Cats either asRight

cats-either-asRight.scala

case class Error(e: String)

/**
  * These “smart constructors” have advantages over Left.apply and Right.apply
  * because they return results of type Either instead of Left and Right.
  * This helps avoid type inference bugs caused by over-narrowing
  */
def sumPositive(l: List[Int]): Either[Error, Int] = {
  l.foldLeft(0.asRight[Error]) { (acc: Either[Error, Int], i: Int) =>
    if (i > 0) {
      acc.map(_ + i)
    } else {
      Left(Error(s"negative value in list $i"))
    }
  }
}


sumPositive(List(1, 2, 3, -1, 4)) // Left(Error(negative value in list -1))
sumPositive(List(1, 2, 3, 4)) // Right(10)