Subroutine

override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)

        val sum = (0..10).toMutableList().sum()
        result_text.text = "Result : $sum" // sum() 연산이 끝난 후 결과 return
}

// Subroutine
private fun MutableList<Int>.sum(): Int = this.sumBy { it }