我正在尝试封装一些代码，但在Scala中，由于某些原因，您必须直接访问变量。为什么这段代码不能编译？

class MyRender {
  var camera_pitch = 0.2d
}

class MyDisplay {
  val render = new MyRender()

  def camera_pitch():Double = {
    render.camera_pitch
  }
  def camera_pitch_=(pitch: Double) = {
    render.camera_pitch = pitch
  }
}

class MyWorld {
  val display = new MyDisplay()
}

class MyEntity {
  val world = new MyWorld()
  val pitch = 4d

  def update() = {
    world.display.camera_pitch = pitch          //this fails
    world.display.render.camera_pitch = pitch   //this works
  }
}

我收到以下错误：

[error] Test.scala:33:32: reassignment to val
[error]     world.display.camera_pitch = this.pitch