Add a module for Emacs daemon

nixos/modules/module-list.nix

@@ -164,6 +164,7 @@
   ./services/desktops/profile-sync-daemon.nix
   ./services/desktops/telepathy.nix
   ./services/development/hoogle.nix
+  ./services/editors/emacs.nix
   ./services/games/factorio.nix
   ./services/games/ghost-one.nix
   ./services/games/minecraft-server.nix

nixos/modules/services/editors/emacs.nix

@@ -0,0 +1,65 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.services.emacs;
+
+in {
+
+  options.services.emacs = {
+    enable = mkOption {
+      type = types.bool;
+      default = false;
+      example = true;
+      description = ''
+        Whether to enable a user service for the Emacs daemon. Use <literal>emacsclient</literal> to connect to the
+        daemon. If <literal>true</literal>, <varname>services.emacs.install</varname> is
+        considered <literal>true</literal>, whatever its value.
+      '';
+    };
+
+    install = mkOption {
+      type = types.bool;
+      default = false;
+      example = true;
+      description = ''
+        Whether to install a user service for the Emacs daemon. Once
+        the service is started, use emacsclient to connect to the
+        daemon.
+
+        The service must be manually started for each user with
+        "systemctl --user start emacs" or globally through
+        <varname>services.emacs.enable</varname>.
+      '';
+    };
+
+
+    package = mkOption {
+      type = types.package;
+      default = pkgs.emacs;
+      defaultText = "pkgs.emacs";
+      description = ''
+        emacs derivation to use.
+      '';
+    };
+
+  };
+
+  config = mkIf (cfg.enable || cfg.install) {
+    systemd.user.services.emacs = {
+      description = "Emacs: the extensible, self-documenting text editor";
+
+      serviceConfig = {
+        Type      = "forking";
+        ExecStart = "${pkgs.bash}/bin/bash -c 'source ${config.system.build.setEnvironment}; exec ${cfg.package}/bin/emacs --daemon'";
+        ExecStop  = "${cfg.package}/bin/emacsclient --eval (kill-emacs)";
+        Restart   = "always";
+      };
+    } // optionalAttrs cfg.enable { wantedBy = [ "default.target" ]; };
+
+    environment.systemPackages = [ cfg.package ];
+  };
+
+}