scala.util

Either

Related Docs: object Either | package util

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

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
})
Projections

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)
Biasing Either values

You may also bias Either values, decorating them so they can be operated upon and used in for comprehensions without extracting projections. If we treat Either as analogous to Option, a right-biased Either treats Right instances as good values while Left instances represent failures, more descriptive variations on None. (Either can also be left-biased, but right-biasing is conventional and should be preferred.)

Biased Either values support pattern matches, variable assignments, and conditionals in for comprehensions, which left- and right-projections do not.

You can right-bias Either very simply via the following import statement:

import Either.RightBias._

val a : Either[String,Int] = Right(1)
val b : Either[String,Int] = Right(99)

for( v <- a; w <- b ) yield v+w          // Right(100)
for( v <- a; w <- b if v > 10) yield v+w // throws NoSuchElementException

In order to support filter operations without throwing an Exception upon failure, a right bias may define a left-side token to indicate emptiness.

val RightBias = Either.RightBias.withEmptyToken[String]("EMPTY")
import RightBias._

val a : Either[String,Int] = Right(1)
val b : Either[String,Int] = Right(99)

for( v <- a; w <- b ) yield v+w          // Right(100)
for( v <- a; w <- b if v > 10) yield v+w // Left(EMPTY)

For very extensive documentation of biased Either, please see RightBias (or, if you wish to defy convention, LeftBias).

For documentation of operations supported by biased Either values, see right-biased ops / left-biased ops.

Source
Either.scala
Version

1.0, 11/10/2008

Since

2.7

Linear Supertypes
Serializable, java.io.Serializable, Product, Equals, AnyRef, Any
Known Subclasses
Left, Right
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Inherited
  1. Either
  2. Serializable
  3. Serializable
  4. Product
  5. Equals
  6. AnyRef
  7. Any
Implicitly
  1. by MergeableEither
  2. by any2stringadd
  3. by StringFormat
  4. by Ensuring
  5. by ArrowAssoc
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Abstract Value Members

  1. abstract def canEqual(that: Any): Boolean

    A method that should be called from every well-designed equals method that is open to be overridden in a subclass.

    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.

    that

    the value being probed for possible equality

    returns

    true if this instance can possibly equal that, otherwise false

    Definition Classes
    Equals

  2. 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

  3. 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

  4. abstract def productArity: Int

    The size of this product.

    The size of this product.

    returns

    for a product A(x1, ..., xk), returns k

    Definition Classes
    Product

  5. abstract def productElement(n: Int): Any

    The nth element of this product, 0-based.

    The nth element of this product, 0-based. In other words, for a product A(x1, ..., xk), returns x(n+1) where 0 < n < k.

    n

    the index of the element to return

    returns

    the element n elements after the first element

    Definition Classes
    Product
    Exceptions thrown

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).

    arg0

    the object to compare against this object for equality.

    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 (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

  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 productIterator: Iterator[Any]

    An iterator over all the elements of this product.

    An iterator over all the elements of this product.

    returns

    in the default implementation, an Iterator[Any]

    Definition Classes
    Product

  28. def productPrefix: String

    A string used in the toString methods of derived classes.

    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.

    returns

    in the default implementation, the empty string

    Definition Classes
    Product

  29. def right: RightProjection[A, B]

    Projects this Either as a Right.

  30. def swap: 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")

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

    Definition Classes
    AnyRef

  32. 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

  33. final def wait(): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws( ... )

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

    Definition Classes
    AnyRef
    Annotations
    @throws( ... )

  35. final def wait(arg0: Long): Unit

    Definition Classes
    AnyRef
    Annotations
    @throws( ... )

  36. 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 Serializable

Inherited from java.io.Serializable

Inherited from Product

Inherited from Equals

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