Elementary functions in kotlin - kotlin

Question

Looking for a way to get elementary functions and their derivatives I approach it like this:
abstract class Fun() {
/**
* i = 0 -- the function itself,
* i = 1, 2, 3, ... -- its successive derivatives
*/
abstract fun d(i: Int, x: Float): Float
}
class Lin(val k: Float) : Fun() {
// y = k*x
override fun d(i: Int, x: Float, p: Float) = when (i) {
0 -> k * x
1 -> k
else -> 0.0f
}
}
class Sum(val fun0: Fun, val fun1: Fun) : Fun() {
// y = fun0(x) + fun1(x)
override fun d(i: Int, x: Float, p: Float) = fun0.d(i, x) + fun1.d(i, x)
}
class Example(val fun1: Fun, val fun2: Fun){
var res = fun1.d(0, 5.25f) // fun1 value at 5.25f
res = fun1.d(1, 3.29f) // fun1 first derivative at 3.29f
val sum = Sum(fun1, fun2) // sum of f1 and f2
res = sum(0, 3.78f) // sum value at 3.78f
res = sum(1, 5.69f) // sum first derivative at 5.69f
}
Is there a more idiomatic way to do it in Kotlin?
I have exposed the problem as I had done in Java, that is, classes that contain functions. My question is if I can do the same with functions, pass them on to a class like:
class ParametricCurveXYZ(val fun_x: Fun, val fun_y: Fun, val fun_z: Fun) {
fun pointToXYZ(s: Float) = VectorXYZ(fun_x.d(0, s), fun_y.d(0, s), fun_z.d(0, s))
fun tangent(s: Float) = VectorXYZ(fun_x.d(1, s), fun_y.d(1, s), fun_z.d(1, s)).normalized()
}

Answer 1

You can use lambdas instead of regular classes and overload operators to combine lambdas.
fun lin(k: Float) = { i: Int, x: Float ->
when (i) {
0 -> k * x
1 -> k
else -> 0.0f
}
}
operator fun ((Int, Float) -> Float).plus(that: (Int, Float) -> Float) =
{ i: Int, x: Float -> this(i, x) + that(i, x) }
fun doSomething() {
val sum = lin(1f) + lin(2f)
val res = sum(0, 3.78f)
}