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Right-Biased Either

Either in Scala prior to 2.12 is not right-biased meaning that you have to call Either.right all the time if you want to use it with for-comprehension.

e.g.) Before 2.12

for {
b <- methodReturningEither(a).right
c <- anotherReturningEither(b).right
} yield c

If you use just-fp, it becomes

import just.fp.syntax._
for {
b <- methodReturningEither(a)
c <- anotherReturningEither(b)
} yield c

Of course, you don't need to do it if you use Scala 2.12 or higher.

Either Constructors

In normal ways, if you want to create Left or Right, you just use the apply methods of their companion objects (i.e. Left() Right()) A problem with this is that what these return is not Either but its data, Left or Right.

You also need to specify not only type parameter for Left but also the one for Right when creating Right.

e.g.) Without type parameters,

// res0: Right[Nothing, Int] = Right(value = 1)

You don't want to have Nothing there. So do it with type parameters,

Right[String, Int](1)
// res1: Right[String, Int] = Right(value = 1)

So it becomes unnecessarily verbose. Right should be inferred as the compiler knows it already yet to specify the left one, you have to put both left and right parameters.

Left, of course, has the same problem.

// res2: Left[String, Nothing] = Left(value = "error")
Left[String, Int]("error")
// res3: Left[String, Int] = Left(value = "error")

Now with just-fp, it's simpler. You can use use left and right constructors as extension methods to the actual data values with only missing type info specified.


1.right[String] // Now you only need to specify
// res4: Either[String, Int] = Right(value = 1)

For Left,

// res5: Either[String, Int] = Left(value = "error")

leftMap and leftFlatMap

So if you Scala 2.12 or higher or just-fp with the older Scala, Either is right-biassed. Then what about the Left case? Can I ever use Left for something useful like transforming the Left value to something else?

For that, just-fp has added leftMap and leftFlatMap to Either. e.g.)

import just.fp.syntax._

final case class ComputeError(error: String)

sealed trait AppError
object AppError {
final case class InvalidNumberError(error: String) extends AppError
final case class ComputationError(computeError: ComputeError) extends AppError

def invalidNumberError(error: String): AppError =
def fromComputeError(computeError: ComputeError): AppError =

def f1(n: Int): Either[String, Int] =
if (n < 0)
Left(s"The number must be non-negative integer - n: $n")

def f2(x: Int, y: Int): Either[ComputeError, Int] = {
val z = x + y
if (x >= 0 && y >= 0 && z < 0)
Left(ComputeError(s"Numbers are too big - x: $x, y: $y"))
Right(x + y)
for {
b <- f1(10).leftMap(AppError.invalidNumberError)
c <- f2(123, b).leftMap(AppError.fromComputeError)
} yield c
// res7: Either[AppError, Int] = Right(value = 133)
for {
b <- f1(-1).leftMap(AppError.invalidNumberError)
c <- f2(123, b).leftMap(AppError.fromComputeError)
} yield c
// res8: Either[AppError, Int] = Left(
// value = InvalidNumberError(
// error = "The number must be non-negative integer - n: -1"
// )
// )
for {
b <- f1(Int.MaxValue).leftMap(AppError.invalidNumberError)
c <- f2(1, b).leftMap(AppError.fromComputeError)
} yield c
// res9: Either[AppError, Int] = Left(
// value = ComputationError(
// computeError = ComputeError(
// error = "Numbers are too big - x: 1, y: 2147483647"
// )
// )
// )