EitherT composition

val f1: Future[Either[Error, Int]] = ???
val f2: Future[Either[Error, Int]] = ???
val f3(x: Int, y: Int): Future[String] = ???

val eitherT = for {
  x <- EitherT(f1)
  y <- EitherT(f2)
  z <- EitherT(f2(x, y).map(_.asRight[Error]))
} yield z
eitherT.value       // Future[Either[Error, String]]