missing Cats Functor[Future] instance - scala

Question

I am trying to use OptionT to combine methods returning Future[Option[T]] in a for-comprehension.
import cats.data._
import cats.implicits._
import cats.instances.future._
for {
data <- OptionT(repo.getData(id))
...
}
The compiler error I am getting:
could not find implicit value for parameter F cats.Functor[scala.concurrent.Future]
This recent example suggests that this is (was?) possible.
so do the docs in the pull request for adding OptionT
and the cats Functor docs
What am I missing here?
Thank you

Answer 1

By importing cats.implicits._ you are actually already importing cats.syntax.AllSyntax and cats.instances.AllInstances
Try using just those imports:
import cats.data._
import cats.implicits._
or (according to your needs):
import cats.data._
import cats.instances.future._
or more specifically:
import cats.data._
import cats.instances.future.catsStdInstancesForFuture
you may also need:
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
Note: of course you have to implicitly provide an actual ExecutionContext in a production environment.

Answer 2

Following imports work for me (also mentioned in approved answer),
import cats.data.OptionT
import cats.instances.future._ // or import cats.implicits._
// as implicits include FutureInstances
import scala.concurrent.Future
import scala.concurrent.ExecutionContext.Implicits.global
Also, the important thing was the dependencies as I was using org.typelevel:cats:0.9.0 along with cats-core-1.1.0 which was causing Symbol 'type cats.kernel.instances.EqInstances' is missing from the classpath.
Had to remove older cats-0.9.0 and use latest cats-core and cats-kernel.
libraryDependencies ++= Seq(
"org.typelevel" %% "cats-core" % "1.1.0",
"org.typelevel" %% "cats-kernel" % "1.2.0",
"org.scalatest" %% "scalatest" % "3.0.4" % Test
)

Answer 3

Seems like causes of this issue can be various; I just ran into this because I had two implicit ExecutionContexts available in scope, so cats was unable to select one to provide the Functor[Future]:
import scala.concurrent.ExecutionContext.Implicits.global
// ^ first implicit ExecutionContext
class MyClass {
def myMethod(e: EitherT[Future, _, _])(implicit ec: ExecutionContext) {
// Second implicit ExecutionContext
e.flatMap(...) // <- Functor unavailable
}
}
In my case I was able to resolve the issue by simply not passing ec into myMethod, though removing the global execution context would have also worked.

Answer 4

Had the same problem. cats actually has the instance.
The real issue is just the implicit ExecutionContext missing which you could just do the following
import scala.concurrent.ExecutionContext.Implicits.global
I realised this when I was trying to provide an instance for it.
implicit val ev: Functor[Future] = new Functor[Future] {
override def map[A, B](fa: Future[A])(f: A => B): Future[B] = fa.map(f)
}
It's at this point the compiler will say the missing the implicit ExecutionContext