Either

sealed abstract
class Either[+A, +B] extends Product with Serializable

Represents a value of one of two possible types (a disjoint union). An instance of Either is an instance of either scala.util.Left or scala.util.Right.

A common use of Either is as an alternative to scala.Option for dealing with possibly missing values. In this usage, scala.None is replaced with a scala.util.Left which can contain useful information. scala.util.Right takes the place of scala.Some. Convention dictates that Left is used for failure and Right is used for success.

For example, you could use Either[String, Int] to indicate whether a received input is a String or an Int.

import scala.io.StdIn._
val in = readLine("Type Either a string or an Int: ")
val result: Either[String,Int] =
  try Right(in.toInt)
  catch {
    case e: NumberFormatException => Left(in)
  }

result match {
  case Right(x) => s"You passed me the Int: $x, which I will increment. $x + 1 = ${x+1}"
  case Left(x)  => s"You passed me the String: $x"
}

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 and flatMap return the Left value unchanged:

def doubled(i: Int) = i * 2
Right(42).map(doubled) // Right(84)
Left(42).map(doubled)  // Left(42)

Since Either defines the methods map and flatMap, it can also be used in for comprehensions:

val right1 = Right(1)   : Right[Double, Int]
val right2 = Right(2)
val right3 = Right(3)
val left23 = Left(23.0) : Left[Double, Int]
val left42 = Left(42.0)

for {
  x <- right1
  y <- right2
  z <- right3
} yield x + y + z // Right(6)

for {
  x <- right1
  y <- right2
  z <- left23
} yield x + y + z // Left(23.0)

for {
  x <- right1
  y <- left23
  z <- right2
} yield x + y + z // Left(23.0)

// Guard expressions are not supported:
for {
  i <- right1
  if i > 0
} yield i
// error: value withFilter is not a member of Right[Double,Int]

// Similarly, refutable patterns are not supported:
for (x: Int <- right1) yield x
// error: value withFilter is not a member of Right[Double,Int]

Since for comprehensions use map and flatMap, the types of function parameters used in the expression must be inferred. These types are constrained by the Either values. In particular, because of right-biasing, Left values may require an explicit type argument for type parameter B, the right value. Otherwise, it might be inferred as Nothing.

for {
  x <- left23
  y <- right1
  z <- left42  // type at this position: Either[Double, Nothing]
} yield x + y + z
//            ^
// error: ambiguous reference to overloaded definition,
// both method + in class Int of type (x: Char)Int
// and  method + in class Int of type (x: Byte)Int
// match argument types (Nothing)

for (x <- right2 ; y <- left23) yield x + y  // Left(23.0)
for (x <- right2 ; y <- left42) yield x + y  // error

for {
  x <- right1
  y <- left42  // type at this position: Either[Double, Nothing]
  z <- left23
} yield x + y + z
// Left(42.0), but unexpectedly a `Either[Double,String]`
Companion
object
Source
Either.scala
trait Product
trait Equals
class Object
trait Matchable
class Any
class Left[A, B]
class Right[A, B]

Value members

Abstract methods

Returns true if this is a Left, false otherwise.

Returns true if this is a Left, false otherwise.

Left("tulip").isLeft // true
Right("venus fly-trap").isLeft // false
Source
Either.scala

Returns true if this is a Right, false otherwise.

Returns true if this is a Right, false otherwise.

Left("tulip").isRight // false
Right("venus fly-trap").isRight // true
Source
Either.scala

Concrete methods

final
def contains[B1 >: B](elem: B1): Boolean

Returns true if this is a Right and its value is equal to elem (as determined by ==), returns false otherwise.

Returns true if this is a Right and its value is equal to elem (as determined by ==), returns false otherwise.

// Returns true because value of Right is "something" which equals "something".
Right("something") contains "something"

// Returns false because value of Right is "something" which does not equal "anything".
Right("something") contains "anything"

// Returns false because it's not a Right value.
Left("something") contains "something"
Value Params
elem

the element to test.

Returns

true if this is a Right value equal to elem.

Source
Either.scala
def exists(p: B => Boolean): Boolean

Returns false if Left or returns the result of the application of the given predicate to the Right value.

Returns false if Left or returns the result of the application of the given predicate to the Right value.

Right(12).exists(_ > 10)   // true
Right(7).exists(_ > 10)    // false
Left(12).exists(_ => true) // false
Source
Either.scala
def filterOrElse[A1 >: A](p: B => Boolean, zero: => A1): Either[A1, B]

Returns Right with the existing value of Right if this is a Right and the given predicate p holds for the right value, or Left(zero) if this is a Right and the given predicate p does not hold for the right value, or Left with the existing value of Left if this is a Left.

