am3.scala

def messageHandler(activations: List[(Variable, Variable)]): Receive = {

  case Validate(x, y) =>
    wire.next ! Validate(module(x), y)

  case Forward(x, y) =>
    val result = module(x)
    wire.next ! Forward(result, y)
    context become messageHandler(activations :+ (x, result))

  case Backward(g) =>
    activations match {
      case (input, output) :: tail =>
        optimizer.zeroGrad()
        output.backward(g)
        wire.prev ! Backward(input.grad)
        optimizer.step()
        context become messageHandler(tail)
    }
}