sketch of a monadic disjoint union (like Either, but a monad)

Result.scala

sealed trait Result[+F, S] {
  def flatMap[FF >: F, SS](f: S => Result[FF, SS]): Result[FF, SS]
  def map[SS](f: S => SS): Result[F, SS]
  def fold[X](ff: F => X, fs: S => X): X
}

case class Failure[+F, S](x: F) extends Result[F, S] {
  def flatMap[FF >: F, SS](f: S => Result[FF, SS]) = Failure[FF, SS](x)
  def map[SS](f: S => SS) = Failure[F, SS](x)
  def fold[X](ff: F => X, fs: S => X) = ff(x)
}

case class Success[+F, S](x: S) extends Result[F, S] {
  def flatMap[FF >: F, SS](f: S => Result[FF, SS]) = f(x)
  def map[SS](f: S => SS) = Success(f(x))
  def fold[X](ff: F => X, fs: S => X) = fs(x)
}