Rounding through type ascription

Question

I would like to introduce a custom class, say Decimal2, so that I would be able to round through type ascription:

val x: Decimal2 = 1.2345
// 1.24

So far I've tried this:

class Decimal2(val value: Double)

object Decimal2 {

  def apply(value: Double) =
    BigDecimal(value).
    setScale(2, BigDecimal.RoundingMode.HALF_UP).
    toDouble

  implicit def doubleToDecimal2(x: Double) = Decimal2(x)

}

val a: Decimal2 = 1.2345

But:

1. the last line apparently fails.
2. return type of apply method is Double, not Decimal2

How can I reach my goal?

Answer 1

One way is by explicitly creating a new instance of Decimal2 from apply method.

object Decimal2 {
  def apply(value: Double): Decimal2 =
    new Decimal2(BigDecimal(value)
      .setScale(2, BigDecimal.RoundingMode.HALF_UP)
      .toDouble)

  implicit def doubleToDecimal2(x: Double): Decimal2 = Decimal2(x)
}

Another option is to tag the type:

trait RoundTo2
type Decimal2 = Double with RoundTo2

object Decimal2 {
  def apply(value: Double): Decimal2 = {
    BigDecimal(value)
      .setScale(2, BigDecimal.RoundingMode.HALF_UP)
      .toDouble
  }.asInstanceOf[Double with RoundTo2]
}
implicit def doubleToDecimal2(x: Double): Decimal2 = Decimal2(x)

val a: Decimal2 = 1.2345