{ lib }:

let
  inherit (lib)
    concatStringsSep defaultFunctor fixedWidthNumber hm imap1 isAttrs isList
    length listToAttrs mapAttrs mkIf mkOrder mkOption mkOptionType nameValuePair
    stringLength types warn;

  dagEntryOf = elemType:
    let
      submoduleType = types.submodule ({ name, ... }: {
        options = {
          data = mkOption { type = elemType; };
          after = mkOption { type = with types; listOf str; };
          before = mkOption { type = with types; listOf str; };
        };
        config = mkIf (elemType.name == "submodule") {
          data._module.args.dagName = name;
        };
      });
      maybeConvert = def:
        if hm.dag.isEntry def.value then
          def.value
        else
          hm.dag.entryAnywhere (if def ? priority then
            mkOrder def.priority def.value
          else
            def.value);
    in mkOptionType {
      name = "dagEntryOf";
      description = "DAG entry of ${elemType.description}";
      # leave the checking to the submodule type
      merge = loc: defs:
        submoduleType.merge loc (map (def: {
          inherit (def) file;
          value = maybeConvert def;
        }) defs);
    };

in rec {
  # A directed acyclic graph of some inner type.
  #
  # Note, if the element type is a submodule then the `name` argument
  # will always be set to the string "data" since it picks up the
  # internal structure of the DAG values. To give access to the
  # "actual" attribute name a new submodule argument is provided with
  # the name `dagName`.
  dagOf = elemType:
    let attrEquivalent = types.attrsOf (dagEntryOf elemType);
    in mkOptionType rec {
      name = "dagOf";
      description = "DAG of ${elemType.description}";
      inherit (attrEquivalent) check merge emptyValue;
      getSubOptions = prefix: elemType.getSubOptions (prefix ++ [ "<name>" ]);
      getSubModules = elemType.getSubModules;
      substSubModules = m: dagOf (elemType.substSubModules m);
      functor = (defaultFunctor name) // { wrapped = elemType; };
      nestedTypes.elemType = elemType;
    };
}