Async2.scala

import scala.concurrent.ExecutionContext

case class Async[A, E](cb: (A => E) => E)

object Async {

  def run[A, E](async: Async[A, E])(
      implicit ec: ExecutionContext): (A => E) => E = async match {
    case Async(callback) =>
      f =>
        callback(f)
  }

  // a bit tricky but doable:
  def map[A, E, B](a: Async[A, E])(f: A => B)(
      implicit ec: ExecutionContext): Async[B, E] = {
    Async { callback =>
      run(a)(ec) { input: A =>
        val lifted: A => Async[B, E] = input => pure(f(input))
        val asyncB: Async[B, E] = lifted(input)
        run(asyncB)(ec)(callback)
      }
    }
  }

  def pure[A, E](a: A): Async[A, E] = Async(x => x(a))
}

  
}