Understanding ErrorOr and EventuallyErrorOr in Scala

In Scala, when dealing with error handling, it’s essential to have robust mechanisms in place. Two common approaches to handle errors are through ErrorOr and EventuallyErrorOr. In this blog post, we’ll explore the differences between these two error-handling strategies and when to use each one.


ErrorOr is a simple error-handling mechanism that is typically implemented using an Either or Try monad. It allows you to represent a computation that can result in either success or failure. The ErrorOr type is straightforward and is often used for synchronous operations where errors are expected to be immediate.

type ErrorOr[A] = Either[String, A]

def divide(a: Int, b: Int): ErrorOr[Double] = {
  if (b == 0) Left("Division by zero")
  else Right(a.toDouble / b.toDouble)

val result: ErrorOr[Double] = divide(10, 2)

In this example, ErrorOr is used to represent the result of a division operation. If the division is successful, it returns a Right with the result; otherwise, it returns a Left with an error message.


EventuallyErrorOr is an error-handling strategy that combines error handling with asynchronicity. It is commonly implemented using a combination of monads like Future and EitherT. This approach is suitable for operations that are expected to be asynchronous and may fail at some point in the future.

import cats.data.EitherT
import cats.implicits._
import scala.concurrent.{ExecutionContext, Future}

type EventuallyErrorOr[A] = EitherT[Future, String, A]

def asyncDivide(a: Int, b: Int)(implicit ec: ExecutionContext): EventuallyErrorOr[Double] = {
  if (b == 0) EitherT.leftT("Division by zero")
  else EitherT(Future(Right(a.toDouble / b.toDouble)))

val result: EventuallyErrorOr[Double] = asyncDivide(10, 0)

In this example, EventuallyErrorOr is used to represent the result of an asynchronous division operation. It combines Future and Either to handle both asynchronicity and errors gracefully.

When to Use Each Approach:

  • Use ErrorOr: Use ErrorOr for synchronous operations where errors are expected to be immediate. It’s a simple and straightforward approach suitable for scenarios where you want to handle errors as soon as they occur.
  • Use EventuallyErrorOr: Use EventuallyErrorOr for asynchronous operations where errors might occur in the future. It provides a way to handle errors gracefully in asynchronous code, making it suitable for scenarios where you need to work with futures and handle errors in a composable manner.

Both ErrorOr and EventuallyErrorOr are valuable tools for error handling in Scala, and the choice between them depends on the nature of your operations. ErrorOr is suitable for synchronous, immediate error handling, while EventuallyErrorOr is designed for asynchronous operations that may fail in the future. Understanding these approaches will help you choose the right one for your specific use cases and write more robust and maintainable code.

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