Monix function cache with TTL

Cache.scala

object Cache {
  def apply[A, B](ttl: FiniteDuration)(f: A => Task[B]): A => Task[B] = {
    val mvar = MVar(Map.empty[A, B])
    a =>
      for {
        cached <- mvar.read.map(_.get(a))
        out <- cached match {
          case Some(b) => Task.pure(b)
          case None =>
            for {
              cache <- mvar.take
              b <- f(a)
              _ <- Task.chooseFirstOf(mvar.put(cache.updated(a, b)),
                                      mvar.take
                                        .flatMap { x =>
                                          mvar.put(x - a)
                                        }
                                        .delayExecution(ttl))
            } yield b
        }
      } yield out
  }
}

So many reasons why this wouldn't work