prob.scala

package prob

import scalaz._
import Scalaz._

import org.scalacheck.{ Arbitrary, Gen }

/** Types for working with probabilities, i.e. values of type `Double` in range `[0.0, 1.0]`. */
object `package` {

  /** a value that contains a probability.
    *
    * See http://eed3si9n.com/learning-scalaz/Tagged+type.html for a tutorial.
    */
  sealed trait Probability

  private val Probability = Tag.of[Probability]

  /** tag a `Double` within `[0.0, 1.0]` with `Probability` */
  val P = (x: Double) => {
    require(0.0 <= x && x <= 1.0)
    Probability(x)
  }

  /** a `Double` that contains a probability value */
  type P = Double @@ Probability

  /** convert a `P` back to its underlying `Double` */
  val `P->Double` = Probability.unwrap(_: P)

  /** `Arbitrary[P]` for ScalaCheck */
  implicit val ArbitraryP = Arbitrary(Gen.choose(0.0, 1.0).map(P(_)))
}