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Either 

Companion object Either

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

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

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

import scala.io.StdIn._
val in = readLine("Type Either a string or an Int: ")
val result: Either[String,Int] =
try Right(in.toInt)
catch {
case e: NumberFormatException => Left(in)
}

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

Either is right-biased, which means that Right is assumed to be the default case to operate on. If it is Left, operations like map and flatMap return the Left value unchanged:

def doubled(i: Int) = i * 2
Right(42).map(doubled) // Right(84)
Left(42).map(doubled)  // Left(42)

Since Either defines the methods map and flatMap, it can also be used in for comprehensions:

val right1 = Right(1)   : Right[Double, Int]
val right2 = Right(2)
val right3 = Right(3)
val left23 = Left(23.0) : Left[Double, Int]
val left42 = Left(42.0)

for {
x <- right1
y <- right2
z <- right3
} yield x + y + z // Right(6)

for {
x <- right1
y <- right2
z <- left23
} yield x + y + z // Left(23.0)

for {
x <- right1
y <- left23
z <- right2
} yield x + y + z // Left(23.0)

// Guard expressions are not supported:
for {
i <- right1
if i > 0
} yield i
// error: value withFilter is not a member of Right[Double,Int]

// Similarly, refutable patterns are not supported:
for (x: Int <- right1) yield x
// error: value withFilter is not a member of Right[Double,Int]

Since for comprehensions use map and flatMap, the types of function parameters used in the expression must be inferred. These types are constrained by the Either values. In particular, because of right-biasing, Left values may require an explicit type argument for type parameter B, the right value. Otherwise, it might be inferred as Nothing.

for {
x <- left23
y <- right1
z <- left42  // type at this position: Either[Double, Nothing]
} yield x + y + z
//            ^
// error: ambiguous reference to overloaded definition,
// both method + in class Int of type (x: Char)Int
// and  method + in class Int of type (x: Byte)Int
// match argument types (Nothing)

for (x <- right2 ; y <- left23) yield x + y  // Left(23.0)
for (x <- right2 ; y <- left42) yield x + y  // error

for {
x <- right1
y <- left42  // type at this position: Either[Double, Nothing]
z <- left23
} yield x + y + z
// Left(42.0), but unexpectedly a `Either[Double,String]`
Source
Either.scala
Version

2.0, 2016-07-15

Since

2.7

Linear Supertypes
Serializable, java.io.Serializable, Product, Equals, AnyRef, Any
Known Subclasses
Ordering
1. Alphabetic
2. By Inheritance
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
1. Hide All
2. Show All
Visibility
1. Public
2. All

Abstract Value Members

1. abstract def canEqual(that: Any)

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

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

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)

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)
Implicit
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
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
()
5. final def ==(arg0: Any)

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()

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
( ... )
Note

not specified by SLS as a member of AnyRef

8. final def contains[B1 >: B](elem: B1)

Returns true if this is a Right and its value is equal to elem (as determined by ==), returns false otherwise.

Returns true if this is a Right and its value is equal to elem (as determined by ==), returns false otherwise.

// Returns true because value of Right is "something" which equals "something".
Right("something") contains "something"

// Returns false because value of Right is "something" which does not equal "anything".
Right("something") contains "anything"

// Returns false because it's not a Right value.
Left("something") contains "something"
elem

the element to test.

returns

true if this is a Right value equal to elem.

9. def ensuring(cond: (Either[A, B]) ⇒ Boolean, msg: ⇒ Any): Either[A, B]
Implicit
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: (Either[A, B]) ⇒ Boolean): Either[A, B]
Implicit
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, msg: ⇒ Any): Either[A, B]
Implicit
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. def ensuring(cond: Boolean): Either[A, B]
Implicit
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
13. final def eq(arg0: AnyRef)

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
14. def equals(arg0: Any)

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
15. def exists(p: (B) ⇒ Boolean)

Returns false if Left or returns the result of the application of the given predicate to the Right value.

Returns false if Left or returns the result of the application of the given predicate to the Right value.

Right(12).exists(_ > 10)   // true
Right(7).exists(_ > 10)    // false
Left(12).exists(_ => true) // false
16. def filterOrElse[A1 >: A](p: (B) ⇒ Boolean, zero: ⇒ A1): Either[A1, B]

Returns Right with the existing value of Right if this is a Right and the given predicate p holds for the right value, or Left(zero) if this is a Right and the given predicate p does not hold for the right value, or Left with the existing value of Left if this is a Left.

Returns Right with the existing value of Right if this is a Right and the given predicate p holds for the right value, or Left(zero) if this is a Right and the given predicate p does not hold for the right value, or Left with the existing value of Left if this is a Left.

Right(12).filterOrElse(_ > 10, -1)   // Right(12)
Right(7).filterOrElse(_ > 10, -1)    // Left(-1)
Left(7).filterOrElse(_ => false, -1) // Left(7)
17. 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
( classOf[java.lang.Throwable] )
Note

not specified by SLS as a member of AnyRef

18. def flatMap[A1 >: A, B1](f: (B) ⇒ Either[A1, B1]): Either[A1, B1]

Binds the given function across Right.

