Optics

Optics are abstractions to update immutable data structures elegantly. One form of optics is Lens (or lenses, depending on the library implementation). A Lens is a functional reference that can focus (hence the name) into a structure and read, write, or modify its target:

typealias GB = Int

data class Memory(val size: GB)
data class MotherBoard(val brand: String, val memory: Memory)
data class Laptop(val price: Double, val motherBoard: MotherBoard)

fun main(args: Array<String>) {
   val laptopX8 = Laptop(500.0, MotherBoard("X", Memory(8)))

   val laptopX16 = laptopX8.copy(
         price = 780.0,
         motherBoard = laptopX8.motherBoard.copy(
               memory = laptopX8.motherBoard.memory.copy(
                     size = laptopX8.motherBoard.memory.size * 2
               )
         )
   )

   println("laptopX16 = $laptopX16")
}

To create a new Laptop value from an existing one, we need to use several nested copy methods and references. In this example, it isn't as...