lense_example.scala

class Lens[A, B](
  val get: A => B,
  val set: (B, A) => A
) {
  def andThen[C](l: Lens[B, C]): Lens[A, C] =
    Lens(
      (a)    => l.get(get(a)),
      (c, a) => set(l.set(c, get(a)), a)
    )
}

object Lens {
  def apply[A, B](
    gf: A => B,
    sf: (B, A) => A
  ): Lens[A, B] = new Lens(gf, sf)
}

/* A simple model. */
case class Address(street: String, zip: String)
case class Person(name: String, address: Address)

/* A Lenses for the Address of a Person and the street of an Address. */
val paL	= Lens[Person, Address](_.address, (a, p) => p.copy(address = a))
val asL	= Lens[Address, String](_.street, (s, a) => a.copy(street = s))
/* Composition of the above eases access to a person.address.street. */
val pasL = paL andThen asL

val chris = Person("Chris", Address("201 B", "32080"))
pasL.get(chris)
pasL.set("67th", chris)