如何实现Future [Either [L，R]]的flatMap方法

Question

Our app uses a lot of Future[Either[Error, R]] to return either a result or an error object. I would like to create a new class that merges the two so it can be use in for-comprehensions, ie if a function is successful (returns a Right), the next function is called, else return Error.

I've done the map method, but has trouble with flatMap .

class FutureEither[L, R](val future: Future[Either[L, R]]) {
  def map[S](f: R => S)(implicit executor: ExecutionContext): FutureEither[L, S] = {
    val result = future.map {
      case Right(r)    => Right(f(r))
      case Left(error) => Left(error)
    }
    new FutureEither(result)
  }

  def flatMap[S](f: R => FutureEither[L, S])(implicit executor: ExecutionContext): FutureEither[L, S] = ???
}

Would be great if someone can help me implement it, or general improvement suggestions thanks.

Answer 1

Pretty much the same, just do future.flatMap instead of future.map . And wrap the left case in a future:

def flatMap[S](f: R => FutureEither[L, S])(implicit executor: ExecutionContext): FutureEither[L, S] = {
  new FutureEither(future.flatMap{
    case Right(r) => f(r).future
    case Left(error) => Future(Left(error))
  })
}