scala

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 Left or Right.

A common use of Either is as an alternative to Option for dealing with possible missing values. In this usage, scala.None is replaced with a Left which can contain useful information. Right takes the place of 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

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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: AnyRef): Boolean

    Definition Classes
    AnyRef
  2. final def !=(arg0: Any): Boolean

    Test two objects for inequality.

    Test two objects for inequality.

    returns

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

    Definition Classes
    Any
  3. final def ##(): Int

    Equivalent to x.hashCode except for boxed numeric types.

    Equivalent to x.hashCode except for boxed numeric types. 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.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  4. final def ==(arg0: AnyRef): Boolean

    Definition Classes
    AnyRef
  5. final def ==(arg0: Any): Boolean

    Test two objects for equality.

    Test two objects for equality.

    returns

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

    Definition Classes
    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[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws()
    Note

    not specified by SLS as a member of AnyRef

  8. final def eq(arg0: AnyRef): Boolean

    Tests whether the argument (arg0) is a reference to the receiver object (this).

    Tests whether the argument (arg0) 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
  9. def equals(arg0: Any): Boolean

    The equality method for reference types.

    The equality method for reference types. See equals in Any.

    returns

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

    Definition Classes
    AnyRef → Any
  10. 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[lang]
    Definition Classes
    AnyRef
    Annotations
    @throws()
    Note

    not specified by SLS as a member of AnyRef

  11. 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
      ))
      
  12. final def getClass(): java.lang.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

  13. def hashCode(): Int

    The hashCode method for reference types.

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

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
  14. 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
  15. 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

  16. 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

  17. def left: LeftProjection[A, B]

    Projects this Either as a Left.

  18. 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
  19. 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

  20. 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

  21. def right: RightProjection[A, B]

    Projects this Either as a Right.

  22. 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")
      
  23. final def synchronized[T0](arg0: ⇒ T0): T0

    Definition Classes
    AnyRef
  24. 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
  25. final def wait(): Unit

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

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

    Definition Classes
    AnyRef
    Annotations
    @throws()

Inherited from AnyRef

Inherited from Any