cats.data.EitherT with traverse - scala

Question

I have a question about using Traverse together with EitherT. Let's say we have a code like this:
def validate(s: String): EitherT[Future, NumberFormatException, Int] = {
EitherT(Future.successful(try { Right(s.toInt) } catch { case e:
NumberFormatException => Left(e)}))
}
List("1", "2").traverse(validate)
Unfortunately this code does not compile, because we are missing something:
error: could not find implicit value for evidence parameter of type cats.Applicative[G] List("1", "2").traverse(validate)
I tried to look this up and found for example this answer: Switching between EitherT and Validation to accumulate error or traverse or Validation versus disjunction
So it seems some solution could exists. But the problem is that both of them are using traverseU, which is no longer an option in scala 2.12. So how can this be done please?
EDIT
This is the code including imports:
import cats.data.EitherT
import cats.syntax.traverse._
import cats.instances.list._
import cats.instances.future._
import scala.concurrent.ExecutionContext.global
import scala.concurrent.Future
def validate(s: String): EitherT[Future, NumberFormatException, Int] = {
EitherT(Future.successful(try { Right(s.toInt) } catch { case e:
NumberFormatException => Left(e)}))
}
List("1", "2").traverse(validate)

Answer 1

Usually cats will pack specific evidence implicits under different imports. In this case, you need the proofs for List and Future.
import cats.data.EitherT
import cats.syntax.traverse._
import cats.instances.list._
import cats.instances.future._
import scala.concurrent.Future
def validate(s: String): EitherT[Future, NumberFormatException, Int] = {
EitherT(Future.successful(try { Right(s.toInt) } catch { case e: NumberFormatException => Left(e) }))
}
List("1", "2").traverse(validate)
This code compiles with cats 1.2.0 for me.

Answer 2

import cats.{Applicative, Monad}
import cats.data.EitherT
import cats.syntax.traverse._
import cats.instances.list._
import cats.instances.future._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future
def validate(s: String): EitherT[Future, NumberFormatException, Int] = {
EitherT(Future.successful(try { Right(s.toInt) } catch { case e: NumberFormatException => Left(e) }))
}
type Tmp[T] = EitherT[Future, NumberFormatException,T]
List("1", "2").traverse[Tmp, Int](validate)
So this is how it works to me. I had to create new type.