//listOf actually creates a List<T>
//student > person List<Student> > List<Person> the relation is variance
// invariant
// why invariant?

open class Person(val name: String)
class Student(name: String): Person(name)

fun main(){
    val names: List<String> = listOf("John", "Sarah", "Mark")
    val ranks: List<Int> = listOf(3, 5, 6)

    val student: Student = Student("John")
    val person: Person = student

    val students: MutableList<Student> = mutableListOf(Student("Jack"), Student("Sarah"))
    val persons: MutableList<Person> = students
}

// why invariant?
open class Person(val name: String)
class Student(name: String): Person(name)
class Teacher(name: String): Person(name)

fun add(group: MutableList<Person>, person: Person) = group.add(person)

fun main(){
    val students: MutableList<Student> = mutableListOf()
    val teacher: Teacher = Teacher("John")
    add(students, teacher)

    val student: Student = students.get(0)
}