joinRight・joinLeftを入れ子ifの替わりに使う ref: http://qiita.com/takaki@github/items/5dba09ff6016cec367f2

file0.scala

package example

object EitherDemo {
  def main(args: Array[String]) {
    println(oddAndPositive(-1))
    println(oddAndPositive(0))
    println(oddAndPositive(1))
    println(oddAndPositive2(-1))
    println(oddAndPositive2(0))
    println(oddAndPositive2(1))

    println(oddOrPositive(-2))
    println(oddOrPositive(-1))
    println(oddOrPositive(0))
    println(oddOrPositive2(-2))
    println(oddOrPositive2(-1))
    println(oddOrPositive2(0))

  }

  def mod(i: Int, j: Int) = (i % j + j) % j

  def oddOrPositive(i: Int): Either[String, Int] = {
    Either.cond(i >= 0, i, Either.cond(mod(i, 2) == 1, i, "even and negative number")).joinLeft
  }

  def oddOrPositive2(i: Int): Either[String, Int] = {
    if (i >= 0) {
      Right(i)
    } else if (mod(i, 2) == 1) {
      Right(i)a
    } else {
      Left("even and negative number")
    }
  }

  def oddAndPositive(i: Int): Either[String, Int] = {
    Either.cond(i >= 0, Either.cond(mod(i, 2) == 1, i, "even number"), "negative number").joinRight
  }

  def oddAndPositive2(i: Int): Either[String, Int] = {
    if (i >= 0) {
      if (mod(i, 2) == 1) {
        Right(i)
      } else {
        Left("even number")
      }
    } else {
      Left("negative number")
    }
  }
}