lu4nm3

trait Set[A] {
  def get(index: Long): A
  def add(value: A): Set[A]
}

lu4nm3

trait Perishable {
  def expired: Boolean
}

case class Baguette() extends Perishable {
  def expired: Boolean = ???
}

case class Croissant() extends Perishable {
  def expired: Boolean = ???

lu4nm3

trait DailyOrderService {
  def needsReplacing(perishables: List[Perishable]): List[Perishable] = {
    perishables.filter(_.expired)
  }
}

lu4nm3

trait PerishableLike[A] {
  def expired: Boolean
}

case class PerishableLikeBaguette(b: Baguette) extends PerishableLike[Baguette] {
  def expired: Boolean = ???
}

case class PerishableLikeCroissant(c: Croissant) extends PerishableLike[Croissant] {
  def expired: Boolean = ???

lu4nm3

def needsReplacing[T](perishables: List[PerishableLike[T]]): List[PerishableLike[T]] = {
  perishables.filter(_.expired)
}

lu4nm3

trait Perishable[T] {
  def expired(item: T): Boolean
}

lu4nm3

case class Baguette()
case class Croissant()

lu4nm3

object Perishables {
  implicit val baguettePerishable: Perishable[Baguette] = new Perishable[Baguette] {
    def expired(b: Baguette): Boolean = false // hard coded, in reality we would examine `b` to figure out if it expired
  }

  implicit val croissantPerishable: Perishable[Croissant] = new Perishable[Croissant] {
    def expired(c: Croissant): Boolean = false
  }
}

lu4nm3

scala> import Perishables._
import Perishables._

scala> DailyOrderService.needsReplacing(List(Baguette(), Baguette(), Baguette()))
res0: List[Baguette] = List()

lu4nm3

object OutsourcedPerishables {
  implicit val iceCreamPerishable: Perishable[IceCream] = new Perishable[IceCream] {
    def expired(i: IceCream): Boolean = true // in reality you would actually calculate the expiration for ice cream
  }
}