Returns Right with the existing value of Right if this is a Right and the given predicate p holds for the right value, or Left(zero) if this is a Right and the given predicate p does not hold for the right value, or Left with the existing value of Left if this is a Left.

Right(12).filterOrElse(_ > 10, -1)   // Right(12)
Right(7).filterOrElse(_ > 10, -1)    // Left(-1)
Left(7).filterOrElse(_ => false, -1) // Left(7)
Source
Either.scala
def flatMap[A1 >: A, B1](f: B => Either[A1, B1]): Either[A1, B1]

Binds the given function across Right.

Binds the given function across Right.

Value Params
f

The function to bind across Right.

Source
Either.scala
def flatten[A1 >: A, B1](implicit ev: B <:< Either[A1, B1]): Either[A1, B1]

Returns the right value if this is right or this value if this is left

Returns the right value if this is right or this value if this is left

Example

val  l: Either[String, Either[String, Int]] = Left("pancake")
val rl: Either[String, Either[String, Int]] = Right(Left("flounder"))
val rr: Either[String, Either[String, Int]] = Right(Right(7))
l.flatten //Either[String, Int]: Left("pancake")
rl.flatten //Either[String, Int]: Left("flounder")
rr.flatten //Either[String, Int]: Right(7)

Equivalent to flatMap(id => id)

Source
Either.scala
def fold[C](fa: A => C, fb: B => C): C

Applies fa if this is a Left or fb if this is a Right.

Applies fa if this is a Left or fb if this is a Right.

Value Params
fa

the function to apply if this is a Left

fb

the function to apply if this is a Right

Returns

the results of applying the function

Example

val result = util.Try("42".toInt).toEither
result.fold(
  e => s"Operation failed with $e",
  v => s"Operation produced value: $v"
)
Source
Either.scala
def forall(f: B => Boolean): Boolean

Returns true if Left or returns the result of the application of the given predicate to the Right value.

Returns true if Left or returns the result of the application of the given predicate to the Right value.

Right(12).forall(_ > 10)    // true
Right(7).forall(_ > 10)     // false
Left(12).forall(_ => false) // true
Source
Either.scala
def foreach[U](f: B => U): Unit

Executes the given side-effecting function if this is a Right.

Executes the given side-effecting function if this is a Right.

Right(12).foreach(println) // prints "12"
Left(12).foreach(println)  // doesn't print
Value Params
f

The side-effecting function to execute.

Source
Either.scala
def getOrElse[B1 >: B](or: => B1): B1

Returns the value from this Right or the given argument if this is a Left.

Returns the value from this Right or the given argument if this is a Left.

Right(12).getOrElse(17) // 12
Left(12).getOrElse(17)  // 17
Source
Either.scala
def joinLeft[A1 >: A, B1 >: B, C](implicit ev: A1 <:< Either[C, B1]): Either[C, B1]

Joins an Either through Left.

Joins an Either through Left.

This method requires that the left side of this Either is itself an Either type. That is, this must be some type like:

Either[Either[C, B], B]

(which respects the type parameter bounds, shown below.)

If this instance is a Left[Either[C, B]] then the contained Either[C, B] will be returned, otherwise this value will be returned unmodified.

Left[Either[Int, String], String](Right("flower")).joinLeft // Result: Right("flower")
Left[Either[Int, String], String](Left(12)).joinLeft // Result: Left(12)
Right[Either[Int, String], String]("daisy").joinLeft // Result: Right("daisy")

This method, and joinRight, are analogous to Option#flatten.

Source
Either.scala
def joinRight[A1 >: A, B1 >: B, C](implicit ev: B1 <:< Either[A1, C]): Either[A1, C]

Joins an Either through Right.

Joins an Either through Right.

This method requires that the right side of this Either is itself an Either type. That is, this must be some type like:

Either[A, Either[A, C]]

(which respects the type parameter bounds, shown below.)

If this instance is a Right[Either[A, C]] then the contained Either[A, C] will be returned, otherwise this value will be returned unmodified.

Example

Right[String, Either[String, Int]](Right(12)).joinRight // Result: Right(12)
Right[String, Either[String, Int]](Left("flower")).joinRight // Result: Left("flower")
Left[String, Either[String, Int]]("flower").joinRight // Result: Left("flower")

This method, and joinLeft, are analogous to Option#flatten

Source
Either.scala

Projects this Either as a Left.

Projects this Either as a Left.

This allows for-comprehensions over the left side of Either instances, reversing Either's usual right-bias.

For example

for (s <- Left("flower").left) yield s.length // Left(6)

Continuing the analogy with scala.Option, a LeftProjection declares that Left should be analogous to Some in some code.

