scala関数型デザイン＆プログラミング P76

Exercise 4.6 Right値を操作するmap,flatMap,orElse,map2をEitherに追加せよ

map2については内包表記を使うと簡潔になる

import scala.{Option => _, Either => _, _}


object Main {
    def main(args:Array[String]) = {
       val x=Right(1)
       val y=Left(22)
      println( x.map(i=>i*2))
      println(x.flatMap(i=>Right(i)))
      println(x.orElse(Left(999)))
      println(y.orElse(Left(999)))
      println(x.map2(Right(2))((a,b)=>a+b))
    }

}

sealed trait Either[+E,+A] {

  def map[B](f:A=>B):Either[E,B] = this match {
case Left(e) => Left(e)
case Right(a) => Right(f(a))
}


   def flatMap[EE>:E,B](f:A=>Either[EE,B]):Either[EE,B] = this match {
     case Left(e) => Left(e)
     case Right(a) => f(a)
   }

 def orElse[EE >: E, AA >: A](b: => Either[EE, AA]): Either[EE, AA] = this match {
   case Left(e) => b
  case a => a
 }

def map2[EE>:E,B,C](b:Either[EE,B])(f:(A,B)=>C):Either[EE,C] =
  for { a <- this; b1 <- b } yield f(a,b1)


}

case class Left[+E](get: E) extends Either[E,Nothing]
case class Right[+A](get: A) extends Either[Nothing,A]