scala.util

# Either

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

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
Type Hierarchy Learn more about scaladoc diagrams
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Inherited

1. Either
2. AnyRef
3. Any

Implicitly

1. by MergeableEither
2. by any2stringadd
3. by StringFormat
4. by Ensuring
5. by ArrowAssoc

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### Instance Constructors

1. #### new Either()

### Abstract Value Members

1. #### 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

2. #### 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

1. #### final def !=(arg0: Any): Boolean

   Definition Classes
     AnyRef → Any

2. #### final def ##(): Int

   Definition Classes
     AnyRef → Any

3. #### def +(other: String): String

   Implicit information
     This member is added by an implicit conversion from Either[A, B] to any2stringadd[Either[A, B]] performed by method any2stringadd in scala.Predef.
   Definition Classes
     any2stringadd

4. #### def ->[B](y: B): (Either[A, B], B)

   Implicit information
     This member is added by an implicit conversion from Either[A, B] to ArrowAssoc[Either[A, B]] performed by method ArrowAssoc in scala.Predef.
   Definition Classes
     ArrowAssoc
   Annotations
     @inline()

5. #### final def ==(arg0: Any): Boolean

   Definition Classes
     AnyRef → Any

6. #### final def asInstanceOf[T0]: T0

   Definition Classes
     Any

7. #### def clone(): AnyRef

   Attributes
     protected[java.lang]
   Definition Classes
     AnyRef
   Annotations
     @throws( ... )

8. #### def ensuring(cond: (Either[A, B]) ⇒ Boolean, msg: ⇒ Any): Either[A, B]

   Implicit information
     This member is added by an implicit conversion from Either[A, B] to Ensuring[Either[A, B]] performed by method Ensuring in scala.Predef.
   Definition Classes
     Ensuring

9. #### def ensuring(cond: (Either[A, B]) ⇒ Boolean): Either[A, B]

   Implicit information
     This member is added by an implicit conversion from Either[A, B] to Ensuring[Either[A, B]] performed by method Ensuring in scala.Predef.
   Definition Classes
     Ensuring

10. #### def ensuring(cond: Boolean, msg: ⇒ Any): Either[A, B]

    Implicit information
      This member is added by an implicit conversion from Either[A, B] to Ensuring[Either[A, B]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring

11. #### def ensuring(cond: Boolean): Either[A, B]

    Implicit information
      This member is added by an implicit conversion from Either[A, B] to Ensuring[Either[A, B]] performed by method Ensuring in scala.Predef.
    Definition Classes
      Ensuring

12. #### final def eq(arg0: AnyRef): Boolean

    Definition Classes
      AnyRef

13. #### def equals(arg0: Any): Boolean

    Definition Classes
      AnyRef → Any

14. #### def finalize(): Unit

    Attributes
      protected[java.lang]
    Definition Classes
      AnyRef
    Annotations
      @throws( classOf[java.lang.Throwable] )

15. #### 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
           ))

16. #### def formatted(fmtstr: String): String

    Returns string formatted according to given `format` string.

    Returns string formatted according to given `format` string. Format strings are as for `String.format` (@see java.lang.String.format).

    Implicit information
      This member is added by an implicit conversion from Either[A, B] to StringFormat[Either[A, B]] performed by method StringFormat in scala.Predef.
    Definition Classes
      StringFormat
    Annotations
      @inline()

17. #### final def getClass(): Class[_]

    Definition Classes
      AnyRef → Any

18. #### def hashCode(): Int

    Definition Classes
      AnyRef → Any

19. #### final def isInstanceOf[T0]: Boolean

    Definition Classes
      Any

20. #### 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`

21. #### 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`

22. #### def left: LeftProjection[A, B]

    Projects this `Either` as a `Left`.

23. #### def merge: B

    Implicit information
      This member is added by an implicit conversion from Either[A, B] to MergeableEither[B] performed by method MergeableEither in scala.util.Either.
    Definition Classes
      MergeableEither

24. #### final def ne(arg0: AnyRef): Boolean

    Definition Classes
      AnyRef

25. #### final def notify(): Unit

    Definition Classes
      AnyRef

26. #### final def notifyAll(): Unit

    Definition Classes
      AnyRef

27. #### def right: RightProjection[A, B]

    Projects this `Either` as a `Right`.

28. #### 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")

29. #### final def synchronized[T0](arg0: ⇒ T0): T0

    Definition Classes
      AnyRef

30. #### def toString(): String

    Definition Classes
      AnyRef → Any

31. #### final def wait(): Unit

    Definition Classes
      AnyRef
    Annotations
      @throws( ... )

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

    Definition Classes
      AnyRef
    Annotations
      @throws( ... )

33. #### final def wait(arg0: Long): Unit

    Definition Classes
      AnyRef
    Annotations
      @throws( ... )

34. #### def →[B](y: B): (Either[A, B], B)

    Implicit information
      This member is added by an implicit conversion from Either[A, B] to ArrowAssoc[Either[A, B]] performed by method ArrowAssoc in scala.Predef.
    Definition Classes
      ArrowAssoc

### Inherited from AnyRef

### Inherited from Any

### Inherited by implicit conversion MergeableEither from Either[A, B] to MergeableEither[B]

### Inherited by implicit conversion any2stringadd from Either[A, B] to any2stringadd[Either[A, B]]

### Inherited by implicit conversion StringFormat from Either[A, B] to StringFormat[Either[A, B]]

### Inherited by implicit conversion Ensuring from Either[A, B] to Ensuring[Either[A, B]]

### Inherited by implicit conversion ArrowAssoc from Either[A, B] to ArrowAssoc[Either[A, B]]

### Ungrouped