SimpleEitherSample.scala

// 失敗する処理
def fail(i: Int): Either[String,Int] = Left(s"error occored from $i")

// 成功する処理
def success(i: Int): Either[String,Int] = Right(i + 1)

println("すべて成功する (e is 5)")
val result0 = for {
  a <- success(0).right
  b <- success(a).right
  c <- success(b).right
  d <- success(c).right
  e <- success(d).right
} yield e
println(result0)

println("成功して最後に失敗する (d is 4)")
val result1 = for {
  a <- success(0).right
  b <- success(a).right
  c <- success(b).right
  d <- success(c).right
  e <- fail(d).right
} yield e
println(result1)

println("途中で失敗する (b is 2)") 
val result2 = for {
  a <- success(0).right
  b <- success(a).right
  c <- fail(b).right
  d <- success(c).right
  e <- success(d).right
} yield e
println(result2)

println("最初で失敗する (a is 0)")
val result3 = for {
  a <- fail(0).right
  b <- success(a).right
  c <- success(b).right
  d <- success(c).right
  e <- success(d).right
} yield e
println(result3)