Monad Transformer for Promise and Validation

monadTransformer.scala

case class ValidationPromised[E, A](promised: Promise[Validation[E, A]]) {
    def map[B](f: A => B): ValidationPromised[E, B] = 
        ValidationPromised(promised map { valid => 
            valid.fold(
                fail => KO(fail),
                suc => OK(f(suc))
            )
        })        

    def flatMap[B](f: A => ValidationPromised[E, B]): ValidationPromised[E, B] = 
        ValidationPromised(promised flatMap { valid => 
            valid.fold (
                bad => Promise.pure(KO(bad)),
                good => f(good).promised
            )
        })

}

with.scala

for {
  ns <- getNode(s); //get source
  nt <- getNode(t); //get target
  l <- link(ns, nt); //link them
} yield l //yield the link

without.scala

for {
  vs <- getNode(s); //get source
  vt <- getNode(t); //get target
  vl <- for {
               ns <- vs;
               nt <- vt
          } yield link(ns, nt); //link them
} yield vl //yield validation of the link