Kotlin and RxJava Cacheのコード part1

kotlin_cache.kt

class UserRepository(
    userDao: UserDao
) {
  private val userDaoWithCache = Cache(userDao::getUser)

  fun getUser(id: Int, name: String): Single<User> {
    return userDaoWithCache.fetchOrCache(id, name)
  }
}

class Cache<K1, K2, V>(
    private val invoker: (K1, K2) -> Single<V>
) {
  private val cache = HashMap<Tuple2<K1, K2>, V>()

  fun fetchOrCache(key1: K1, key2: K2): Single<V> {
    val key = Tuple2(key1, key2)
    val value = cache[key] ?: return invoker(key1, key2).doOnSuccess { cache[key] = it }
    return Single.just(value)
  }
}

data class Tuple2<out V1, out V2>(val v1: V1, val v2: V2)

class UserDao {
  fun getUser(id: Int, name: String): Single<User> {
    return Single.just(User(id, name))
        .delay(500, TimeUnit.MILLISECONDS)
  }
}

data class User(val id: Int, val name: String)

引数の数が増えたらTuple2, 3, ... Nと際限なく増えていくのでここをなんとかしたい