Cli.scala

object Cli {
  sealed abstract class EvalResult
  object Exit extends EvalResult
  case class ReadMore(lines: List[String]) extends EvalResult
  case class PrintMe(result: (String => Unit) => Unit) extends EvalResult

  def prompt(ps1: String, ps2: String)(eval: List[String] => EvalResult): Unit = {
    def run(ps: String, acc: List[String]): Unit = eval(readLine(ps) :: acc) match {
      case Exit => ()
      case ReadMore(lines) => run(ps2, lines)
      case PrintMe(result) => result(print _); run(ps1, Nil)
    }
    run(ps1, Nil)
  }
}

Usage.scala

object More {
  def unapply(x: String): Option[String] =
    if (x.endsWith("\\"))
      Some(x.substring(0, x.length - 1))
    else
      None
}

Cli.prompt("Enter a sentence: ", ">>> ") {
  case "quit" :: Nil => Cli.Exit
  case More(x) :: xs => Cli.ReadMore( x :: xs)
  case xs => Cli.PrintMe { callback =>
    callback("Your input is so long. Please wait...\n")
    Thread.sleep(10000)
    callback("Ok. After hard work. ")
    callback {
      "That string contains %d vowels.\n".format {
        xs.foldLeft(0) {
          (acc, x) => acc + "(?i)[aeiou]".r.findAllIn(x).collect.size
        }
      }
    }
  }
}