cats-effect: How to obtain an implicit NonEmptyParallel - scala

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