Either Mither

Either has changed. A superficial Google might suggest that Either represents just that - either A or B.

"You cannot, at least not directly, use an Either instance like a collection, the way you are familiar with from Option and Try. This is because Either is designed to be unbiased.

"Try is success-biased: it offers you map, flatMap and other methods that all work under the assumption that the Try is a Success, and if that’s not the case, they effectively don’t do anything, returning the Failure as-is." (from here)

And: "if you use Either for error reporting, then it is true that you want it to be biased to one side, but that is only one usecase of many, and hardcoding a single special usecase into a general interface smells of bad design" from here.

But then you see this in the Scala documentation: "Either is right-biased, which means that Right is assumed to be the default case to operate on. If it is Left, operations like map, flatMap, ... return the Left value unchanged".

This apparent contradiction arises as Scala 2.12 changed Either. It has become biased.

Let's demonstrate using ScalaTest. First, we define an Either and some functions to act on it:

    type LeftType   = List[Int]
    type RightType  = Int
    type EitherType = Either[LeftType, RightType]
    val left        = Left(List[Int]())
    val right       = Right(1)

    val rightFn: (RightType) => RightType = _ + 1
    val leftFn:  (LeftType)  => LeftType  = _ :+ 1

Then, the test looks like:

    "Either" should {
      "be agnostic if left or right has been explicitly stated on an Either that's a Left" in {
        val either: EitherType = left
        either.left.map(leftFn) shouldEqual Left(List(1))
        either.right.map(rightFn) shouldEqual left // unchanged
      }
      "be agnostic if left or right has been explicitly stated on an Either that's a Right" in {
        val either: EitherType = right
        either.right.map(rightFn) shouldEqual Right(2)
        either.left.map(leftFn) shouldEqual right // unchanged
      }
.
.

So far, so good. But this following code is new in 2.12 (it won't compile in earlier versions):

      "ignore the unbiased side" in {
        val either: EitherType = left
        either.map(rightFn) shouldEqual left
      }
      "map the biased side" in {
        val either: EitherType = right
        either.map(rightFn) shouldEqual Right(2)
      }

Either's new monadic functions cannot take anything other than a function of type (RightType) => ...

The mnemonic is that if you're using this for error reporting, then Right is the right answer.