Define newtype with smart constructor in scala

newtype.scala

import scalaz.{@@, Show, Tag, \/}
import scalaz.Scalaz.ToEitherOps

/**
  * Double représentant un pourcentage.
  */
package object percentagedouble {

	private[percentagedouble] sealed trait PositiveOrNegativePercentageDoubleTag
	type PositiveOrNegativePercentageDouble = Double @@ PositiveOrNegativePercentageDoubleTag
	
	sealed trait PositiveOrNegativePercentageDoubleError
	case class InvalidRawValue(error: String) extends PositiveOrNegativePercentageDoubleError
	case class OperationError(error: String) extends PositiveOrNegativePercentageDoubleError

	/**
	  * Représente un pourcentage qui peut être positive ou négatif
	  */
	object PositiveOrNegativePercentageDouble {
		def apply(rawValue: Double): InvalidRawValue \/ PositiveOrNegativePercentageDouble = {
			if(rawValue >= -1 && rawValue <= 1) buildType(rawValue).right
			else InvalidRawValue("must be a -1 <= double <= 1").left
		}

		/**
		  * a.k.a unwrap function
		  *
		  * @param i
		  * @return the unwrapped string value
		  */
		def rawValue(i: PositiveOrNegativePercentageDouble): Double = Tag.unwrap(i)

		/**
		  * Allow to use scalaz Show typeclass
		  */
		implicit val showPositiveOrNegativePercentageDouble: Show[PositiveOrNegativePercentageDouble] = Show shows { i => s"PositiveOrNegativePercentageDouble(${Tag.unwrap(i)})" }

		private def buildType(i: Double) = Tag.apply[Double, PositiveOrNegativePercentageDoubleTag](i)

		val MINUS_ONE = buildType(-1)
		val ZERO = buildType(0)
		val ONE = buildType(1)
	}
}