Enumeratee is also defined using a trait and its companion object with the same Enumeratee name.

It is defined as follows:

trait Enumeratee[From, To] {
...
def applyOn[A](inner: Iteratee[To, A]): Iteratee[From, Iteratee[To, A]]

def apply[A](inner: Iteratee[To, A]): Iteratee[From, Iteratee[To, A]] = applyOn[A](inner)
...
}

An Enumeratee transforms the Iteratee given to it as input and returns a new Iteratee. Let's look at a method that defines an Enumeratee by implementing the applyOn method. An Enumeratee's flatten method accepts Future[Enumeratee] and returns an another Enumeratee, which is defined as follows:

  def flatten[From, To](futureOfEnumeratee: Future[Enumeratee[From, To]]) = new Enumeratee[From, To] {
    def applyOn[A](it: Iteratee[To, A]): Iteratee[From, Iteratee[To, A]] =
      Iteratee.flatten(futureOfEnumeratee.map(_.applyOn[A](it))(dec))
  }

In the preceding snippet, applyOn is called on the Enumeratee whose future is passed and dec is defaultExecutionContext.

Defining...