Either

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

A common use of Either is as an alternative to scala.Option for dealing with possible 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 detect whether a received input is a String or an Int.

val in = Console.readLine("Type Either a string or an Int: ")
val result: Either[String,Int] = try {
    Right(in.toInt)
  } catch {
    case e: Exception =>
      Left(in)
}

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

A projection can be used to selectively operate on a value of type Either, depending on whether it is of type Left or Right. For example, to transform an Either using a function, in the case where it's a Left, one can first apply the left projection and invoke map on that projected Either. If a right projection is applied to that Left, the original Left is returned, unmodified.

val l: Either[String, Int] = Left("flower")
val r: Either[String, Int] = Right(12)
l.left.map(_.size): Either[Int, Int] // Left(6)
r.left.map(_.size): Either[Int, Int] // Right(12)
l.right.map(_.toDouble): Either[String, Double] // Left("flower")
r.right.map(_.toDouble): Either[String, Double] // Right(12.0)

Like with other types which define a map method, the same can be achieved using a for-comprehension:

for (s <- l.left) yield s.size // Left(6)

To support multiple projections as generators in for-comprehensions, the Either type also defines a flatMap method.

Version: 1.0, 11/10/2008
Since: 2.7

Linear Supertypes

AnyRef, Any

Known Subclasses

Left, Right

Abstract Value Members

abstract def isLeft: Boolean

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
```
abstract def isRight: Boolean

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
```

Concrete Value Members

final def !=(arg0: Any): Boolean

Definition Classes
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final def ##(): Int

Definition Classes
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final def ==(arg0: Any): Boolean

Definition Classes
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final def asInstanceOf[T0]: T0

Definition Classes
Any
def clone(): AnyRef

Attributes
protected[java.lang]
Definition Classes
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Annotations
@throws( ... )
final def eq(arg0: AnyRef): Boolean

Definition Classes
AnyRef
def equals(arg0: Any): Boolean

Definition Classes
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def finalize(): Unit

Attributes
protected[java.lang]
Definition Classes
AnyRef
Annotations
@throws( classOf[java.lang.Throwable] )
def fold[X](fa: (A) ⇒ X, fb: (B) ⇒ X): X

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.
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:
1. val result: Either[Exception, Value] = possiblyFailingOperation() log(result.fold( ex => "Operation failed with " + ex, v => "Operation produced value: " + v ))
final def getClass(): Class[_]

Definition Classes
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def hashCode(): Int

Definition Classes
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final def isInstanceOf[T0]: Boolean

Definition Classes
Any
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
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:
1. 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
def left: LeftProjection[A, B]

Projects this Either as a Left.
final def ne(arg0: AnyRef): Boolean

Definition Classes
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final def notify(): Unit

Definition Classes
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final def notifyAll(): Unit

Definition Classes
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def right: RightProjection[A, B]

Projects this Either as a Right.
def swap: Product with Serializable with 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:
1. val l: Either[String, Int] = Left("left") val r: Either[Int, String] = l.swap // Result: Right("left")
final def synchronized[T0](arg0: ⇒ T0): T0

Definition Classes
AnyRef
def toString(): String

Definition Classes
AnyRef → Any
final def wait(): Unit

Definition Classes
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Annotations
@throws( ... )
final def wait(arg0: Long, arg1: Int): Unit

Definition Classes
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Annotations
@throws( ... )
final def wait(arg0: Long): Unit

Definition Classes
AnyRef
Annotations
@throws( ... )

Related Docs: object Either | package util

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

Abstract Value Members

abstract def isLeft: Boolean

abstract def isRight: Boolean

Concrete Value Members

final def !=(arg0: Any): Boolean

final def ##(): Int

final def ==(arg0: Any): Boolean

final def asInstanceOf[T0]: T0

def clone(): AnyRef

final def eq(arg0: AnyRef): Boolean

def equals(arg0: Any): Boolean

def finalize(): Unit

def fold[X](fa: (A) ⇒ X, fb: (B) ⇒ X): X

final def getClass(): Class[_]

def hashCode(): Int

final def isInstanceOf[T0]: Boolean

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

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

def left: LeftProjection[A, B]

final def ne(arg0: AnyRef): Boolean

final def notify(): Unit

final def notifyAll(): Unit

def right: RightProjection[A, B]

def swap: Product with Serializable with Either[B, A]

final def synchronized[T0](arg0: ⇒ T0): T0

def toString(): String

final def wait(): Unit

final def wait(arg0: Long, arg1: Int): Unit

final def wait(arg0: Long): Unit

Inherited from AnyRef

Inherited from Any

Ungrouped