Kotlin nested property reference

Question

data class House(var name: String = "House", var door: Door = Door())
data class Door(var name: String = "Door")

fun test() {
   val testHouse = House("My house", Door(name = "My door"))
}

How could I get nested property reference nice and safe, ideally like this (this doesn't work though):

   val houseDoorName = House::door::name
   println(houseDoorName.get(testHouse)) //My door

I figured I could maybe do extension function, something like: House::door.nested(Door::name) but Im stuck with the implementation.

Answer 1

For your hypothetical nested function, try this:

fun <A, B, C> ((A) -> B).nested(getter : (B) -> C) : (A) -> C = { getter(this(it)) }

Now you can do exactly what you asked:

val houseDoorName = House::door.nested(Door::name)
val house = House(door = Door(name = "My door"))
println(houseDoorName(house)) // prints "My door"

You can chain it, too:

val doorNameLength = House::door.nested(Door::name).nested(String::length)

---

The neat trick here is the way Kotlin allows a property reference to be treated as a function.

The nested function is essentially a functional composition. It takes a function a -> b and a function b -> c, and composes them into a new function a -> c. You'll often find it called compose in standard libraries.

Kotlin doesn't have function composition as standard, but there are libraries out there if you need anything more complex than this.