lu4nm3

sealed trait Hygiene extends Item
case class Shampoo(price: Double) extends Hygiene
case class ToothPaste(price: Double) extends Hygiene

lu4nm3

trait Price[A] {
  def price(a: A): Double
}

lu4nm3

object Cake {
  implicit val price: Price[Cake] = new Price[Cake] {
    def price(cake: Cake): Double = cake.price
  }
}

object Apple {
  val PricePerLb: Double = 3.0

  implicit val price: Price[Apple] = new Price[Apple] {

lu4nm3

def orderTotal(shoppingCart: List[Item]): Double = {
  shoppingCart.foldLeft(0.0) { case (total, item) =>
    total + implicitly[Price[Item]].price(item)
  }
}

lu4nm3

[error] ...could not find implicit value for parameter e: Price[Item]
[error] total + implicitly[Price[Item]].price(item)
[error]                   ^

lu4nm3

object Item {
  implicit val price: Price[Item] = new Price[Item] {
    def price(item: Item): Double = item match {
      case cake: Cake => Cake.price.price(cake)
      case baguette: Baguette => Baguette.price.price(baguette)
      case apple: Apple => Apple.price.price(apple)
      case orange: Orange => Orange.price.price(orange)
      case shampoo: Shampoo => Shampoo.price.price(shampoo)
      case toothPaste: ToothPaste => ToothPaste.price.price(toothPaste)
    }

lu4nm3

import shapeless._

object PriceCompanion extends LabelledTypeClassCompanion[Price] {

  object typeClass extends LabelledTypeClass[Price] {
    def emptyProduct: Price[HNil] = new Price[HNil] {
      def price(t: HNil) = 0.0
    }

    def product[F, T <: HList](name: String, sh: Price[F], st: Price[T]): Price[F :: T] = new Price[F :: T] {

lu4nm3

def orderTotal(shoppingCart: List[Item]): Double = {
  import PriceCompanion._
  
  shoppingCart.foldLeft(0.0) { case (total, item) =>
    total + implicitly[Price[Item]].price(item)
  }
}

lu4nm3

val shoppingCart: List[Item] = List(
  Orange(0.25),
  Orange(0.33),
  Apple(0.22),
  Apple(0.27),
  Apple(0.30),
  Baguette(2),
  ToothPaste(4)
)

lu4nm3

val shoppingCart: List[Fruit] = List(
  Orange(0.25),
  Orange(0.33),
  Apple(0.22),
  Apple(0.27),
  Apple(0.30)
)

orderTotal(shoppingCart) // 4.69