dart で可変長引数を実現するためのパッケージ(dynamic_function)を作成しました。

とりあえず、デモ用のソースを提示しておきます(dart-sdk-3.7.2 と dart-sdk-3.9.0-14.0.dev で動作を確認しました。また flutter web, flutter windows desktop でも動作しました(flutter-sdk-3.29.3))。

pubspec..yaml

name: qiita
description: A sample command-line application
version: 0.0.1
publish_to: none

environment:
  sdk: ^3.7.2

dependencies:
  dynamic_function: ^2025.427.1341

dev_dependencies:

func_demo.dart

import 'package:dynamic_function/dynamic_function.dart';

final dynamic sum = DynamicFunction((
  List<dynamic> positional,
  Map<Symbol, dynamic> named
) {
  print('positional.length=${positional.length}');
  int result = 0;
  for (int i = 0; i < positional.length; i++) {
    result += positional[i] as int;
  }
  return result;
});

void main() {
  int answer = sum(11, 22, 33);
  print('answer=$answer');
}

/*
【実行結果】
positional.length=3
answer=66
 */

class_demo.dart

import 'package:dynamic_function/dynamic_function.dart';

class MyNumber {
  final int _num;

  MyNumber(this._num);

  late final dynamic addNumbers = DynamicFunction((
    List<dynamic> positional,
    Map<Symbol, dynamic> named
  ) {
    print('positional.length=${positional.length}');
    int result = _num;
    for (int i = 0; i < positional.length; i++) {
      result += positional[i] as int;
    }
    return result;
  });
}

void main() {
  final myNum = MyNumber(11);
  int answer = myNum.addNumbers(22, 33);
  print('answer=$answer');
}

/*
【実行結果】
positional.length=2
answer=66
 */