Here is another approach. It uses a sealed class. The area is a abstract val. An else branch of a when expression is not needed. The compiler knows the three possible subclasses of the sealed class Shapes.

sealed class Shapes {

    abstract val area : Float

    fun nameType() : String {
        return when(this) {
            is Circle -> "Circle"
            is Square -> "Square"
            is Rectangle -> "Rectangle"
        }
    }

    data class Circle(val radius : Float) : Shapes() {
        override val area: Float
            get() = Math.PI.toFloat() * radius * radius
    }
    data class Square(val side: Float) : Shapes() {
        override val area: Float
            get() = side * side
    }
    data class Rectangle(val length : Float, val width: Float) : Shapes() {
        override val area: Float
            get() = length * width
    }
}

fun main(args: Array<String>) {
    val circle = Shapes.Circle(radius = 8.8f)
    println(circle.area) // 243.28496
    println(circle.nameType()) // Prints Circle

}