Additional example for FreeT in docs

[mtsokol]

Hi,
I have recently used FreeT and found that docs provide only pretty specific example with State monad.

Would you mind accepting docs PR with additional basic Future Option example?

After consulting on gitter I ended up with additional transformation for Option ~> OptionT, which I am not sure.

import cats.free._
import cats._
import cats.data._
import cats.implicits._
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

sealed trait Ctx[A]

case class Action(value: Int) extends Ctx[Int]

def op1: FreeT[Ctx, Option, Int] =
  FreeT.liftF[Ctx, Option, Int](Action(7))

def op2: FreeT[Ctx, Option, Int] =
  FreeT.liftT[Ctx, Option, Int](Some(4))

def op3: FreeT[Ctx, Option, Int] =
  FreeT.pure[Ctx, Option, Int](1)

val opComplete: FreeT[Ctx, Option, Int] =
  for {
    a <- op1
    b <- op2
    c <- op3
  } yield a + b + c


// ----interpreters----

type OptFut[A] = OptionT[Future, A]

def futureInterpreter: Ctx ~> OptFut = new (Ctx ~> OptFut) {
  def apply[A](fa: Ctx[A]): OptFut[A] = {
    fa match {
      case Action(value) => OptionT.liftF(Future(value))
    }
  }
}

def optFutLift: Option ~> OptFut = new (Option ~> OptFut) {
  def apply[A](fa: Option[A]): OptFut[A] = {
    fa match {
      case Some(value) =>
        OptionT(Future(Option(value)))
      case None =>
        OptionT.none
    }
  }
}

val hoisted = opComplete.hoist(optFutLift)
val evaluated = hoisted.foldMap(futureInterpreter)

val future = evaluated.value
// Some(12)

[kailuowang]

👍 for Option. Mix feelings for Future. Future is widely used but we should be encouraging people to move away from it. Maybe we should add a FAQ item on this topic?