Referring to abstract type outside of trait for class construction

Question

Premise:

I want to separate the information necessary to instantiate a class from the information necessary to "run" the class. However, the information neccesary to "run" the class may differ from class to class. Thus, I imagine the class "has" specific information to run it, and the two go together.

Here is an example of my code:

trait Machine {
  type Params <: BaseParams

  def start(machineParams: Params): Unit
}

trait BaseParams {
  def speed: Int
  def power: Int
}

class FlyingMachine() extends Machine {
  type Params = FlyingParams

  override def start(machineParams: Params): Unit = {
    println(s"I'm flying with $machineParams")
  }
}

trait FlyingParams extends BaseParams {
  def height: Int
}


abstract class MachineOwner{

  val machine: Machine

  def params: machine.Params

  def startMachine(): Unit = {
    machine.start(params)
  }
}

This compiles, passes tests, I'm happy.

Problem: I'm using val machine: Machine in order to define def params: machine.Params. I've been told to make this a def to let the implementer have more freedom. If I do so, I can no longer refer to machine.Params

At this point, I'm at a loss for how to continue. I keep thinking that if this should be a def and definitely not a val, then my architecture is wrong.

So

1. Is my approach to this problem wrong, given the premise I set out with?
2. If it's not wrong, is there a way to still achieve this while using def instead of val in the MachineOwner class?

EDIT Given Alexey Romanov's answer, the last bit of the code would look like this

abstract class MachineOwner{

  type Params1 <: BaseParams

  def machine: Machine { type Params = Params1 }

  def params: Params1

  def startMachine(): Unit = {
    machine.start(params)
  }
}

class FlyingMachineOwner(
  machine: FlyingMachine
) extends MachineOwner {

  override type Params1 = FlyingParams

  override def params = FlyingParams(1,1,1)
}

But this doesn't compile because it expects an override specifically for def machine: Machine { type Params = Params1 }. How does one define that?

Answer 1

It really can't be answered without knowing desired semantics.

If MachineOwner is supposed to own a single machine, then "to make this a def to let the implementer have more freedom" is bad advice: the freedom it gives is exactly to return different machines from different calls to def machine and not to hold references to machines it gives out.

If it is supposed to have multiple machines, should all of them have the same Params type? Then you would do something like

abstract class MachineOwner{

  type Params1 <: BaseParams

  def machine: Machine { type Params = Params1 }

  def params: Params1

  def startMachine(): Unit = {
    machine.start(params)
  }
}

Or if not, then you need a different design again, maybe def params(machine: Machine): machine.Params. Etc. etc.

For the edit: you can do

class FlyingMachineOwner(
  _machine: FlyingMachine
) extends MachineOwner {

  override type Params1 = FlyingParams

  override def params = FlyingParams(1,1,1)

  override def machine = _machine
}

but it really seems unnecessarily complicated compared to what you get with type parameters.