Convert EitherT[Future, A, Future[B]] to EitherT[Future, A, B]

Question

I am trying to change an EitherT[Future, A, B] to EitherT[Future, C, D] and for this, I am using bimap to map left and right parts appropriately. While I am transforming the right part of this EitherT, I am making a service call which returns me a Future[D]… I am having trouble converting this Future[D] to D in my bimap. Not sure how to proceed now. Any help here would be appreciated.

Psuedo code:

val myResult: EitherT[Future, C, D] = EitherT[Future, A, B](myService.doStuff())
    .bimap({ err => /*deal with errors and give me C*/ }
      ,{ success => someService.doSomething(success) // This is returing a Future[D]. But I want a D 
       })

Answer 1

Try .flatMap aka for-comprehension

import cats.data.EitherT
import cats.instances.future._
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

val myResult: EitherT[Future, C, D] = for {
  d <- EitherT.right(someService.doSomething())
  res <- EitherT[Future, A, B](myService.doStuff())
    .bimap({ err => ??? : C //deal with errors and give me C
    }, { success => {
      d
    }
    })
} yield res

---

Try .biSemiflatMap

val myResult: EitherT[Future, C, D] =
  EitherT[Future, A, B](myService.doStuff())
    .biSemiflatMap({ err => Future.successful(??? : C)
    }, { success => {
      someService.doSomething(success)
    }
    })