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Either

scala.util.Either

See theEither companion object

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

// To use a filtered value, convert to an Option first,
// which drops the Left case, as None contains no value:
for {
  i <- right1.toOption
  if i > 0
} yield i

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]`

Type parameters

A: the type of the Left value
B: the type of the Right value

Attributes

Companion: object
Source: Either.scala
Graph
Supertypes: trait Serializable

trait Product

trait Equals

class Object

trait Matchable

class Any
Show all
Known subtypes: class Left[A, B]

class Right[A, B]

Members list

Value members

Abstract methods

Returns true if this is a Left, false otherwise.

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

Attributes

Returns: true if this is a Left, false otherwise
Source: Either.scala

Returns true if this is a Right, false otherwise.

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

Attributes

Returns: true if this is a Right, false otherwise
Source: Either.scala

Concrete methods

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"

Type parameters

B1: the supertype of B used to widen the comparison type

Value parameters

elem: the element to test.

Attributes

Returns: true if this is a Right value equal to elem.
Source: Either.scala

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

Value parameters

p: the predicate to apply to the Right value

Attributes

Returns: true if this is a Right and the predicate holds for its value
Source: Either.scala

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)

Type parameters

A1: the supertype of A used to widen the left type

Value parameters

p: the predicate to test the Right value against
zero: the value to use as Left if the predicate does not hold, evaluated lazily

Attributes

Returns: this Either if it is a Left or the predicate holds, otherwise Left(zero)
Source: Either.scala

Binds the given function across Right.

Type parameters

A1: the supertype of A used to widen the left type
B1: the right type of the resulting Either

Value parameters

f: the function to bind across Right

Attributes

Source: Either.scala

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

Type parameters

A1: the supertype of A used to widen the left type
B1: the right type of the inner Either

Value parameters

ev: evidence that B is a subtype of Either[A1, B1]

Attributes

Returns

the inner Either if this is a Right, otherwise the outer 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

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

Type parameters

C: the result type of the fold

Value parameters

fa: the function to apply to the Left value
fb: the function to apply to the Right value

Attributes

Returns

the result of applying fa or fb to the contained value

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

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

Value parameters

f: the predicate to apply to the Right value

Attributes

Returns: true if this is a Left or the predicate holds for the Right value
Source: Either.scala

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

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

Type parameters

U: the return type of the side-effecting function (discarded)

Value parameters

f: the side-effecting function to execute

Attributes

Source: Either.scala

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

Type parameters

B1: the supertype of B used to widen the return type

Value parameters

or: the default value to return if this is a Left, evaluated lazily

Attributes

Returns: the Right value if present, otherwise or
Source: Either.scala

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.

@tparam A1 the supertype of A constrained to be an Either[C, B1] @tparam B1 the supertype of B used to widen the right type @tparam C the left type of the inner Either @param ev evidence that A1 is a subtype of Either[C, B1] @return the inner Either if this is a Left, otherwise this Right value

Attributes

Source: Either.scala

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

@tparam A1 the supertype of A used to widen the left type @tparam B1 the supertype of B, evidenced to be an Either[A1, C] @tparam C the right type of the inner Either @param ev evidence that B1 is a subtype of Either[A1, C] @return the inner Either if this is a Right, otherwise this Left value

Attributes

Source: Either.scala

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")

Attributes

Source: Either.scala

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)

Type parameters

B1: the result type of the mapping function

Value parameters

f: the function to apply to the Right value

Attributes

Returns: a new Either with the function applied if this is a Right, otherwise the unchanged Left
Source: Either.scala

Implicitly added by MergeableEither

Attributes

Source: Either.scala

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)

Type parameters

A1: the supertype of A used to widen the left type
B1: the supertype of B used to widen the right type

Value parameters

or: the alternative Either to return if this is a Left, evaluated lazily

Attributes

Returns: this Either if it is a Right, otherwise or
Source: Either.scala

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

Attributes

Returns

an Either with the left and right values swapped

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.

Right(12).toOption // Some(12)
Left(12).toOption  // None

Attributes

Returns: a Some containing the Right value, or None if this is a Left
Source: Either.scala

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()

Attributes

Returns: a Seq containing the Right value, or an empty Seq if this is a Left
Source: Either.scala

Attributes

Source: Either.scala

Deprecated methods

Projects this Either as a Right.

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

Attributes

Deprecated: [Since version 2.13.0] Either is now right-biased, use methods directly on Either
Source: Either.scala

Inherited methods

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

Value parameters

n: the index of the element name to return

Attributes

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.

Attributes

Inherited from:: Product
Source: Product.scala

An iterator over all the elements of this product.

Attributes

Returns: in the default implementation, an Iterator[Any]
Inherited from:: Product
Source: Product.scala

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.

Attributes

Returns: in the default implementation, the empty string
Inherited from:: Product
Source: Product.scala

Inherited and Abstract methods

Checks whether this instance can possibly equal that.

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 parameters

that: the value being probed for possible equality

Attributes

Returns: true if this instance can possibly equal that, otherwise false
Inherited from:: Equals
Source: Equals.scala

The size of this product.

Attributes

Returns: for a product A(x,,1,,, ..., x,,k,,), returns k
Inherited from:: Product
Source: Product.scala

The n^th^ element of this product, 0-based. In other words, for a product A(x,,1,,, ..., x,,k,,), returns x,,(n+1),, where `0 = productArity).

Attributes

Returns: the element n elements after the first element
Inherited from:: Product
Source: Product.scala

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