Lift Future[A] to F[A]

FutureLift.scala

/*
  adapted from:
  https://github.com/monix/monix/blob/4f1862c4bcad7d111aa8cf64510c2fc5dcee6b77/monix-catnap/shared/src/main/scala/monix/catnap/FutureLift.scala
  Monix dependency removed.
*/
package async {
  import cats.effect.Async
  import scala.concurrent.Future
  import scala.language.higherKinds

  trait FutureLift[F[_]] {
    def futureLift[MF[T] <: Future[T], A](fa: F[MF[A]]): F[A]
  }

  object FutureLift {
    def apply[F[_]](implicit F: FutureLift[F]): FutureLift[F] = F

    def toAsync[F[_], MF[T] <: Future[T], A](fa: F[MF[A]])
                                            (implicit F: Async[F]): F[A] = {
      F.flatMap(fa) { future =>
        future.value match {
          case Some(value) => F.fromTry(value)
          case _ => startAsync(future)
        }
      }
    }

    private def start[A](fa: Future[A], cb: Either[Throwable, A] => Unit): Unit = {
      import concurrent.ExecutionContext.Implicits.global // TODO
      fa.onComplete { r =>
        cb(r match {
          case scala.util.Success(a) => Right(a)
          case scala.util.Failure(t) => Left(t)
        })
      }
    }

    private def startAsync[F[_], A](fa: Future[A])(implicit F: Async[F]): F[A] =
      F.async { cb => start(fa, cb) }

  }
}