fs2_concat.scala

import cats.effect.concurrent.Ref
import cats.effect.{IO, Sync}
import cats._
import cats.implicits._
import fs2.Stream

def g(r: => IO[Ref[IO, Int]]): Stream[IO, Int] =
  Stream
    .eval(r)
    .flatMap(
      x =>
        Stream.eval(for {
          i <- x.get
          _ <- x.set(i + 1)
        } yield i)
    )

def f: Stream[IO, Int] = {
  val j = Ref.of[IO, Int](1)

  val r = for {
    xs <- IO(g(j))
    ys <- IO(g(j))
  } yield xs ++ ys

  Stream.eval(r).flatten
}

f.map(println).compile.drain.unsafeRunSync()

// Result:
// 1
// 1
// Desire:
// 1
// 2