when / unless / otherwise implementation in Scala with some Implicit Value Classes and call by name (deferred evaluation only when necessary).

WhenUnlessOtherwise.scala

// --- Shorter with Option[T] --- //

implicit class WhenUnless[T](x: => T) {
  lazy val xx = x
  def when(cond: => Boolean):Option[T] = if(cond) Some(xx) else None
  def unless(cond: => Boolean):Option[T] = if(cond) None else Some(xx)
}

implicit class Otherwise[+T](val x: Option[T]) extends AnyVal {
  def otherwise[S >: T](z: => S):S = if(x.isDefined) x.get else z //could not use getOrElse
}

trait Mammal
case object Cat extends Mammal
case object Dog extends Mammal

val uri = "/news/index.html"

val file = "index.html" when uri == "/" otherwise uri

val result = {println("eval when");Cat} when {println("eval cond");false} otherwise {println("eval otherwise");Dog}

/* --- output
eval cond
eval otherwise
result: Product with Serializable with Mammal = Dog

file: String = /news/index.html
*/