Binds the given function across Right.

f

The function to bind across Right.

19. def fold[C](fa: (A) ⇒ C, fb: (B) ⇒ C): C

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 = util.Try("42".toInt).toEither
result.fold(
e => s"Operation failed with \$e",
v => s"Operation produced value: \$v"
)
20. def forall(f: (B) ⇒ Boolean)

Returns true if Left or returns the result of the application of the given predicate to the Right value.

Returns true if Left or returns the result of the application of the given predicate to the Right value.

Right(12).forall(_ > 10)    // true
Right(7).forall(_ > 10)     // false
Left(12).forall(_ => false) // true
21. def foreach[U](f: (B) ⇒ U): Unit

Executes the given side-effecting function if this is a Right.

Executes the given side-effecting function if this is a Right.

Right(12).foreach(println) // prints "12"
Left(12).foreach(println)  // doesn't print
f

The side-effecting function to execute.

22. def formatted(fmtstr: 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
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
()
23. final def getClass(): Class[_]

Returns the runtime class representation of the object.

Returns the runtime class representation of the object.

returns

a class object corresponding to the runtime type of the receiver.

Definition Classes
AnyRef → Any
24. def getOrElse[B1 >: B](or: ⇒ B1): B1

Returns the value from this Right or the given argument if this is a Left.

Returns the value from this Right or the given argument if this is a Left.

Right(12).getOrElse(17) // 12
Left(12).getOrElse(17)  // 17
25. 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
26. final def isInstanceOf[T0]

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
27. 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.

28. 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

29. def left: LeftProjection[A, B]

Projects this Either as a Left.

Projects this Either as a Left.

This allows for-comprehensions over the left side of Either instances, reversing Either's usual right-bias.

For example

for (s <- Left("flower").left) yield s.length // Left(6)

Continuing the analogy with scala.Option, a LeftProjection declares that Left should be analogous to Some in some code.

// using Option
def interactWithDB(x: Query): Option[Result] =
try Some(getResultFromDatabase(x))
catch {
case _: SQLException => None
}

// this will only be executed if interactWithDB returns a Some
val report = for (result <- interactWithDB(someQuery)) yield generateReport(result)
report match {
case Some(r) => send(r)
case None    => log("report not generated, not sure why...")
}

// using Either
def interactWithDB(x: Query): Either[Exception, Result] =
try Right(getResultFromDatabase(x))
catch {
case e: SQLException => Left(e)
}

// run a report only if interactWithDB returns a Right
val report = for (result <- interactWithDB(someQuery)) yield generateReport(result)
report match {
case Right(r) => send(r)
case Left(e)  => log(s"report not generated, reason was \$e")
}
// only report errors
for (e <- interactWithDB(someQuery).left) log(s"query failed, reason was \$e")
30. def map[B1](f: (B) ⇒ B1): Either[A, B1]

The given function is applied if this is a Right.

The given function is applied if this is a Right.

Right(12).map(x => "flower") // Result: Right("flower")
Left(12).map(x => "flower")  // Result: Left(12)
31. def merge: B
Implicit
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
32. final def ne(arg0: AnyRef)

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
33. 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

34. 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

35. 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
36. 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
37. def right: RightProjection[A, B]

Projects this Either as a Right.

Projects this Either as a Right.

Because Either is right-biased, this method is not normally needed.

38. 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.

Examples:
1. val right = Right(2)
val left  = Left(3)
for {
r1 <- right
r2 <- left.swap
} yield r1 * r2 // Right(6)
2. ,
3. val left: Either[String, Int]  = Left("left")
val right: Either[Int, String] = left.swap // Result: Right("left")
39. final def synchronized[T0](arg0: ⇒ T0): T0
Definition Classes
AnyRef
40. def toOption: Option[B]

Returns a Some containing the Right value if it exists or a None if this is a Left.

Returns a Some containing the Right value if it exists or a None if this is a Left.

Right(12).toOption // Some(12)
Left(12).toOption  // None
41. def toSeq

Returns a Seq containing the Right value if it exists or an empty Seq if this is a Left.

Returns a Seq containing the Right value if it exists or an empty Seq if this is a Left.

Right(12).toSeq // Seq(12)
Left(12).toSeq  // Seq()
42. 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
43. def toTry(implicit ev: <:<[A, Throwable]): Try[B]
44. final def wait(): Unit
Definition Classes
AnyRef
Annotations
( ... )
45. final def wait(arg0: Long, arg1: Int): Unit
Definition Classes
AnyRef
Annotations
( ... )
46. final def wait(arg0: Long): Unit
Definition Classes
AnyRef
Annotations
( ... )
47. def [B](y: B): (Either[A, B], B)
Implicit
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