Option型(Maybeモナド)の自作

gistfile1.scala

sealed trait MyOption[+A] {

  def map[B](f: A => B): MyOption[B] = 
    if (isEmpty) MyNone else MySome(f(this.get))
    
  def flatMap[B](f: A => MyOption[B]): MyOption[B] = 
    if (isEmpty) MyNone else f(this.get)

  def get: A

  def getOrElse[B >: A](elseValue: B):B = 
    if (isEmpty) elseValue else this.get

  def isEmpty: Boolean

}

case class Some[A](private val value: A) 
  extends MyOption[A]{

  def isEmpty = false

  def get = value

}

case object None
  extends MyOption[Nothing] {

  def isEmpty = true

  def get = throw new java.util.NoSuchElementException

}

object Main {

  def testMatch(option: MyOption[Int]) {
    val value = option match {
      case MySome(v) => println("testMatch value = " + v)
      case MyNone => ()
    }
  }

  def testFlatMap(option: MyOption[Int]) {
    val value = option.flatMap{ e =>
      MyNone
    }.getOrElse(0)
    println("testFlatMap value = " + value)
  }

  def testMap(option: MyOption[Int]) {
    val value = option.map{ e =>
      e * 2
    }.getOrElse(0)
    println("testMap value = " + value)
  }

  def forTest(option: MyOption[MyOption[Int]]) {
    val value = for{
      a <- option
      b <- a
    } yield b
    println("forTest = " + value.getOrElse(0))
  }

  def main(args : Array[String]) {
    testMatch(MySome(2))
    testFlatMap(MySome(1))
    testMap(MySome(1))
    forTest(MySome(MySome(1)))
  }

}