EitherT

Either can be used for error handling in most situations. However, when Either is placed into effectful types such as Option orFuture, a large amount of boilerplate is required to handle errors. For example, consider the following program:

import scala.util.Try
// import scala.util.Try

import cats.implicits._
// import cats.implicits._

def parseDouble(s: String): Either[String, Double] =
  Try(s.toDouble).map(Right(_)).getOrElse(Left(s"$s is not a number"))
// parseDouble: (s: String)Either[String,Double]

def divide(a: Double, b: Double): Either[String, Double] =
  Either.cond(b != 0, a / b, "Cannot divide by zero")
// divide: (a: Double, b: Double)Either[String,Double]

def divisionProgram(inputA: String, inputB: String): Either[String, Double] =
  for {
    a <- parseDouble(inputA)
    b <- parseDouble(inputB)
    result <- divide(a, b)
  } yield result
// divisionProgram: (inputA: String, inputB: String)Either[String,Double]

divisionProgram("4", "2") // Right(2.0)
// res0: Either[String,Double] = Right(2.0)

divisionProgram("a", "b") // Left("a is not a number")
// res1: Either[String,Double] = Left(a is not a number)

Suppose parseDouble and divide are rewritten to be asynchronous and return Future[Either[String, Double]] instead. The for-comprehension can no longer be used since divisionProgram must now compose Future and Either together, which means that the error handling must be performed explicitly to ensure that the proper types are returned:

import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.Future

def parseDoubleAsync(s: String): Future[Either[String, Double]] =
  Future.successful(parseDouble(s))
def divideAsync(a: Double, b: Double): Future[Either[String, Double]] =
  Future.successful(divide(a, b))

def divisionProgramAsync(inputA: String, inputB: String): Future[Either[String, Double]] =
  parseDoubleAsync(inputA) flatMap { eitherA =>
    parseDoubleAsync(inputB) flatMap { eitherB =>
      (eitherA, eitherB) match {
        case (Right(a), Right(b)) => divideAsync(a, b)
        case (Left(err), _) => Future.successful(Left(err))
        case (_, Left(err)) => Future.successful(Left(err))
      }
    }
  }

Clearly, the updated code is less readable and more verbose: the details of the program are now mixed with the error handling. In addition, as more Eithers and Futures are included, the amount of boilerplate required to properly handle the errors will increase dramatically.

EitherT

EitherT[F[_], A, B] is a lightweight wrapper for F[Either[A, B]] that makes it easy to compose Eithers and Fs together. To use EitherT, values of Either, F, A, and B are first converted into EitherT, and the resulting EitherT values are then composed using combinators. For example, the asynchronous division program can be rewritten as follows:

import cats.data.EitherT
// import cats.data.EitherT

import cats.implicits._
// import cats.implicits._

def divisionProgramAsync(inputA: String, inputB: String): EitherT[Future, String, Double] =
  for {
    a <- EitherT(parseDoubleAsync(inputA))
    b <- EitherT(parseDoubleAsync(inputB))
    result <- EitherT(divideAsync(a, b))
  } yield result
// divisionProgramAsync: (inputA: String, inputB: String)cats.data.EitherT[scala.concurrent.Future,String,Double]

divisionProgramAsync("4", "2").value
// res4: scala.concurrent.Future[Either[String,Double]] = Future(<not completed>)

divisionProgramAsync("a", "b").value
// res5: scala.concurrent.Future[Either[String,Double]] = Future(<not completed>)

Note that when F is a monad, then EitherT will also form a monad, allowing monadic combinators such as flatMap to be used in composing EitherT values.

From A or B to EitherT[F, A, B]

To obtain a left version or a right version of EitherT when given an A or a B, use EitherT.leftT and EitherT.rightT (which is an alias for EitherT.pure), respectively.

val number: EitherT[Option, String, Int] = EitherT.rightT(5)
val error: EitherT[Option, String, Int] = EitherT.leftT("Not a number")

From F[A] or F[B] to EitherT[F, A, B]

Similary, use EitherT.left and EitherT.right to convert an F[A] or an F[B] into an EitherT. It is also possible to use EitherT.liftF as an alias for EitherT.right.

val numberO: Option[Int] = Some(5)
val errorO: Option[String] = Some("Not a number")

val number: EitherT[Option, String, Int] = EitherT.right(numberO)
val error: EitherT[Option, String, Int] = EitherT.left(errorO)

From Either[A, B] or F[Either[A, B]] to EitherT[F, A, B]

Use EitherT.fromEither to a lift a value of Either[A, B] into EitherT[F, A, B]. An F[Either[A, B]] can be converted into EitherT using the EitherT constructor.

val numberE: Either[String, Int] = Right(100)
val errorE: Either[String, Int] = Left("Not a number")
val numberFE: List[Either[String, Int]] = List(Right(250))

val numberET: EitherT[List, String, Int] = EitherT.fromEither(numberE)
val errorET: EitherT[List, String, Int] = EitherT.fromEither(errorE)
val numberFET: EitherT[List, String, Int] = EitherT(numberFE)

From Option[B] or F[Option[B]] to EitherT[F, A, B]

An Option[B] or an F[Option[B]], along with a default value, can be passed to EitherT.fromOption and EitherT.fromOptionF, respectively, to produce an EitherT.

val myOption: Option[Int] = None
// myOption: Option[Int] = None

val myOptionList: List[Option[Int]] = List(None, Some(2), Some(3), None, Some(5))
// myOptionList: List[Option[Int]] = List(None, Some(2), Some(3), None, Some(5))

val myOptionET = EitherT.fromOption[Future](myOption, "option not defined")
// myOptionET: cats.data.EitherT[scala.concurrent.Future,String,Int] = EitherT(Future(Success(Left(option not defined))))

val myOptionListET = EitherT.fromOptionF(myOptionList, "option not defined")
// myOptionListET: cats.data.EitherT[List,String,Int] = EitherT(List(Left(option not defined), Right(2), Right(3), Left(option not defined), Right(5)))

Extracting an F[Either[A, B]] from an EitherT[F, A, B]

Use the value method defined on EitherT to retrieve the underlying F[Either[A, B]]:

val errorT: EitherT[Future, String, Int] = EitherT.leftT("foo")
// errorT: cats.data.EitherT[scala.concurrent.Future,String,Int] = EitherT(Future(Success(Left(foo))))

val error: Future[Either[String, Int]] = errorT.value
// error: scala.concurrent.Future[Either[String,Int]] = Future(Success(Left(foo)))