Invoke some block roughly some proportion of the time

roughly.scala

def sumProbabilities[U] (options: Seq[(Int, () => U)]): Int = {
    options.map {case (p, f) => p}.sum
  }
  def roughly [U] (chance: Int) (f: => U) = chance -> (() => f)

  def chancesOf [U] (options: (Int, () => U)*): () => Option[U] = {
    type Chance = PartialFunction[Int,Option[U]]
    val fallback: Chance = {
      case x => None
    }
    val sumProbs = sumProbabilities(options)
    require (sumProbs <= 100, s"Sum of probabilities must not be more than 100% ($sumProbs)")
    var max = 0
    val chances: (Int) => Option[U] = options.map {
      case (p, f) =>
        println(s"$p => $f")
        val min = max
        max += p
        (min to max) -> f
    }.map {
      case (range, func) =>
        println (s"$range -> $func")
        val pf: Chance = {
          case x if range contains x => Some(func())
        }
        pf
    }.foldRight (fallback) (_ orElse _)

    val random = new Random()
    def result () = chances(random.nextInt(100))
    result
  }