List[EitherT[Future, String, CustomObj]] => EitherT[Future, String, List[CustomObj]]

Question

How to use sequence function for List[EitherT[Future, String, CustomObj]] with custom class CustomObj ? I want something like that :

import scala.language.postfixOps
import cats.instances.list._
import cats.syntax.traverse._
import cats.data.EitherT
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

case class CustomObj(int: Int)

private val fst: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(1))
private val snd: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(2))

val source: List[EitherT[Future, String, CustomObj]] = fst :: snd :: Nil

val result: EitherT[Future, String, List[CustomObj]] = source.sequence

import scala.concurrent.duration._
val res = scala.concurrent.Await.result(result.value, 1 second)
println(res) // Right(List(CustomObj(1), CustomObj(2)))

Every time during compile I get

 error: Cannot prove that EitherT[Future,String,CustomObj] <:< G[A].

What does it mean <:< G[A] ?

Answer 1

This is an edited version, code @ https://scastie.scala-lang.org/Yaneeve/Jro89ZHwS3G23Aveanxc5A:

import scala.language.postfixOps
import cats.implicits._
import cats.data.EitherT
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global

case class CustomObj(int: Int)

private val fst: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(1))
private val snd: EitherT[Future, String, CustomObj] =
  EitherT.pure[Future, String](CustomObj(2))

val source: List[EitherT[Future, String, CustomObj]] = fst :: snd :: Nil

val result: EitherT[Future, String, List[CustomObj]] = source.sequence

import scala.concurrent.duration._
val res = scala.concurrent.Await.result(result.value, 1 second)
println(res) // Right(List(CustomObj(1), CustomObj(2)))

Initially I wrote:

Notice please that it didn't compile and produced odd compile messages until I added import scala.concurrent.ExecutionContext.Implicits.global

And that holds true, BUT your trouble did not lie there. When I went to reproduce again, I got the error you had specified. The thing that was missing is an SBT/scalac flag: scalacOptions += "-Ypartial-unification"