minBy

public inline fun <T, R : Comparable<R>> Iterable<T>.minBy(selector: (T) -> R): T? {
    val iterator = iterator()
    if (!iterator.hasNext()) return null
    var minElem = iterator.next()
    var minValue = selector(minElem)
    while (iterator.hasNext()) {
        val e = iterator.next()
        val v = selector(e)
        if (minValue > v) {
            minElem = e
            minValue = v
        }
    }
    return minElem
}


val list = listOf(Person("a", 10), Person("b", 20), Person("c", 30))
println(list.minBy{ it.age })  // Person(name=a, age=10)