{
  config,
  lib,
  pkgs,
  ...
}:
let
  cfg = config.services.rss2email;
in
{

  ###### interface

  options = {

    services.rss2email = {

      enable = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = "Whether to enable rss2email.";
      };

      to = lib.mkOption {
        type = lib.types.str;
        description = "Mail address to which to send emails";
      };

      interval = lib.mkOption {
        type = lib.types.str;
        default = "12h";
        description = "How often to check the feeds, in systemd interval format";
      };

      config = lib.mkOption {
        type =
          with lib.types;
          attrsOf (oneOf [
            str
            int
            bool
          ]);
        default = { };
        description = ''
          The configuration to give rss2email.

          Default will use system-wide `sendmail` to send the
          email. This is rss2email's default when running
          `r2e new`.

          This set contains key-value associations that will be set in the
          `[DEFAULT]` block along with the
          `to` parameter.

          See `man r2e` for more information on which
          parameters are accepted.
        '';
      };

      feeds = lib.mkOption {
        description = "The feeds to watch.";
        type = lib.types.attrsOf (
          lib.types.submodule {
            options = {
              url = lib.mkOption {
                type = lib.types.str;
                description = "The URL at which to fetch the feed.";
              };

              to = lib.mkOption {
                type = with lib.types; nullOr str;
                default = null;
                description = ''
                  Email address to which to send feed items.

                  If `null`, this will not be set in the
                  configuration file, and rss2email will make it default to
                  `rss2email.to`.
                '';
              };
            };
          }
        );
      };
    };

  };

  ###### implementation

  config = lib.mkIf cfg.enable {
    users.groups = {
      rss2email.gid = config.ids.gids.rss2email;
    };

    users.users = {
      rss2email = {
        description = "rss2email user";
        uid = config.ids.uids.rss2email;
        group = "rss2email";
      };
    };

    environment.systemPackages = with pkgs; [ rss2email ];

    services.rss2email.config.to = cfg.to;

    systemd.tmpfiles.settings."10-rss2email"."/var/rss2email".d = {
      user = "rss2email";
      group = "rss2email";
      mode = "0700";
    };

    systemd.services.rss2email =
      let
        conf = pkgs.writeText "rss2email.cfg" (
          lib.generators.toINI { } (
            {
              DEFAULT = cfg.config;
            }
            // lib.mapAttrs' (
              name: feed:
              lib.nameValuePair "feed.${name}" (
                { inherit (feed) url; } // lib.optionalAttrs (feed.to != null) { inherit (feed) to; }
              )
            ) cfg.feeds
          )
        );
      in
      {
        preStart = ''
          if [ ! -f /var/rss2email/db.json ]; then
            echo '{"version":2,"feeds":[]}' > /var/rss2email/db.json
          fi
        '';
        path = [ pkgs.system-sendmail ];
        serviceConfig = {
          ExecStart = "${pkgs.rss2email}/bin/r2e -c ${conf} -d /var/rss2email/db.json run";
          User = "rss2email";
        };
      };

    systemd.timers.rss2email = {
      partOf = [ "rss2email.service" ];
      wantedBy = [ "timers.target" ];
      timerConfig.OnBootSec = "0";
      timerConfig.OnUnitActiveSec = cfg.interval;
    };
  };

}
