How to avoid calling asInstanceOf in Scala with family polymorphism

user_1437346 0
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By design, we know for sure that we have an instance of HourlyDateFormat

How to avoid calling asInstanceOf in this case (i.e how to help the compiler to infer the type) ?

  sealed trait StorageLayout extends Product with Serializable
  case object Hourly         extends StorageLayout
  case object Daily          extends StorageLayout

  sealed trait DateFormat[S <: StorageLayout]

  sealed abstract class HourlyDateFormat extends DateFormat[Hourly.type] {
    def format(localDate: LocalDate): String         = ???
    def format(localDateTime: LocalDateTime): String = ???
  }

  sealed abstract class DailyDateFormat extends DateFormat[Daily.type] {
    def format(localDate: LocalDate): String = ???
  }

  class Log[S <: StorageLayout](storageLayout: S, dateFormat: DateFormat[S]) {
    def getPath(date: LocalDate): String =
      dateFormat match {
        case hdf: HourlyDateFormat => hdf.format(date)
        case ddf: DailyDateFormat  => ddf.format(date)
      }
    @SuppressWarnings(Array("org.wartremover.warts.AsInstanceOf"))
    def getPath(date: LocalDateTime)(implicit ev: S =:= Hourly.type): String = {
      assert(ev == ev)
      dateFormat.asInstanceOf[HourlyDateFormat].format(date)
    }
  }
casting scala
3 Answers

Try to add one more implicit parameter

def getPath(date: LocalDateTime)(implicit ev: S =:= Hourly.type, ev1: DateFormat[S] =:= HourlyDateFormat): String = {
  //assert(ev == ev)
  dateFormat.format(date)
}

Assertion looks strange: assert(ev == ev).

Or just

def getPath(date: LocalDateTime)(implicit ev1: DateFormat[S] =:= HourlyDateFormat): String

Fixed version (I added one more type parameter, it's now similar to the first @user's version)

class Log[S <: StorageLayout, D <: DateFormat[S]](storageLayout: S, dateFormat: D) {
  def getPath(date: LocalDate): String =
    dateFormat match {
      case hdf: HourlyDateFormat => hdf.format(date)
      case ddf: DailyDateFormat  => ddf.format(date)
    }
  def getPath(date: LocalDateTime)(implicit
                                   ev: S =:= Hourly.type,
                                   ev1: D <:< HourlyDateFormat,
  ): String = {
    dateFormat.format(date)
  }
}

val log = new Log(Hourly, new HourlyDateFormat(){})
print(log.getPath(LocalDateTime.now()))
user_52496210

Generally things like that are kind-of type-classy, so I would do it this way instead:

trait DailyFormatter[S] {
  def formatDate(localDate: LocalDate): String
}
trait HourlyFormatter[S] {
  def formatDateTime(localDateTime: LocalDateTime): String
}

implicit val dailyFormats: DailyFormatter[Daily]
implicit val hourFormats: DailyFormatter[Hourly] with HourlyFormatter[Hourly]

class Log[S <: StorageLayout](storageLayout: S, dateFormat: DateFormat[S]) {

  def getPath(date: LocalDate)(implicit formater: DailyFormatter[S]): String =
    formater.formatDate(date)

  def getPath(date: LocalDateTime)(implicit formater: HourlyFormatter[S]): String =
    formater.formatDateTime(date)
}

It has the advantage that you don't have to be aware of existence of types HourlyDateFormat and DailyDateFormat to make it work.

user_13051210

A quick fix would be something like this:

class Log[S <: StorageLayout, D <: DateFormat[S]](storageLayout: S, dateFormat: D) {
  def getPath(date: LocalDateTime)(implicit ev: D <:< HourlyDateFormat): String =
    dateFormat.format(date)
}

However, I don't think you've designed this the right way. It'd probably be better to have a separate trait for each type of format. This makes it more scalable, since you don't need to add a case in your match expression for each different class, the right method gets selected automatically at runtime. You still have to use those evidence parameters, which I don't like, but it still feels cleaner to me.

Edit: I've updated the code so that everything extends FormatLocalDate and you only need an evidence parameter for getPath(LocalDateTime)

sealed trait FormatLocalDate[S <: StorageLayout] {
  def format(localDate: LocalDate): String
}
sealed trait FormatLocalDateTime[S <: StorageLayout] extends FormatLocalDate[S] {
  def format(localDate: LocalDateTime): String
}

sealed abstract class HourlyDateFormat extends FormatLocalDateTime[Hourly.type] {
  def format(localDate: LocalDate): String = ???
  def format(localDateTime: LocalDateTime): String = ???
}
sealed abstract class DailyDateFormat extends FormatLocalDate[Daily.type] {
  def format(localDate: LocalDate): String = ???
}

class Log[S <: StorageLayout, D <: FormatLocalDate[S]](storageLayout: S, dateFormat: D) {
  def getPath(date: LocalDate): String =
    dateFormat.format(date)

  def getPath(date: LocalDateTime)(implicit ev: D <:< FormatLocalDateTime[S]): String =
    dateFormat.format(date)
}

Scastie

user_118820020
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