If you’ve gotten this far reading my blog, you’re probably very familiar with Either. Of course Im also assuming someone is reading this blog in the first place 😛

Anyway, Either[A,B] allows us to return either an A or a B depending on some conditions… In other words, its a XOR (exclusive or) A value can either be A or it can be B but it cannot be both.

You probably see where this is going, IOR is a cats datatype that stands for Inclusive Or. In other words, Ior[A,B] can either be A or it can be B or it can be Both A and B.

Another syntax for defining an Ior is: A Ior B

Ior is often used when we want to handle errors and correct values as well as correct values but wth a warning.


Here is an example using IOR:

  import cats.data.Ior

  val right: Ior[String, Int] = Ior.right[String, Int](3)
  val left: Ior[String, Int] = Ior.left[String, Int]("Error")
  val both: Ior[String, Int] = Ior.both[String, Int]("Warning", 0)

Of course things can be made to look nicer when using the cats syntax:

  import cats.syntax.ior._

  val x: Ior[String, Nothing] = "NOPE".leftIor
  // Or
  val z: Ior[String, Int] = "Better Nope".leftIor[Int]

Accumulating Errors

Another benefit of Ior is its ability to accumulate errors on the left side. Similar to Cats Validated (Will be discussed in a later article)

How does it work

Ior will accumulate warnings on the left side but will short circuit the computations as soon as it encounters a left only value. Lets look at an example for a clearer image.

  import cats.data.Ior
  import cats.syntax.ior._
  import cats.data.{NonEmptyList => Nel}
  type Failures = Nel[String]

  final case class UserName(private val name: String) extends AnyVal {
    def validateUserName: Failures Ior UserName = {

      if (name.isEmpty) Nel.one("Error, Username cannot be empty").leftIor
      else if (name == "admin")
            "In the future, protected words will no longer be considered valid"),
      else UserName(name).rightIor

  final case class UserPassword(private val value: String) extends AnyVal {
    def validatePassword: Failures Ior UserPassword = {
      if (value.isEmpty) Nel.one("Passwords cannot be empty").leftIor
      else if (value.length < 10)
        Ior.both(Nel.one("Password is not very secure"), UserPassword(value))
      else UserPassword(value).rightIor

which would allow us to create a valid User:

Note: If you’re confused by mapN and the various imports, it’ll be explained in another post along with validated and validatedN.

  import cats.syntax.apply._

  final case class User(name: UserName, pass: UserPassword)

  val username1 = UserName("Fares")
  val pass1 = UserPassword("qwertyazerty")
  val user1 = (username1.validateUserName, pass1.validatePassword).mapN(User)

  // user1 ==> Right(User(UserName(fares),UserPassword(qwertyazerty)))

But what if the username and password contained non blocking errors?

  val username2 = UserName("admin")
  val pass2 = UserPassword("pass")
  val user2 = (username2.validateUserName, pass2.validatePassword).mapN(User)

  //user2 ==> Both(NonEmptyList(In the future, protected words will no longer be considered valid, Password is not very secure),User(UserName(admin),UserPassword(pass)))

This is what we meant by accumulating warnings or nonblocking errors.

But what if we encountered a blocking error?

  val username3 = UserName("")
  val pass3 = UserPassword("")
  val user3 = (username3.validateUserName, pass3.validatePassword).mapN(User)

  //user3 ==> Left(NonEmptyList(Error, Username cannot be empty))

Notice it short circuited the computations without accumulating the errors on the left side (Password was empty but the error was not added to that of Username)


Ior Can be converted to an Either a Validate or even an Option.

Keep in mind however that when doing so, the Both will drop its left side and only keep the right value and transform it to a Some or a Right ect…

Here are the transformation codes:

  final def toEither: Either[A, B] = fold(Left(_), Right(_), (_, b) => Right(b))
  final def toValidated: Validated[A, B] = fold(Invalid(_), Valid(_), (_, b) => Valid(b))
  final def toOption: Option[B] = fold(_ => None, b => Some(b), (_, b) => Some(b))