guix.texi (Desktop Services): Document it.

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

doc/guix.texi

@@ -42626,6 +42626,47 @@ A timeout in seconds after which to hide cursor.
 
 @end deftp
 
+@defvar home-xmodmap-service-type
+This is the service type for the
+@uref{https://gitlab.freedesktop.org/xorg/app/xmodmap,xmodmap} utility
+to modify keymaps and pointer button mappings under the Xorg display
+server.  Its associated value must be a
+@code{home-xmodmap-configuration} record, as shown below.
+
+The @code{key-map} field takes a list of objects, each of which is
+either a @dfn{statement} (a string) or an @dfn{assignment} (a pair of
+strings).  As an example, the snippet below swaps around the
+@kbd{Caps_Lock} and the @kbd{Control_L} keys, by first removing the
+keysyms (on the right-hand side) from the corresponding modifier maps
+(on the left-hand side), re-assigning them by swapping each other out,
+and finally adding back the keysyms to the modifier maps.
+
+@lisp
+(service home-xmodmap-service-type
+         (home-xmodmap-configuration
+          (key-map '(("remove Lock" . "Caps_Lock")
+                     ("remove Control" . "Control_L")
+                     ("keysym Control_L" . "Caps_Lock")
+                     ("keysym Caps_Lock" . "Control_L")
+                     ("add Lock" . "Caps_Lock")
+                     ("add Control" . "Control_L")))))
+@end lisp
+@end defvar
+
+@deftp {Data Type} home-xmodmap-configuration
+The configuration record for @code{home-xmodmap-service-type}.  Its
+available fields are:
+
+@table @asis
+@item @code{xmodmap} (default: @code{xmodmap}) (type: file-like)
+The @code{xmodmap} package to use.
+
+@item @code{key-map} (default: @code{'()}) (type: list)
+The list of expressions to be read by @code{xmodmap} on service startup.
+
+@end table
+@end deftp
+
 @node Guix Home Services
 @subsection Guix Home Services
 

gnu/home/services/desktop.scm

@@ -24,6 +24,7 @@ (define-module (gnu home services desktop)
   #:use-module (gnu services configuration)
   #:autoload   (gnu packages glib)    (dbus)
   #:autoload   (gnu packages xdisorg) (redshift unclutter)
+  #:autoload   (gnu packages xorg) (setxkbmap xmodmap)
   #:use-module (guix records)
   #:use-module (guix gexp)
   #:use-module (srfi srfi-1)
@@ -275,3 +276,59 @@ (define home-unclutter-service-type
    (description "Run the @code{unclutter} daemon, which, on systems using the
 Xorg graphical display server, automatically hides the cursor after a
 user-defined timeout has expired.")))
+
+
+;;;
+;;; Xmodmap.
+;;;
+
+(define-configuration/no-serialization home-xmodmap-configuration
+  (xmodmap
+   (file-like xmodmap)
+   "The @code{xmodmap} package to use.")
+  (key-map
+   (list '())
+   "List of expressions to be read by @code{xmodmap} on service startup."))
+
+(define (serialize-xmodmap-configuration field-name val)
+  (define serialize-field
+    (match-lambda
+      ((key . value)
+       (format #f "~a = ~a" key value))
+      (e e)))
+
+  #~(string-append
+     #$@(interpose (map serialize-field val) "\n" 'suffix)))
+
+(define (xmodmap-shepherd-service config)
+  (define config-file
+    (mixed-text-file
+     "config"
+     (serialize-xmodmap-configuration
+      #f (home-xmodmap-configuration-key-map config))))
+
+  (list
+   (shepherd-service
+    (provision '(xmodmap))
+    (start #~(make-system-constructor
+              (string-join
+               (list #$(file-append
+                        (home-xmodmap-configuration-xmodmap config)
+                        "/bin/xmodmap")
+                     #$config-file))))
+    (stop #~(make-system-constructor
+             #$(file-append setxkbmap "/bin/setxkbmap")))
+    (documentation "On startup, run @code{xmodmap} and read the expressions in
+the configuration file.  On stop, reset all the mappings back to the
+defaults."))))
+
+(define home-xmodmap-service-type
+  (service-type
+   (name 'home-xmodmap)
+   (extensions
+    (list
+     (service-extension home-shepherd-service-type
+                        xmodmap-shepherd-service)))
+   (default-value (home-xmodmap-configuration))
+   (description "Run the @code{xmodmap} utility to modify keymaps and pointer
+buttons under the Xorg display server via user-defined expressions.")))