OneWayImplicits.scala

object OneWayImplicits extends App {

  //outside implicits visible, but T is not visible inside
  case class Outwards[T](value: T) extends AnyVal

  //outside implicits invisible, T is visible inside
  case class Inwards[T](value: T) extends AnyVal

  implicit def outwardsFromT[T](implicit t: T): Outwards[T] = Outwards(t)
  implicit def tFromInwards[T](implicit inw: Inwards[T]): T = inw.value

  type X = String

  def printX(implicit x: X) = println(x)

  def usesOutwards()(implicit ow: Outwards[X]) = {
    //X is not implicitly visible here
    val x: X = ow.value
    println("usesOutwards")
    printX(x)
  }

  def testOutwards() = {
    implicit val x: X = "iks out"
    usesOutwards()
  }

  testOutwards()

  def usesInwards()(implicit iw: Inwards[X]) = {
    println("usesInwards")
    //doesn't get X from implicit scope
    //but X is implicitly visible here
    printX
  }

  def testInwards() = {
    implicit val iw: Inwards[X] = Inwards("iks in")
    usesInwards()
  }

  testInwards()

  def composability()(implicit x: Inwards[X]) = {
    println("composability")
    usesOutwards() //gets `Outwards(x.value)`
  }

  def testComposability() = {
    implicit val iw: Inwards[X] = Inwards("iks compose")
    composability()
  }

  testComposability()

  // prints:
  // ==============
  
  // usesOutwards
  // iks out
  // usesInwards
  // iks in
  // composability
  // usesOutwards
  // iks compose
}