Some snippet for an explanation

Snippet.scala

package futures

import scala.concurrent.Future

object Snippet {

  type A = String
  type B = Int

  def first(x: String): Future[Option[A]] = ???

  def second(a: A): Future[List[B]] = ???

  def fallback: Future[List[B]] = ???

  val justFlatMap: Future[List[B]] = first("arg").flatMap(_.fold(fallback)(second))

  val comprehension: Future[List[B]] = for {
    maybeA <- first("arg")
    results <- maybeA.fold(fallback)(second)
  } yield results
}