Accessing nested type parameters in Dotty Scala

Question

I would like to do something like this with the new Scala Dotty compiler:

trait Monad[M[A]] (underlyingValue:A) {
  def bind[A, B](f: A => M[B]): M[B]
}

or at least

class Monad[M[A]] (underlyingValue:A) {
  def bind[A, B](f: A => M[B]): M[B]
}

but the compiler complains that Not found: type A

is there someway to access the type parameter of a type parameter? Or something with the same end result but done differently?

I know that you can create a Monad like here: https://dotty.epfl.ch/docs/reference/contextual/type-classes.html but having a Monad class would allow me to declare a class a Monad in the same place it is defined, and would also make more sense to my stylistically.

Is there some way to do this?

Answer 1

For what it's worth, The following solution fulfills the criteria I had in mind:

trait Monad[F[_], A](underlyingValue: A) {

  def flatMap[B](f: A => F[B]):F[B]

}

And usage would look like:

class Opt[A](underlyingValue: A) extends Monad[Opt, A](underlyingValue: A) {
   def flatMap[B](f: A => Opt[B]):Opt[B] = {
     ...
   }
} 

While it does require two type parameters, the type parameter "A" is not repeated twice, and as such there's no logic duplication.