{ lib }:
let
  checkType = lib.types.submodule {
    options = {
      assertion = lib.mkOption {
        type = lib.types.functionTo lib.types.bool;
        description = ''
          Predicate run against this registry's flattened collected entries.
        '';
      };

      message = lib.mkOption {
        type = lib.types.str;
        description = ''
          Error message shown when this registry validation check fails.
        '';
      };
    };
  };

  predicates = {
    uniqueValue =
      value: entries: builtins.length (lib.unique (map value entries)) == builtins.length entries;
  };

  registryLib = lib // {
    registry = (lib.registry or { }) // {
      inherit predicates;
    };
  };

  registryDefinitionModule = {
    options = {
      type = lib.mkOption {
        type = lib.types.raw;
        description = ''
          NixOS option type for one registry entry.
        '';
      };

      replicate = lib.mkOption {
        type = lib.types.bool;
        default = false;
        description = ''
          Whether collected entries are merged and published under every host key.
        '';
      };

      checks = lib.mkOption {
        type = lib.types.listOf checkType;
        default = [ ];
        description = ''
          Validation checks for this registry's flattened collected entries.
        '';
      };
    };
  };
in
{
  inherit
    checkType
    predicates
    registryDefinitionModule
    registryLib
    ;
}