{
  lib,
  pkgs,
  config,
  ...
}: let
  inherit (lib) filter assertMsg head length escapeShellArg types mkOption concatStringsSep;

  envToBash = {
    name,
    value,
    eval,
    prefix,
    ...
  } @ args: let
    vals = filter (key: args.${key} != null && args.${key} != false) [
      "eval"
      "prefix"
      "unset"
      "value"
    ];
    valType = head vals;
  in
    assert assertMsg (
      (length vals) > 0
    ) "[devshell/env]: ${name} expected one of (value|eval|prefix|unset) to be set.";
    assert assertMsg ((length vals) < 2)
    "[devshell/env]: ${name} expected only one of (value|eval|prefix|unset) to be set. Not ${toString vals}";
    assert assertMsg (!(name == "PATH" && valType == "value")) "[devshell/env]: ${name} should not override the value. Use 'prefix' instead.";
      if valType == "value"
      then "export ${name}=${escapeShellArg (toString value)}"
      else if valType == "eval"
      then "export ${name}=${eval}"
      else if valType == "prefix"
      then ''export ${name}=$(${pkgs.coreutils}/bin/realpath --canonicalize-missing "${prefix}")''${${name}+:''${${name}}}''
      else if valType == "unset"
      then ''unset ${name}''
      else throw "[devshell] BUG in the env module. This should never be reached.";

  envType = types.submodule ({name, ...}: {
    options = {
      name = mkOption {
        type = types.str;
        default = name;
        description = ''
          Name of the environment variable, usually already set to `<name>`.
        '';
      };
      desc = mkOption {
        type = types.nullOr types.str;
        default = null;
        description = ''
          Description of the env var, can be shown in the menu.
        '';
      };
      visible = mkOption {
        type = types.bool;
        default = true;
        description = ''
          Whether to include this env var and it's description in the menu.
        '';
      };
      value = mkOption {
        type = with types;
          nullOr (oneOf [
            str
            int
            bool
            package
          ]);
        default = null;
        example = "hello";
        description = ''
          Any value to convert to a string and set the env variable to.
          There is no evaluation of the value, for that see [eval](#envnameeval).
        '';
      };

      eval = mkOption {
        type = types.nullOr types.str;
        default = null;
        description = ''
          Like value but evaluated by Bash. This allows to inject other
          variable names or even commands using the `$()` notation.
        '';
        example = "$OTHER_VAR";
      };

      prefix = mkOption {
        type = types.nullOr types.str;
        default = null;
        description = ''
          Prepend to PATH-like environment variables.

          For example `name = "PATH"; prefix = "bin";` will expand the path of
          `./bin` and prepend it to the `PATH`, separated by `:`.
        '';
        example = "bin";
      };

      unset = mkOption {
        type = types.bool;
        default = false;
        description = ''
          Unset the env variable.
        '';
      };
    };
  });
in {
  options.env = mkOption {
    type = types.attrsOf envType;
    default = {};
    description = ''
      Manage environment variables.
    '';
  };

  config = {
    env = {
      "XDG_DATA_DIRS".eval = "$DEVSHELL_DIR/share:\${XDG_DATA_DIRS-}";
    };
    enterShellCommands."env".text = concatStringsSep "\n" (map envToBash (builtins.attrValues config.env));
    # TODO: collect all env vars, then add them to the menu if they have a description and "visibile == true"
  };
}