ContextShift.scala

import java.util.concurrent.Executors
import scala.concurrent.ExecutionContext
import cats.effect._

object Foo extends IOApp {
  val cs = implicitly[ContextShift[IO]]
  val ex = Executors.newCachedThreadPool()
  val ec = ExecutionContext.fromExecutor(ex)

  def run(args: List[String]) =
    for {
      _ <- cs.evalOn(ec)(IO(println("foo")))
      _ = ex.shutdown()
    } yield ExitCode.Success
}