{ pkgs, lib, config, options, ... }:
let
  inherit (lib.modules)
    mkIf
  ;
  inherit (lib.options)
    mkOption
    mkEnableOption
    literalExpression
    showOption
  ;
  t = lib.types;
  cfg = config.dynamicism;
  opts = options.dynamicism;
  subOpts = lib.mapAttrs (_: metaAttr: metaAttr.configuration.options) options.dynamicism.for.valueMeta.attrs;
  settingsFormat = pkgs.formats.yaml { };

  concatFoldl = f: list: lib.foldl' (acc: value: acc ++ (f value)) [ ] list;
  recUpdateFoldlAttrs = f: attrs: lib.foldlAttrs (acc: name: value: lib.recursiveUpdate acc (f name value)) { } attrs;

  finalSettingsFor = { ... }@submod: lib.foldl (acc: optPath: let
    next =
      assert lib.isList optPath;
      lib.setAttrByPath optPath (lib.getAttrFromPath optPath config);
  in lib.recursiveUpdate acc next) { } submod.source-options;

  ourAssertions = lib.concatAttrValues {
    unitsExist = concatFoldl (submod: let
      next = lib.map (unit: assert lib.isString unit; {
        assertion = config.systemd.units.${unit}.enable or false;
        message = ''
          '${showOption submod.systemd-services-updated.loc}' specified non-existent unit '${unit}'
        '';
      }) submod.systemd-services-updated.value;
    in lib.optionals submod.enable.value next) (lib.attrValues subOpts);

    optsExist = concatFoldl (submod: lib.optionals submod.enable.value (lib.map (optPath: {
      assertion = lib.hasAttrByPath optPath options;
      message = "'${showOption submod.source-options.loc}' specified non-existent option '${showOption optPath}'";
    }) submod.source-options.value)) (lib.attrValues subOpts);
  };
in
{
  #
  # Interface.
  #
  options.dynamicism = {
    for = mkOption {
      type = t.attrsOf (t.submoduleWith {
        modules = [ ./submodule.nix ];
        shorthandOnlyDefinesConfig = false;
        specialArgs = { inherit pkgs; };
      });
      default = { };
    };

    finalSettings = mkOption {
      type = t.attrsOf t.raw;
      internal = true;
      readOnly = true;
      description = ''
        Attrset of each `source-options` tree to their actual values.
      '';
    };
  };

  # Assertions.
  config.assertions = ourAssertions;

  #
  # Generic implementation.
  #
  config.dynamicism.finalSettings = lib.asserts.checkAssertWarn ourAssertions [ ] (
    recUpdateFoldlAttrs (name: { ... }@submod: finalSettingsFor submod) config.dynamicism.for
  );

  # Implementations.
  config.dynamicism.for.gotosocial = let
    cfg = config.dynamicism.for.gotosocial;
  in {
    source-options = [
      "services.gotosocial.settings"
    ];

    configFile = settingsFormat.generate "gotosocial-override.yml" config.services.gotosocial.settings;

    unitDropins."gotosocial.service" = pkgs.writeText "gotosocial-override.conf" ''
      [Service]
      ExecStart=
      ExecStart=${lib.getExe' pkgs.gotosocial "gotosocial"} --config-path ${cfg.configFile} start
    '';
  };
}