list(추가 불가능함)에 add 를 달아보자

fun appendList<T>(listOfLists: List<List<T>>, toAppend: List<T>): List<List<T>> {
}

Of course both of the following work:

fun appendList<T>(listOfLists: List<List<T>>, toAppend: List<T>): List<List<T>> {
    // Use spread operator
    return listOf(*listOfLists.toTypedArray(), toAppend);
}

fun appendList<T>(listOfLists: List<List<T>>, toAppend: List<T>): List<List<T>> {

  // Turn to mutable list

  val ret = listOfLists.toArrayList()

  ret.add(toAppend)

  return ret

}

But this is confusing to users who might expect + to work. Due to type erasure, + will always have quirks like this. I had to add the following extension:

operator fun <T, C : Collection<T>> List<C>.plus(toAppend: C): List<C> {
    val ret = java.util.ArrayList<C>(this.size + 1)
    ret.addAll(this)
    ret.add(toAppend)
    return ret
}