Simple lenses in Scala.

lenses.scala

object Lenses {
  // Main API
  case class Lens[A, B](get: A => B, set: (A, B) => A) {
    def compose[C](other: Lens[B, C]): Lens[A, C] = {
      Lens[A, C](
        get = (a: A) => other.get(get(a)),
        set = (a: A, c: C) => set(a, other.set(get(a), c))
      )
    }
  }

  // Example structures
  case class Address(street: String)
  case class Person(name: String, address: Address)

  // Lenses for above structures
  object Address {
    val street: Lens[Address, String] = Lens(
      get = (a) => a.street,
      set = (a,s) => a.copy(street = s)
    )
  }

  object Person {
    val name: Lens[Person, String] = Lens(
      get = (p) => p.name,
      set = (p,n) => p.copy(name = n)
    )
    val address: Lens[Person, Address] = Lens(
      get = (p) => p.address,
      set = (p,a) => p.copy(address = a)
    )
  }

  // Usage
  def example(): Unit = {
    val personStreet = Person.address compose Address.street
    val john = Person("John", Address("Șelari"))

    println(personStreet.get(john))
    println(personStreet.set(john, "Smârdan"))
  }
}