AsyncFn.scala

/**
 * Something similar to Kleisli[Future, A, B], but doesn't require an
 * ExecutionContext up front.
 */
case class AsyncFn[A, B](fn: ExecutionContext => A => Future[B]) {
  def map[B0](f: B => B0): AsyncFn[A, B0] =
    AsyncFn[A, B0] { implicit ec => a =>
      fn(ec)(a).map(f)
    }

  def flatMap[B0](f: B => AsyncFn[A, B0]): AsyncFn[A, B0] =
    AsyncFn[A, B0] { implicit ec => a =>
      fn(ec)(a).flatMap { b =>
        f(b).fn(ec)(a)
      }
    }

  def contramap[A0](f: A0 => A): AsyncFn[A0, B] =
    AsyncFn[A0, B] { implicit ec => a0 => fn(ec)(f(a0)) }

  def apply(a: A)(implicit ec: ExecutionContext): Future[B] =
    fn(ec)(a)
}

object AsyncFn {
  def lift[A, B](f: A => Future[B]): AsyncFn[A, B] = AsyncFn(_ => f)
}