MyLens

MyLens.scala

scala> :paste

object MyLens {

  trait Lens[A, B] {
    self =>

    def get(a: A): B
    def set(a: A, b: B): A
    def modify(f: B => B, a: A): A

    def compose[C](that: Lens[C, A]) = new Lens[C, B] {
      def get(c: C): B = self.get(that.get(c))
      def set(c: C, b: B): C = that.set(c, self.set(that.get(c), b))
      def modify(f: B => B, c: C) = that.set(c, self.set(that.get(c), f(self.get(that.get(c)))))
    }
  }

  def mkLens[A, B](setter: (A, B) => A, getter: A => B) = new Lens[A, B] {
    def get(a: A): B = getter(a)
    def set(a: A, b: B): A = setter(a, b)
    def modify(f: B => B, a: A): A = setter(a, f(getter(a)))
  }
}

// --- Position Lens
val xLens = MyLens.mkLens[Position, Int]((pos, _x) => pos.copy(x = _x), _.x)
val yLens = MyLens.mkLens[Position, Int]((pos, _y) => pos.copy(y = _y), _.y)

// --- Player Lens
val nameLens = MyLens.mkLens[Player, String]((pl, n) => pl.copy(name = n), _.name)
val positionLens = MyLens.mkLens[Player, Position]((pl, pos) => pl.copy(position = pos), _.position)

// --- Compose
val pxLens = xLens compose positionLens
val pyLens = yLens compose positionLens



// --- Example
case class Position(x: Int, y: Int)
case class Player(name: String, position: Position)

val startPosition = Position(0, 1)
val player = Player("Aoino", startPosition)


scala> xLens.get(startPosition)
res0: Int = 0

scala> yLens.set(startPosition, 100)
res1: Position = Position(0,100)

scala> nameLens.modify("Aoiro" + _, player)
res2: Player = Player(AoiroAoino,Position(0,1))

scala> pxLens.get(player)
res3: Int = 0

scala> pyLens.set(player, 100)
res4: Player = Player(Aoino,Position(0,100))

scala> pyLens.modify(_ * 20, player)
res5: Player = Player(Aoino,Position(0,20))