Can there be Parallel for Async in cats effect?

Question

I noticed that cats-effect typeclass hierarchy doesn't inherit Parallel from cats core even at their most powerful typeclass ConcurrentEffect . The only instance provided for parallel exists only if you use IO directly.

But shouldn't there be one? I kinda feel like Sync[F] and Async[F] should be a nice duo for Parallel[F] .

Answer 1

The Parallel behavior is really different to what Sync and Async hierarchy promises (that is sequential but (a)synchronous execution). ConcurrentEffect promises that your computation runs on a thread pool and can be cancelled (+ everything its smaller elements promise) - still not enable combining parallel computations but allows you to implement races. Parallel is an orthogonal semantics which is why it is passed as a separate type class/type constraint. So just add it as a separate type constraint.

def toStringParallel[F[_]: Sync: Parallel](list: List[F[Int]]): F[List[String]] =
  list.parTraverse(a => a.toString.pure[F])

object App1 extends IOApp {
  def run(args: List[String]) = toStringParallel[IO](List(IO(1), IO(2)))
    .as(ExitCode.Success)
}

If you cannot instantiate Parallel[IO] , remember that it requires ContextShift[IO] to create an instant of Parallel[IO] .

// example from docs
implicit val contextShift: ContextShift[IO] = IO.contextShift(ExecutionContext.global)

val ioA = IO(println("Running ioA"))
val ioB = IO(println("Running ioB"))
val ioC = IO(println("Running ioC"))

// make sure that you have an implicit ContextShift[IO] in scope. 
val program = (ioA, ioB, ioC).parMapN { (_, _, _) => () }