gistfile1.scala

  def yipWhenL[I, O, O2](ord: (I, O) => Ordering)(f: (I, Option[O]) => O2): Wye[I, O, O2] = {
    val fbL: Tee[I, O, O2] = tee.passL[I] map (f(_, None))
    def go(io: Option[I], oo: Option[O]): Wye[I, O, O2] = {
      io match {
        case None => awaitL[I].flatMap { i => go(Some(i), oo) }
        case Some(i) => oo match {
          case None => awaitR[O]
            .flatMap { o => go(io, Some(o)) }
            .orElse { emit(f(i, None)) fby go(None, None) }
          case Some(o) => ord(i, o) match {
            case Ordering.LT => emit(f(i, None)) fby go(None, oo)
            case Ordering.EQ => emit(f(i, oo)) fby go(None, None)
            case Ordering.GT => go(io, None)
          }
        }
      }
    }
    go(None, None)
  }