[scala] Boolean-ifThenElse (coursera 강의 발췌)

coursera의 강의 Functional Programming Principles in Scala 4장 강의 중에 Funtional 언어의 특징을 유난히 보였던 예제가 있어서 끄적거려 봤다..

abstract class Boolean {
  def ifThenElse[T](t: => T, e: => T): T

  def unary_! : Boolean = ifThenElse(False , True)

  def && (x: => Boolean): Boolean = ifThenElse(x, False)
  def || (x: => Boolean): Boolean = ifThenElse(True, x)

  def == (x: Boolean): Boolean = ifThenElse(x, x.unary_!)
  def != (x: Boolean): Boolean = ifThenElse(x.unary_! , x)

  def < (x: Boolean): Boolean = ifThenElse(False, x)
  def > (x: Boolean): Boolean = ifThenElse(x, False)

}

object False extends Boolean {
  def ifThenElse[T](t: => T, e: => T) = e
}

object True extends Boolean {
  def ifThenElse[T](t: => T, e: => T) = t
}

테스트 결과

True.ifThenElse(println("yes"), println("no"))       // yes
False.ifThenElse(println("yes"), println("no"))      // no
(! True).ifThenElse(println("yes"), println("no"))   // no
(! False).ifThenElse(println("yes"), println("no"))   // yes

println
(True && False).ifThenElse(println("yes"), println("no"))  // no
(True && True).ifThenElse(println("yes"), println("no"))  // yes
(False && True).ifThenElse(println("yes"), println("no"))  // no
(False && False).ifThenElse(println("yes"), println("no"))  // no

println
(True == True).ifThenElse(println("yes"), println("no")) // yes
(True && True == True).ifThenElse(println("yes"), println("no")) //yes
(False || True != True).ifThenElse(println("yes"), println("no")) //no

println
(False < True).ifThenElse(println("yes"), println("no")) // yes
(False > True).ifThenElse(println("yes"), println("no")) // no