{pkgs, config, ...}:

###### interface
let
  inherit (pkgs.lib) mkOption;

  slimOptions = {

    theme = mkOption {
      default = null;
      example = pkgs.fetchurl {
        url = http://download.berlios.de/slim/slim-wave.tar.gz;
        sha256 = "0ndr419i5myzcylvxb89m9grl2xyq6fbnyc3lkd711mzlmnnfxdy";
      };
      description = "
        The theme for the SLiM login manager.  If not specified, SLiM's
        default theme is used.  See <link
        xlink:href='http://slim.berlios.de/themes01.php'/> for a
        collection of themes.
      ";
    };

    defaultUser = mkOption {
      default = "";
      example = "login";
      description = "
        The default user to load. If you put a username here you
        get it automatically loaded into the username field, and
        the focus is placed on the password.
      ";
    };

    hideCursor = mkOption {
      default = false;
      example = true;
      description = "
        Hide the mouse cursor on the login screen.
      ";
    };

  };

  options = { services = { xserver = { displayManager = {

    slim = slimOptions // {

      enable = mkOption {
        default = true;
        description = "
          Whether to enable slim as the display manager.
        ";
      };

    };

  }; /* displayManager */ }; /* xserver */ }; /* services */ };

  copyOldOptions = { services = { xserver = {

    # Declare old options.
    slim = slimOptions;

    # Copy the old options into the new options.
    displayManager = {
      slim = config.services.xserver.slim;
    };

  }; /* xserver */ }; /* services */ };

in

###### implementation
let
  xcfg = config.services.xserver;
  dmcfg = xcfg.displayManager;
  cfg = dmcfg.slim;

  inherit (pkgs.lib) mkIf;

  slimConfig = pkgs.writeText "slim.cfg" ''
    xauth_path ${dmcfg.xauthBin}
    default_xserver ${dmcfg.xserverBin}
    xserver_arguments ${dmcfg.xserverArgs}
    sessions ${pkgs.lib.concatStringsSep "," dmcfg.session.names}
    login_cmd exec ${pkgs.stdenv.bash}/bin/sh ${dmcfg.session.script} "%session"
    halt_cmd ${pkgs.upstart}/sbin/shutdown -h now
    reboot_cmd ${pkgs.upstart}/sbin/shutdown -r now
    ${if cfg.defaultUser != "" then "default_user " + cfg.defaultUser else ""}
    ${if cfg.hideCursor then "hidecursor true" else ""}
  '';


  # Unpack the SLiM theme, or use the default.
  slimThemesDir =
    let
      unpackedTheme = pkgs.stdenv.mkDerivation {
        name = "slim-theme";
        buildCommand = ''
          ensureDir $out
          cd $out
          unpackFile ${cfg.theme}
          ln -s * default
        '';
      };
    in if cfg.theme == null then "${pkgs.slim}/share/slim/themes" else unpackedTheme;

in

mkIf cfg.enable {
  require = [
    options
    copyOldOptions
  ];

  services = {
    xserver = {
      displayManager = {
        job = {
          beforeScript = ''
            rm -f /var/log/slim.log
          '';
          env = ''
            env SLIM_CFGFILE=${slimConfig}
            env SLIM_THEMESDIR=${slimThemesDir}
          '';
          execCmd = "${pkgs.slim}/bin/slim";
        };
      };
    };
  };

  environment = {
    etc = [
      { source = ./slim.pam;
        target = "pam.d/slim";
      }
    ];
  };
}