scala.util

# Either

###
Related Docs: object Either | package util

#### 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.

Source
  Either.scala
Version

  1.0, 11/10/2008

Since

  2.7

Linear Supertypes
AnyRef, Any
Known Subclasses
Left, Right
Ordering

1. Alphabetic
2. By Inheritance

Inherited

1. Either
2. AnyRef
3. Any

Implicitly

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

1. Hide All
2. Show All

Visibility

1. Public
2. All

### 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

   Test two objects for inequality.

   Test two objects for inequality.

   returns

     `true` if !(this == that), false otherwise.

   Definition Classes
     AnyRef → Any

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

   Equivalent to `x.hashCode` except for boxed numeric types and `null`.

   Equivalent to `x.hashCode` except for boxed numeric types and `null`. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For `null` returns a hashcode where `null.hashCode` throws a `NullPointerException`.

   returns

     a hash value consistent with ==

   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

   The expression `x == that` is equivalent to `if (x eq null) that eq null else x.equals(that)`.

   The expression `x == that` is equivalent to `if (x eq null) that eq null else x.equals(that)`.

   returns

     `true` if the receiver object is equivalent to the argument; `false` otherwise.

   Definition Classes
     AnyRef → Any

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

   Cast the receiver object to be of type `T0`.

   Cast the receiver object to be of type `T0`.

   Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression `1.asInstanceOf[String]` will throw a `ClassCastException` at runtime, while the expression `List(1).asInstanceOf[List[String]]` will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

   returns

     the receiver object.

   Definition Classes
     Any
   Exceptions thrown

     `ClassCastException` if the receiver object is not an instance of the erasure of type `T0`.

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

   Create a copy of the receiver object.

   Create a copy of the receiver object.

   The default implementation of the `clone` method is platform dependent.

   returns

     a copy of the receiver object.

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

     not specified by SLS as a member of AnyRef

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

    Tests whether the argument (`that`) is a reference to the receiver object (`this`).

    Tests whether the argument (`that`) is a reference to the receiver object (`this`).

    The `eq` method implements an equivalence relation on non-null instances of `AnyRef`, and has three additional properties:

    - It is consistent: for any non-null instances `x` and `y` of type `AnyRef`, multiple invocations of `x.eq(y)` consistently returns `true` or consistently returns `false`.
    - For any non-null instance `x` of type `AnyRef`, `x.eq(null)` and `null.eq(x)` returns `false`.
    - `null.eq(null)` returns `true`.

    When overriding the `equals` or `hashCode` methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (`o1 eq o2`), they should be equal to each other (`o1 == o2`) and they should hash to the same value (`o1.hashCode == o2.hashCode`).

    returns

      `true` if the argument is a reference to the receiver object; `false` otherwise.

    Definition Classes
      AnyRef

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

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to `eq`.

    See also `equals` in scala.Any.

    returns

      `true` if the receiver object is equivalent to the argument; `false` otherwise.

    Definition Classes
      AnyRef → Any

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

    Called by the garbage collector on the receiver object when there are no more references to the object.

    Called by the garbage collector on the receiver object when there are no more references to the object.

    The details of when and if the `finalize` method is invoked, as well as the interaction between `finalize` and non-local returns and exceptions, are all platform dependent.

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

      not specified by SLS as a member of AnyRef

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[_]

    A representation that corresponds to the dynamic class of the receiver object.

    A representation that corresponds to the dynamic class of the receiver object.

    The nature of the representation is platform dependent.

    returns

      a representation that corresponds to the dynamic class of the receiver object.

    Definition Classes
      AnyRef → Any
    Note

      not specified by SLS as a member of AnyRef

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

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

      the hash code value for this object.

    Definition Classes
      AnyRef → Any

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

    Test whether the dynamic type of the receiver object is `T0`.

    Test whether the dynamic type of the receiver object is `T0`.

    Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression `1.isInstanceOf[String]` will return `false`, while the expression `List(1).isInstanceOf[List[String]]` will return `true`. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

    returns

      `true` if the receiver object is an instance of erasure of type `T0`; `false` otherwise.

    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

    Equivalent to `!(this eq that)`.

    Equivalent to `!(this eq that)`.

    returns

      `true` if the argument is not a reference to the receiver object; `false` otherwise.

    Definition Classes
      AnyRef

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

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Definition Classes
      AnyRef
    Note

      not specified by SLS as a member of AnyRef

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

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    Definition Classes
      AnyRef
    Note

      not specified by SLS as a member of AnyRef

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

    Projects this `Either` as a `Right`.

28. #### def swap: Serializable with Product 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

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

      a String representation of the object.

    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