These

these.scala

import scalaz._
import Scalaz._
import \&/._

sealed trait Error
case class NotFatalError(reason: String) extends Error
case class FatalError(reason: String) extends Error

def function(i: Int): List[Error] \&/ Int = i match {
  case 0 => This(List(NotFatalError("Zero returned")))
  case 1 => This(List(FatalError("One is fatal")))
  case 2 => Both(List(NotFatalError("Both?")), 2)
  case x => That(x)
}

val first = for {
  a <- function(10)
  b <- function(11)
} yield a + b

val second = for {
  a <- function(10)
  b <- function(1)
} yield a + b

val third = for {
  a <- function(1)
  b <- function(1)
  c <- function(1)
  d <- function(2)
} yield a + b

val fourth = for {
  a <- function(2)
  b <- function(2)
} yield a + b