Passing parameters to a trait - scala

Question

I want to model the game of chess.
For that, I want to make an abstract class, Piece which takes a player and a position as arguments. From that, I want to extend to other classes such as Pawn:
trait Piece(player: Int, pos: Pos) = {
def spaces(destination: Pos): List[Pos]
}
case class Pawn extends Piece = {
//some other code
}
However, I think I'm not allowed to pass parameters to a trait, like this trait Piece(player: Int, pos: Pos).
So how can I have an abstract class Piece that has fields?

Answer 1

You could use an abstract class
abstract class Piece(player: Int, pos: Pos) {
...
}
case class Pawn(player: Int, pos: Pos) extends Piece(player, pos)
Or (probably better) you define those members abstractly in a trait
trait Piece {
def player: Int
def pos: Pos
...
}
case class Pawn(player: Int, pos: Pos) extends Piece

Answer 2

Dotty allows traits to have parameters, just like classes have parameters.
trait Greeting(val name: String) {
def msg = s"How are you, $name"
}
class C extends Greeting("Bob") {
println(msg)
}

Answer 3

I wasn't happy with the accepted answer for my use-case so I did the following. Note that you have two possibilities here for this same approach:
trait Piece {
// these can be referred to within this trait to implement reusable code
val player: Int
val pos: Pos
def spaces(destination: Pos): List[Pos] = {
// use player and pos at will e.g.
List(pos, destination)
}
}
case class Pawn(playerArg: Int, posArg: Pos) extends Piece = {
// now override those with whatever you like
override val player: Int = playerArg
override val pos: Pos = posArg
//some other code
}
The second alternative is to use and override methods instead e.g. def getPlayer: Int.
Yet another possibility is to use implicit on the trait methods that would require accessing those attributes but I'm not a big fan of this approach.
That said, apparently they have been thinking about it SIP-25 Trait Parameters.