Free Monad Stream

build.sbt

name := "example"

scalaVersion := "2.12.4"

scalacOptions ++= Seq(
  "-feature",
  "-deprecation",
  "-Ypartial-unification",
//    "-Xlog-implicits",
)

libraryDependencies ++= {
  val fs2Version = "0.10.0-RC1"
  val catsVersion = "1.0.1"

  Seq(
    "org.typelevel"     %% "cats-free"                    % catsVersion,
    "co.fs2"            %% "fs2-core"                     % fs2Version,
  )
}

compiler output.txt

// Exiting paste mode, now interpreting.

<pastie>:39: error: type mismatch;
 found   : fs2.Stream[F,MetricAlarm]
 required: FreeCloudWatch.this.StreamF[A]
    (which expands to)  fs2.Stream[F,A]
      case DescribeAlarms() ⇒ Stream.emit(MetricAlarm("my-alarm")).covary[F]

free.scala

// cats 1.0.1, fs2 0.10.0-RC1

import cats._
import cats.data._
import cats.implicits._
import cats.effect.Effect
import cats.free.Free
import cats.free.Free.liftF
import fs2._

import scala.language.higherKinds

case class MetricAlarm(alarmName: String)

class FreeCloudWatch[F[_] : Effect] {
  sealed trait CloudWatchA[A]
  case class DescribeAlarms() extends CloudWatchA[StreamF[MetricAlarm]]
  case class DeleteAlarm(alarmNames: StreamF[MetricAlarm]) extends CloudWatchA[Unit]

  type CloudWatch[A] = Free[CloudWatchA, A]
  type StreamF[A] = Stream[F, A]

  def describeAlarms: CloudWatch[StreamF[MetricAlarm]] =
    liftF[CloudWatchA, StreamF[MetricAlarm]](DescribeAlarms())

  def deleteAlarms(alarms: StreamF[MetricAlarm]): CloudWatch[Unit] =
    liftF[CloudWatchA, Unit](DeleteAlarm(alarms))

  def javaSdkCompiler: CloudWatchA ~> StreamF = new (CloudWatchA ~> StreamF) {
    override def apply[A](fa: CloudWatchA[A]): StreamF[A] = fa match {
      case DescribeAlarms() ⇒ Stream.emit(MetricAlarm("my-alarm")).covary[F]
      case DeleteAlarm(_) ⇒ Stream.emit(()).covary[F]
    }
  }

}

free.scala:31 should have been

      case DescribeAlarms() ⇒ Stream.emit(Stream.emit(MetricAlarm("my-alarm")).covary[F])

With that change, the gist compiles.