scala3-ContextFunctions.scala

import scala.concurrent.{Await, ExecutionContext, Future}
import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.duration.*

object FutureCF:
  type Executable[T] = ExecutionContext ?=> T                        

  // Compare Future.apply's signature:
  // apply[T](body: => T)(implicit executor: ExecutionContext): Future[T]
  def apply[T](body: => T): Executable[Future[T]] = Future(body)     

def sleepN(dur: Duration): Duration =                                
  val start = System.currentTimeMillis()
  Thread.sleep(dur.toMillis)
  Duration(System.currentTimeMillis - start, MILLISECONDS)

val future1 = FutureCF(sleepN(1.second))                             
val future2 = FutureCF(sleepN(1.second))(using global)
val duration1 = Await.result(future1, 2.seconds)                     
val duration2 = Await.result(future2, 2.seconds)

// REPL results for the last four lines:
// val future1: concurrent.Future[concurrent.duration.Duration] = 
//   Future(Success(1004 milliseconds))
// val future2: concurrent.Future[concurrent.duration.Duration] = 
//   Future(Success(1004 milliseconds))
// val duration1: concurrent.duration.Duration = 1004 milliseconds
// val duration2: concurrent.duration.Duration = 1004 milliseconds