cats-effect: How to obtain an implicit NonEmptyParallel

Question

I am trying to use parMapN function and I am not able to compile the code. If my type is IO then there is not problem, but when I use types on my functions, then I cannot manage to make it work.

In the snippet below, there is randomMessage which compiles and runs correctly but calling to randomMessageF does not compiles since there is not a NonEmptyParallel implicit on the scope. But then, which implicit is using randomMessage? Passing contextShift doest not work either.

    import cats.NonEmptyParallel
    import cats.effect._
    import cats.syntax.all._
    import fs2._

    import scala.util.Random

    object Test  extends IOApp {

      def randomMessageF[F[_], A, B, C](toA: => F[A],
                                       toB: => F[B],
                                       toC: (A, B) => C)(implicit nep: NonEmptyParallel[F, F]): Stream[F, C] = Stream.eval {
        val funcA   = toA
        val funcB = toB
        (funcA, funcB).parMapN {
          case (a, b) =>
            toC(a, b)
        }
      }

      def randomMessage[A, B, C](toA: => IO[A],
                                 toB: => IO[B],
                                 toC: (A, B) => C): Stream[IO, C] = Stream.eval {
        val funcA   = toA
        val funcB = toB
        (funcA, funcB).parMapN {
          case (a, b) =>
            toC(a, b)
        }
      }

      def run(args: List[String]): IO[ExitCode] =  {
        println(
          randomMessage(
            IO(Random.nextInt(1000).toString),
            IO(Random.nextString(10)),
            (k: String, v: String) => s"$k:$v"
          ).compile.toList.unsafeRunSync().head)


        println(
          randomMessageF[IO, String, String, String](
            IO(Random.nextInt(1000).toString),
            IO(Random.nextString(10)),
            (k, v) => s"$k:$v"
          )(???).compile.toList.unsafeRunSync().head)

        IO(ExitCode(0))
      }

    }

Answer 1

Try

def randomMessageF[M[_], F[_], A, B, C](toA: => M[A],
                                        toB: => M[B],
                                        toC: (A, B) => C)(implicit 
                                        nep: NonEmptyParallel[M, F]): Stream[M, C] = Stream.eval {
  val funcA = toA
  val funcB = toB
  (funcA, funcB).parMapN {
    case (a, b) =>
      toC(a, b)
  }
}

println(
  randomMessageF/*[IO, IO.Par, String, String, String]*/(
    IO(Random.nextInt(1000).toString),
    IO(Random.nextString(10)),
    (k: String, v: String) => s"$k:$v"
  ).compile.toList.unsafeRunSync().head)

In randomMessage the implicit used is NonEmptyParallel[IO, IO.Par] .

https://github.com/typelevel/cats-effect/blob/master/core/shared/src/main/scala/cats/effect/IO.scala#L834