gistfile1.scala

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

I think the ord function is fine. You definitely want to allow the two types to be different, since you might be using this to join like a (A,B,C) with a B, using only the B for comparison.

Thanks, for showing me how to get to the fallback state. Also what is the meaning of the fby name?

OK, I'll work with those names and we can discuss it once again when I send the PR.

fby is short for 'followedBy'. It's a semi-standard name in stream processing / dataflow programming. :)