weapon.scala

case class Weapon(name: String, ammo: Long)

case class Soldier(rank: Int, weapon: Weapon)

case class Lens[R, F](get: R => F, set: (R, F) => R) {
  def compose[Q](g: Lens[Q, R]): Lens[Q, F] =
    Lens(get = get compose g.get,
      set = (q, f) => g set(q, set(g get q, f)))
}

val weaponL = Lens[Soldier, Weapon](
  get = _.weapon,
  set = (s, w) => s.copy(weapon = w)
)

val ammoL = Lens[Weapon, Long](
  get = _.ammo,
  set = (w, a) => w.copy(ammo = a)
)

val nameL = Lens[Weapon, String](
  get = _.name,
  set = (w, n) => w.copy(name = n)
)

val soldierAmmoL = ammoL compose weaponL
val soldierWeaponName = nameL compose weaponL