Different monads in for comprehension

Question

I have below code


def processInfoAndReturnResponse(input: Input]): EitherT[Future, CustomException, A] = ???

def sendMessage(message: A): monix.eval.Task[Boolean] = ???

def anotherMethod(message: Input): Future[Either[CustomException, Unit]]= ???
def integrate(): Future[Either[GoogleException, A]] = {
(for {
  response <- processInfoAndModelResponse(input)
  _ <- EitherT.liftF[Future, CustomException, A](sendMessage(response).map(_ => response).runToFuture
}yield response).value

so far all this is good. But now, I want to get boolean from sendMessage and then only if sendMessage returns true, I want to call anotherMethod.

I understand they are different monads. Please let me know a cleaner way how I can add all three calls in for comprehension. Appreciate help

Probably something like: `for { response <- processInfoAndModelResponse(inp); int <- EitherT(sendMessage(response).runAsyncAndForget); foo < -bar(int) } yield ???` — Luis Miguel Mejía Suárez, Sep 25 '20 at 18:06
Your question is very messy and hard to read. Can you clean it up (fix formatting, make sure it compiles etc.)? If you want to provide just the signature of the method, but not the whole implementation just use ???. — Krzysztof Atłasik, Sep 25 '20 at 20:03

Krzysztof Atłasik · Accepted Answer · 2020-09-26T21:16:10.530

Unfortunately, EitherT and Task are different monads and monads doesn't compose so you can't use them directly in the same for comprehension.

What you could do is lifting Task into EitherT but in this case type parameter F of EitherT would have to be Task and in your case it's Future.

So you have to do 2 things:

Transform Task into Future
Lift Future to EitherT

Let's say your another method looks like this:

def anotherMethod(input: Integer): EitherT[Future, Exception, Unit] = EitherT.rightT[Future, Exception](())

So your for-comprehension could look like this:

import cats.implicits._
import scala.concurrent.ExecutionContext.Implicits._

val prog = for {
    //you need to use leftWiden adjust left of either to common type
    response <- processInfoAndReturnResponse(inp).leftWiden[Exception]
    //running task to Future and then lifting to EitherT
    i <- EitherT.liftF[Future, Exception, Integer](sendMessage(response).runToFuture)
    _ <- anotherMethod(i)
} yield ()

//prog is of type EitherT so we have to unwrap it to regular Future with rethrowT
val future: Future[Unit] = prog.rethrowT

To answer your question after edit, you can use whenA to conditionaly use effect in for-comprehension:

def integrate(): Future[Either[GoogleException, A]] ={
  (for {
    response <- processInfoAndModelResponse(input)
    sendStatus <- EitherT.liftF[Future, CustomException, Boolean](sendMessage(response).runToFuture)
    finalresult <- anotherMethod(input).whenA(sendStatus)
  } yield finalresult).value
}

thanks edited above question.Please advice solution if I want to call anotherMethod only when sendMessage returns true — ApprenticeWST, Sep 26 '20 at 09:31
I edited my anwser, so it attempts to solve your problem. If you feel it was helpful, please accept. — Krzysztof Atłasik, Sep 26 '20 at 21:17

Different monads in for comprehension

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