Scala function to parse json with time fields to appropriate case classes. How can I write this better?

Let this be the definition of your Date types:

sealed trait Date {
  def year: Int
}

case class Year(year: Int) extends Date
case class YearMonth(year: Int, month: Int) extends Date
case class YearWeek(year: Int, week: Int) extends Date
case class YearMonthDay(year: Int, month: Int, day: Int) extends Date

In your question there are problems when value is not defined, or when it is empty string, or when value is present, but it is not a number. So you can use common function readInt to read optional integer from JSON. In case if value is not present or if value is empty or non-integer, the result of function will be scala.util.Failure.

  def parseDate(dateJson: JsValue): Try[Date] = {
    def readInt(fieldName: String): Try[Int] =
      (dateJson \ fieldName).asOpt[String].filter(_.nonEmpty).map(x => Try(x.toInt))
        .getOrElse(Failure(new IllegalArgumentException(s"$fieldName is empty")))

    val maybeDay: Try[Int] = readInt("day")
    val maybeWeek: Try[Int] = readInt("week")
    val maybeMonth: Try[Int] = readInt("month")
    val maybeYear: Try[Int] = readInt("year")

    maybeYear.map { year: Int =>
      maybeMonth match {
        case Success(month) => maybeDay match {
          case Success(day) => YearMonthDay(year, month, day)
          case _ => YearMonth(year, month)
        }
        case _ => maybeWeek match {
          case Success(week) => YearWeek(year, week)
          case _ => Year(year)
        }
      }
    }
  }

My tests:

val date1 = JsObject(Map("year" -> JsString("2001"), "month" -> JsString("11"), "day" -> JsString("30")))
println(parseDate(date1)) // => Success(YearMonthDay(2001,11,30))
val date2 = JsObject(Map("year" -> JsString("2001"), "month" -> JsString("11")))
println(parseDate(date2)) // => Success(YearMonth(2001,11))
val date3 = JsObject(Map("year" -> JsString("2001"), "week" -> JsString("51")))
println(parseDate(date3)) // => Success(YearWeek(2001,51))
val date4 = JsObject(Map("year" -> JsString("")))
println(parseDate(date4)) // => Failure(java.lang.IllegalArgumentException: year is empty)

Sorry for my English