{ lib, config, pkgs, ... }:
{
  options.var = with lib.types; {
    # host-specific options 
    username = lib.mkOption { type = str; };
    hostname = lib.mkOption { type = str;}; 
    #[...]
    # Variables shared by all hosts
    locale = lib.mkOption { type = str;};
    timezone = lib.mkOption { type = str;};
    #[...]
  };
  config.var = {
    # Variables shared by all hosts
    locale = "de_DE.UTF-8";
    timezone = "Europe/Berlin";
  };
}