Another data structure that’s compared to Either in Scalaz is Validation:

sealed trait Validation[+E, +A] {
  /** Return `true` if this validation is success. */
  def isSuccess: Boolean = this match {
    case Success(_) => true
    case Failure(_) => false
  /** Return `true` if this validation is failure. */
  def isFailure: Boolean = !isSuccess


final case class Success[E, A](a: A) extends Validation[E, A]
final case class Failure[E, A](e: E) extends Validation[E, A]

At the first glance Validation looks similar to \/. They can even be converted back and forth using validation method and disjunction method.

ValidationV introduces success[X], successNel[X], failure[X], and failureNel[X] methods to all data types (don’t worry about the Nel thing for now):

scala> "event 1 ok".success[String]
res36: scalaz.Validation[String,String] = Success(event 1 ok)

scala> "event 1 failed!".failure[String]
res38: scalaz.Validation[String,String] = Failure(event 1 failed!)

What’s different about Validation is that it is not a monad, but it’s an applicative functor. Instead of chaining the result from first event to the next, Validation validates all events:

scala> ("event 1 ok".success[String] |@| "event 2 failed!".failure[String] |@| "event 3 failed!".failure[String]) {_ + _ + _}
res44: scalaz.Unapply[scalaz.Apply,scalaz.Validation[String,String]]{type M[X] = scalaz.Validation[String,X]; type A = String}#M[String] = Failure(event 2 failed!event 3 failed!)

It’s a bit difficult to see, but the final result is Failure(event 2 failed!event 3 failed!). Unlike \/ monad which cut the calculation short, Validation keeps going and reports back all failures. This probably would be useful for validating user’s input on an online bacon shop.

The problem, however, is that the error messages are mushed together into one string. Shouldn’t it be something like a list?


This is where NonEmptyList (or Nel for short) comes in:

/** A singly-linked list that is guaranteed to be non-empty. */
sealed trait NonEmptyList[+A] {
  val head: A
  val tail: List[A]
  def <::[AA >: A](b: AA): NonEmptyList[AA] = nel(b, head :: tail)

This is a wrapper trait for plain List that’s guaranteed to be non-empty. Since there’s at least one item in the list, head always works. IdOps adds wrapNel to all data types to create a Nel.

scala> 1.wrapNel
res47: scalaz.NonEmptyList[Int] = NonEmptyList(1)

Now does successNel[X] and failureNel[X] make sense?

scala> "event 1 ok".successNel[String]
res48: scalaz.ValidationNEL[String,String] = Success(event 1 ok)

scala> "event 1 failed!".failureNel[String]
res49: scalaz.ValidationNEL[String,String] = Failure(NonEmptyList(event 1 failed!))

scala> ("event 1 ok".successNel[String] |@| "event 2 failed!".failureNel[String] |@| "event 3 failed!".failureNel[String]) {_ + _ + _}
res50: scalaz.Unapply[scalaz.Apply,scalaz.ValidationNEL[String,String]]{type M[X] = scalaz.ValidationNEL[String,X]; type A = String}#M[String] = Failure(NonEmptyList(event 2 failed!, event 3 failed!))

In Failure, we were able to accumulate all failed messages.

We will pick it up from here later.