home-manager/modules/lib/generators.nix

{ lib }:

{
  toKDL = { }:
    let
      inherit (lib) concatStringsSep splitString mapAttrsToList any;
      inherit (builtins) typeOf replaceStrings elem;

      # ListOf String -> String
      indentStrings = let
        # Although the input of this function is a list of strings,
        # the strings themselves *will* contain newlines, so you need
        # to normalize the list by joining and resplitting them.
        unlines = lib.splitString "\n";
        lines = lib.concatStringsSep "\n";
        indentAll = lines: concatStringsSep "\n" (map (x: "	" + x) lines);
      in stringsWithNewlines: indentAll (unlines (lines stringsWithNewlines));

      # String -> String
      sanitizeString = replaceStrings [ "\n" ''"'' ] [ "\\n" ''\"'' ];

      # OneOf [Int Float String Bool Null] -> String
      literalValueToString = element:
        lib.throwIfNot
        (elem (typeOf element) [ "int" "float" "string" "bool" "null" ])
        "Cannot convert value of type ${typeOf element} to KDL literal."
        (if typeOf element == "null" then
          "null"
        else if element == false then
          "false"
        else if element == true then
          "true"
        else if typeOf element == "string" then
          ''"${sanitizeString element}"''
        else
          toString element);

      # Attrset Conversion
      # String -> AttrsOf Anything -> String
      convertAttrsToKDL = name: attrs:
        let
          optArgsString = lib.optionalString (attrs ? "_args")
            (lib.pipe attrs._args [
              (map literalValueToString)
              (lib.concatStringsSep " ")
              (s: s + " ")
            ]);

          optPropsString = lib.optionalString (attrs ? "_props")
            (lib.pipe attrs._props [
              (lib.mapAttrsToList
                (name: value: "${name}=${literalValueToString value}"))
              (lib.concatStringsSep " ")
              (s: s + " ")
            ]);

          children =
            lib.filterAttrs (name: _: !(elem name [ "_args" "_props" ])) attrs;
        in ''
          ${name} ${optArgsString}${optPropsString}{
          ${indentStrings (mapAttrsToList convertAttributeToKDL children)}
          }'';

      # List Conversion
      # String -> ListOf (OneOf [Int Float String Bool Null])  -> String
      convertListOfFlatAttrsToKDL = name: list:
        let flatElements = map literalValueToString list;
        in "${name} ${concatStringsSep " " flatElements}";

      # String -> ListOf Anything -> String
      convertListOfNonFlatAttrsToKDL = name: list: ''
        ${name} {
        ${indentStrings (map (x: convertAttributeToKDL "-" x) list)}
        }'';

      # String -> ListOf Anything  -> String
      convertListToKDL = name: list:
        let elementsAreFlat = !any (el: elem (typeOf el) [ "list" "set" ]) list;
        in if elementsAreFlat then
          convertListOfFlatAttrsToKDL name list
        else
          convertListOfNonFlatAttrsToKDL name list;

      # Combined Conversion
      # String -> Anything  -> String
      convertAttributeToKDL = name: value:
        let vType = typeOf value;
        in if elem vType [ "int" "float" "bool" "null" "string" ] then
          "${name} ${literalValueToString value}"
        else if vType == "set" then
          convertAttrsToKDL name value
        else if vType == "list" then
          convertListToKDL name value
        else
          throw ''
            Cannot convert type `(${typeOf value})` to KDL:
              ${name} = ${toString value}
          '';
    in attrs: ''
      ${concatStringsSep "\n" (mapAttrsToList convertAttributeToKDL attrs)}
    '';
}