Class scala.util.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]`

Abstract methods

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def isLeft: Boolean

Returns true if this is a Left, false otherwise.

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

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def isRight: Boolean

Returns true if this is a Right, false otherwise.

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

Concrete methods

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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 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"

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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.

Right(12).exists(_ > 10)   // true
Right(7).exists(_ > 10)    // false
Left(12).exists(_ => true) // false

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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.

Right(12).filterOrElse(_ > 10, -1)   // Right(12)
Right(7).filterOrElse(_ > 10, -1)    // Left(-1)
Left(7).filterOrElse(_ => false, -1) // Left(7)

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def flatMap[A1 >: A, B1](f: B => Either[A1, B1]): Either[A1, B1]

Binds the given function across Right.

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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

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)

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def fold[C](fa: A => C, fb: B => C): C

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

val result = util.Try("42".toInt).toEither
result.fold(
  e => s"Operation failed with $e",
  v => s"Operation produced value: $v"
)

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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.

Right(12).forall(_ > 10)    // true
Right(7).forall(_ > 10)     // false
Left(12).forall(_ => false) // true

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def foreach[U](f: B => U): Unit

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

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

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def getOrElse[B1 >: B](or: => B1): B1

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

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def joinLeft[A1 >: A, B1 >: B, C](implicit ev: A1 <:< Either[C, B1]): Either[C, B1]

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.

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

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.

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")

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def left: LeftProjection[A, B]

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")

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def map[B1](f: B => B1): Either[A, B1]

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)

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def merge: A

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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.

Right(1) orElse Left(2) // Right(1)
Left(1) orElse Left(2)  // Left(2)
Left(1) orElse Left(2) orElse Right(3) // Right(3)

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def swap: Either[B, A]

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

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)

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def toOption: Option[B]

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

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def toSeq: Seq[B]

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()

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def toTry(implicit ev: A <:< Throwable): Try[B]

Inherited methods

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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. See Programming in Scala, Chapter 28 for discussion and design.

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def productArity: Int

The size of this product.

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def productElement(n: Int): Any

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.

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def productElementName(n: Int): String

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

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def productElementNames: Iterator[String]

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

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def productIterator: Iterator[Any]

An iterator over all the elements of this product.

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def productPrefix: String

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.