flatMap for Enumerateess

flatMap.scala

object KloutEnumeratee {
    def flatMapInput[From] = new {
        def apply[To](f: Input[From] => PlayPromise[Input[To]]) = 
            new Enumeratee.CheckDone[From, To] { //Checkdone is part of the Play Iteratee library
                def step[A](k: K[To, A]): K[From, Iteratee[To, A]] = {
                    case in @ (Input.El(_) | Input.Empty) =>
                        val promiseOfInput = f(in)
                        Iteratee.flatten(promiseOfInput map { input =>
                            new CheckDone[From, To] {
                                def continue[A](k: K[To, A]) = Cont(step(k))
                            } &> k(input)
                        })
    
                    case Input.EOF => Done(k(Input.EOF), Input.EOF)
                }
    
                def continue[A](k: K[To, A]) = Cont(step(k))
            }
    }

    def flatMap[E] = new {
        def apply[NE](f: E => Promise[NE]): Enumeratee[E, NE] = flatMapInput[E]{
            case Input.El(e) => f(e) map (Input.El(_))
            case Input.Empty => Promise pure Input.Empty
            case Input.EOF   => Promise pure Input.EOF
        }

    }
}