IndexOptional.scala

import monocle.function._
import monocle.std.list._
import monocle.Lens
import monocle.syntax.apply._

case class Person(name: String, age: Int)

val ps = List(Person("John", 25), Person("Paul", 32))

val _age = Lens[Person, Int](_.age)(a => p => p.copy(age = a))
// or val _age = GenLens[Person](_.age)

(ps applyOptional index(1) composeLens _age).set(10) == List(Person("John", 25), Person("Paul", 10))

// we only need the apply syntax to guide type inference in the choice of index instance
// if instead you collection is inside of an object we would not need it

case class Familly(name: String, persons: List[Person])
val _persons = Lens[Familly, List[Person]](_.persons)(p => f => f.copy(persons = p))

(_persons composeOptional index(1) composeLens _age)
  .set(10)(Familly("Doe", ps)) == Familly("Doe", List(Person("John", 25), Person("Paul", 10)))