Java's CompletionStage to Cats Effect F[_]: Async / IO

completion-stage-to-io.scala

import java.util.concurrent.CompletionStage

import cats.effect.Async
import cats.syntax.flatMap._

case object EmptyValue extends Throwable

def to[F[_], A](fa: F[CompletionStage[A]])(implicit F: Async[F]): F[A] = {
  fa.flatMap { f =>
    F.async[A] { cb =>
      f.handle[Unit] { (value: A, t: Throwable) =>
        if (t != null) cb(Left(t))
        else if (value != null) cb(Right(value))
        else cb(Left(EmptyValue))
      }
    }
  }
}