// using Option
def interactWithDB(x: Query): Option[Result] =
  try Some(getResultFromDatabase(x))
  catch {
    case _: SQLException => None
  }

// this will only be executed if interactWithDB returns a Some
val report = for (result <- interactWithDB(someQuery)) yield generateReport(result)
report match {
  case Some(r) => send(r)
  case None    => log("report not generated, not sure why...")
}

// using Either
def interactWithDB(x: Query): Either[Exception, Result] =
  try Right(getResultFromDatabase(x))
  catch {
    case e: SQLException => Left(e)
  }

 // run a report only if interactWithDB returns a Right
 val report = for (result <- interactWithDB(someQuery)) yield generateReport(result)
 report match {
   case Right(r) => send(r)
   case Left(e)  => log(s"report not generated, reason was $e")
 }
 // only report errors
 for (e <- interactWithDB(someQuery).left) log(s"query failed, reason was $e")
Source
Either.scala
def map[B1](f: B => B1): Either[A, B1]

The given function is applied if this is a Right.

The given function is applied if this is a Right.

Right(12).map(x => "flower") // Result: Right("flower")
Left(12).map(x => "flower")  // Result: Left(12)
Source
Either.scala
def orElse[A1 >: A, B1 >: B](or: => Either[A1, B1]): Either[A1, B1]

Returns this Right or the given argument if this is a Left.

Returns this Right or the given argument if this is a Left.

Right(1) orElse Left(2) // Right(1)
Left(1) orElse Left(2)  // Left(2)
Left(1) orElse Left(2) orElse Right(3) // Right(3)
Source
Either.scala
def swap: Either[B, A]

If this is a Left, then return the left value in Right or vice versa.

If this is a Left, then return the left value in Right or vice versa.

Example

val left: Either[String, Int]  = Left("left")
val right: Either[Int, String] = left.swap // Result: Right("left")

val right = Right(2)
val left  = Left(3)
for {
  r1 <- right
  r2 <- left.swap
} yield r1 * r2 // Right(6)
Source
Either.scala

Returns a Some containing the Right value if it exists or a None if this is a Left.

Returns a Some containing the Right value if it exists or a None if this is a Left.

Right(12).toOption // Some(12)
Left(12).toOption  // None
Source
Either.scala
def toSeq: Seq[B]

Returns a Seq containing the Right value if it exists or an empty Seq if this is a Left.

Returns a Seq containing the Right value if it exists or an empty Seq if this is a Left.

Right(12).toSeq // Seq(12)
Left(12).toSeq  // Seq()
Source
Either.scala
def toTry(implicit ev: A <:< Throwable): Try[B]

Deprecated methods

@deprecated("Either is now right-biased, use methods directly on Either", "2.13.0")

Projects this Either as a Right.

Projects this Either as a Right.

Because Either is right-biased, this method is not normally needed.

Deprecated
Source
Either.scala

Inherited methods

def canEqual(that: Any): Boolean

A method that should be called from every well-designed equals method that is open to be overridden in a subclass.

A method that should be called from every well-designed equals method that is open to be overridden in a subclass. See Programming in Scala, Chapter 28 for discussion and design.

Value Params
that

the value being probed for possible equality

Returns

true if this instance can possibly equal that, otherwise false

Inherited from
Equals
Source
Equals.scala

The size of this product.

The size of this product.

Returns

for a product A(x1, ..., xk), returns k

Inherited from
Product
Source
Product.scala

The nth element of this product, 0-based.

The nth element of this product, 0-based. In other words, for a product A(x1, ..., xk), returns x(n+1) where 0 <= n < k.

Value Params
n

the index of the element to return

Returns

the element n elements after the first element

Throws
IndexOutOfBoundsException

if the n is out of range(n < 0 || n >= productArity).

Inherited from
Product
Source
Product.scala

The name of the nth element of this product, 0-based.

The name of the nth element of this product, 0-based. In the default implementation, an empty string.

Value Params
n

the index of the element name to return

Returns

the name of the specified element

Throws
IndexOutOfBoundsException

if the n is out of range(n < 0 || n >= productArity).

Inherited from
Product
Source
Product.scala

An iterator over the names of all the elements of this product.

An iterator over the names of all the elements of this product.

Inherited from
Product
Source
Product.scala

An iterator over all the elements of this product.

An iterator over all the elements of this product.

Returns

in the default implementation, an Iterator[Any]

Inherited from
Product
Source
Product.scala

A string used in the toString methods of derived classes.

A string used in the toString methods of derived classes. Implementations may override this method to prepend a string prefix to the result of toString methods.

Returns

in the default implementation, the empty string

Inherited from
Product
Source
Product.scala