Item.scala

package example.j5ik2o.dop.domain

import example.j5ik2o.common.domain.ItemType

import com.github.tarao.record4s.{%, Tag}
import java.net.URL

type Item = %{
  val id: ItemId
  val name: ItemName
  val price: Money
  val itemType: ItemType
} & Tag[Item.Tag]

opaque type ItemId = String

object ItemId {
  def apply(value: String): ItemId          = value
  def unapply(self: ItemId): Option[String] = Some(self)

  given Conversion[String, ItemId] = ItemId(_)

  extension (self: ItemId) {
    def value: String = self
  }
}

object Item {
  def apply(id: ItemId, name: ItemName, price: Money, itemType: ItemType): Item =
    %(id = id, name = name, price = price, itemType = itemType).tag[Item.Tag]

  def id(self: Item): ItemId         = self.id
  def name(self: Item): ItemName     = self.name
  def price(self: Item): Money       = self.price
  def itemType(self: Item): ItemType = self.itemType

  trait Tag
}

type GenericItem = Item & Tag[GenericItem.Tag]

object GenericItem {

  def apply(id: ItemId, name: ItemName, price: Money): GenericItem =
    Item(id, name, price, ItemType.Generic).tag[GenericItem.Tag]

  def unapply(self: GenericItem): Option[(ItemId, ItemName, Money)] =
    Some((self.id, self.name, self.price))

  trait Tag
}

type DownloadableItem = Item & { val url: URL } & Tag[DownloadableItem.Tag]

object DownloadableItem {

  def apply(id: ItemId, name: ItemName, url: URL, price: Money): DownloadableItem =
    (Item(id, name, price, ItemType.Download) + (url = url))
      .tag[DownloadableItem.Tag]
      .asInstanceOf[DownloadableItem] // record4s bug

  def unapply(self: DownloadableItem): Option[(ItemId, ItemName, URL, Money)] =
    Some(
      (
        self.id,
        self.name,
        self.url,
        self.price,
      )
    )

  trait Tag
  object Tag {
    // (demo) Defining method on DownloadableItem
    //
    // ```
    // val item: DownloadableItem = ???
    // item.hoge 
    // ```
    extension(self: DownloadableItem) {
      def hoge: String = "hoge"
    }
  }
}

type CarItem = Item & Tag[CarItem.Tag]

object CarItem {

  def apply(id: ItemId, name: ItemName, price: Money): CarItem =
    Item(id, name, price, ItemType.Car).tag[CarItem.Tag]

  def unapply(self: CarItem): Option[(ItemId, ItemName, Money)] =
    Some((self.id, self.name, self.price))

  trait Tag
}

original:
https://github.com/j5ik2o/oop-dop-other/blob/2ac61d3bf6486ab5a4e7bf42b8d66ce9969e5ac6/src/main/scala/example/j5ik2o/dop/domain/Item.scala