parTraverse with a limit of N using a Semaphore

parTraverseN.scala

import cats.Traverse
import cats.effect._
import cats.effect.concurrent.Semaphore
import cats.temp.par._
import cats.syntax.all._

import scala.concurrent.duration._

object Main extends IOApp {
  import ParTask._

  def putStrLn[A](a: A): IO[Unit] = IO(println(a))

  override def run(args: List[String]): IO[ExitCode] = {
    import cats.instances.list._
    parTraverseN(3, List.range(1, 10))(x => putStrLn(x) *> IO.sleep(1.second)).as(ExitCode.Success)
  }
}

object ParTask {

  def parTraverseN[F[_]: Concurrent: Par, G[_]: Traverse, A, B](
      n: Int,
      ga: G[A]
  )(f: A => F[B]) =
    Semaphore[F](n).flatMap { s =>
      ga.parTraverse(a => s.withPermit(f(a)))
    }

}

Brilliant!