Extend case class with typeclass

Question

I have this code

  case class Salary(employee: String, amount: Double){

  }

  trait XN[M] {
    def x2(m: M): M
    def x3(m: M): M
    def x4(m: M): M
  }

    

I want to extend Salary with XN trait in order for the following test to work:

test("Salary is extended with xN") {

    val bobSalary = Salary("Bob", 100.0)

    bobSalary.x2 shouldBe Salary("Bob", 200.0)
    bobSalary.x3 shouldBe Salary("Bob", 300.0)
    bobSalary.x4 shouldBe Salary("Bob", 400.0)

  }

My attempts:

#1

  implicit val SalaryXN: XN[Salary] = new XN[Salary] {
    override def x2(m: Salary): Salary = m.copy(amount = m.amount * 2)

    override def x3(m: Salary): Salary = m.copy(amount = m.amount * 3)

    override def x4(m: Salary): Salary = m.copy(amount = m.amount * 4)
  }

#2

  object Salary extends XN[Salary] {
    override def x2(m: Salary): Salary = new Salary(employee = m.employee, amount = m.amount * 2)

    override def x3(m: Salary): Salary = new Salary(employee = m.employee, amount = m.amount * 3)

    override def x4(m: Salary): Salary = new Salary(employee = m.employee, amount = m.amount * 4)
  }

How to do that?

Online code

Answer 1

Since it seems that XN is a typeclass, it would be better to properly use that pattern instead of relying on a (discouraged) implicit conversion.

trait XN[M] {
  def x2(m: M): M
  def x3(m: M): M
  def x4(m: M): M
}

object XN {
  object syntax {
    implicit class XNOp[M](private val m: M) extends AnyVal {
      @inline final def x2(implicit ev: XN[M]): M = ev.x2(m)
      @inline final def x3(implicit ev: XN[M]): M = ev.x3(m)
      @inline final def x4(implicit ev: XN[M]): M = ev.x4(m)
    }
  }
}

final case class Salary(employee: String, amount: Double)
object Salary {
  implicit final val SalaryXN: XN[Salary] =
    new XN[Salary] {
      override def x2(s: Salary): Salary = s.copy(amount = s.amount * 2)
      override def x3(s: Salary): Salary = s.copy(amount = s.amount * 3)
      override def x4(s: Salary): Salary = s.copy(amount = s.amount * 4)
    }
}

Which can be used like this:

import XN.syntax._

val bobSalary = Salary("Bob", 100.0)

bobSalary.x2
// res: Salary = Salary("Bob", 200.0)

You can see the code running here.