Non-strict evaluation in Scala

non_strict.scala

/**
 * Try to solve this puzzle only by modifying the function [[doubleMessage]].
 * All checks should succeed.
 *
 * This exercise must be run in a worksheet.
 *  - One is available on the Web with Scastie (https://scastie.scala-lang.org)
 *  - One is available in IntelliJ IDEA
 *    (https://www.jetbrains.com/help/idea/work-with-scala-worksheet-and-ammonite.html)
 *
 * Click on "> Run". See the results in the console. And try to solve this
 * puzzle by using call-by-value and lazy declaration.
 */

def doubleMessage(displayIt: Boolean, message: => String): String =
  if (displayIt)
    s"$message $message"
  else
    "(nope)"


// vvvv DON'T TOUCH THIS vvvv

object MessageCheck:
  def check(description: String, condition: Boolean): Unit =
    if (condition)
      println(s"$description: ${Console.GREEN}Success${Console.RESET}")
    else
      println(s"$description: ${Console.RED}Failure${Console.RESET}")

  private var x = 0

  def message = {
    x += 1

    "hello"
  }

  def run: Unit =
    check("should answer (nope) when no display", doubleMessage(displayIt = false, message) == "(nope)")
    check("x should not have increased", x == 0)

    check("should answer message twice when display", doubleMessage(displayIt = true, message) == "hello hello")
    check("x should have increased only once", x == 1)

MessageCheck.run