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
- Since
2.7
- Alphabetic
- By Inheritance
- Either
- Serializable
- Serializable
- Product
- Equals
- AnyRef
- Any
- by MergeableEither
- by any2stringadd
- by StringFormat
- by Ensuring
- by ArrowAssoc
- Hide All
- Show All
- Public
- All
Abstract Value Members
-
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
-
abstract
def
isLeft: Boolean
Returns
true
if this is aLeft
,false
otherwise.Returns
true
if this is aLeft
,false
otherwise.Left("tulip").isLeft // true Right("venus fly-trap").isLeft // false
-
abstract
def
isRight: Boolean
Returns
true
if this is aRight
,false
otherwise.Returns
true
if this is aRight
,false
otherwise.Left("tulip").isRight // false Right("venus fly-trap").isRight // true
-
abstract
def
productArity: Int
The size of this product.
-
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)
, returnsx(n+1)
where0 <= 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
-
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
-
final
def
##(): Int
Equivalent to
x.hashCode
except for boxed numeric types andnull
.Equivalent to
x.hashCode
except for boxed numeric types andnull
. 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. Fornull
returns a hashcode wherenull.hashCode
throws aNullPointerException
.- returns
a hash value consistent with ==
- Definition Classes
- AnyRef → Any
-
def
+(other: String): 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
-
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
- @inline()
-
final
def
==(arg0: Any): Boolean
The expression
x == that
is equivalent toif (x eq null) that eq null else x.equals(that)
.The expression
x == that
is equivalent toif (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
-
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 aClassCastException
at runtime, while the expressionList(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 typeT0
.
-
def
clone(): AnyRef
Create a copy of the receiver object.
-
final
def
contains[B1 >: B](elem: B1): Boolean
Returns
true
if this is aRight
and its value is equal toelem
(as determined by==
), returnsfalse
otherwise.Returns
true
if this is aRight
and its value is equal toelem
(as determined by==
), returnsfalse
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 aRight
value equal toelem
.
-
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
-
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
-
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
-
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
-
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 ofAnyRef
, and has three additional properties:- It is consistent: for any non-null instances
x
andy
of typeAnyRef
, multiple invocations ofx.eq(y)
consistently returnstrue
or consistently returnsfalse
. - For any non-null instance
x
of typeAnyRef
,x.eq(null)
andnull.eq(x)
returnsfalse
. null.eq(null)
returnstrue
.
When overriding the
equals
orhashCode
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
- It is consistent: for any non-null instances
-
def
equals(arg0: Any): Boolean
The equality method for reference types.
-
def
exists(p: (B) ⇒ Boolean): Boolean
Returns
false
ifLeft
or returns the result of the application of the given predicate to theRight
value.Returns
false
ifLeft
or returns the result of the application of the given predicate to theRight
value.Right(12).exists(_ > 10) // true Right(7).exists(_ > 10) // false Left(12).exists(_ => true) // false
-
def
filterOrElse[A1 >: A](p: (B) ⇒ Boolean, zero: ⇒ A1): Either[A1, B]
Returns
Right
with the existing value ofRight
if this is aRight
and the given predicatep
holds for the right value, orLeft(zero)
if this is aRight
and the given predicatep
does not hold for the right value, orLeft
with the existing value ofLeft
if this is aLeft
.Returns
Right
with the existing value ofRight
if this is aRight
and the given predicatep
holds for the right value, orLeft(zero)
if this is aRight
and the given predicatep
does not hold for the right value, orLeft
with the existing value ofLeft
if this is aLeft
.Right(12).filterOrElse(_ > 10, -1) // Right(12) Right(7).filterOrElse(_ > 10, -1) // Left(-1) Left(7).filterOrElse(_ => false, -1) // Left(7)
-
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 betweenfinalize
and non-local returns and exceptions, are all platform dependent.- Attributes
- protected[lang]
- Definition Classes
- AnyRef
- Annotations
- @throws( classOf[java.lang.Throwable] )
- Note
not specified by SLS as a member of AnyRef
-
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
.
-
def
fold[C](fa: (A) ⇒ C, fb: (B) ⇒ C): C
Applies
fa
if this is aLeft
orfb
if this is aRight
.Applies
fa
if this is aLeft
orfb
if this is aRight
.- 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
val result = util.Try("42".toInt).toEither result.fold( e => s"Operation failed with $e", v => s"Operation produced value: $v" )
Example: -
def
forall(f: (B) ⇒ Boolean): Boolean
Returns
true
ifLeft
or returns the result of the application of the given predicate to theRight
value.Returns
true
ifLeft
or returns the result of the application of the given predicate to theRight
value.Right(12).forall(_ > 10) // true Right(7).forall(_ > 10) // false Left(12).forall(_ => false) // true
-
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.
-
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 forString.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
- @inline()
-
final
def
getClass(): Class[_]
Returns the runtime class representation of the object.
-
def
getOrElse[B1 >: B](or: ⇒ B1): B1
Returns the value from this
Right
or the given argument if this is aLeft
.Returns the value from this
Right
or the given argument if this is aLeft
.Right(12).getOrElse(17) // 12 Left(12).getOrElse(17) // 17
-
def
hashCode(): Int
The hashCode method for reference types.
-
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 returnfalse
, while the expressionList(1).isInstanceOf[List[String]]
will returntrue
. 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 typeT0
;false
otherwise.
- Definition Classes
- Any
-
def
joinLeft[A1 >: A, B1 >: B, C](implicit ev: <:<[A1, Either[C, B1]]): Either[C, B1]
Joins an
Either
throughLeft
.Joins an
Either
throughLeft
.This method requires that the left side of this
Either
is itself anEither
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 containedEither[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 toOption#flatten
. -
def
joinRight[A1 >: A, B1 >: B, C](implicit ev: <:<[B1, Either[A1, C]]): Either[A1, C]
Joins an
Either
throughRight
.Joins an
Either
throughRight
.This method requires that the right side of this
Either
is itself anEither
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 containedEither[A, C]
will be returned, otherwise this value will be returned unmodified.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 toOption#flatten
Example: -
def
left: LeftProjection[A, B]
Projects this
Either
as aLeft
.Projects this
Either
as aLeft
.This allows for-comprehensions over the left side of
Either
instances, reversingEither
'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 thatLeft
should be analogous toSome
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")
-
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)
-
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
-
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
-
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
- Annotations
- @native()
- Note
not specified by SLS as a member of AnyRef
-
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
- Annotations
- @native()
- Note
not specified by SLS as a member of AnyRef
-
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
-
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 oftoString
methods.- returns
in the default implementation, the empty string
- Definition Classes
- Product
-
def
right: RightProjection[A, B]
Projects this
Either
as aRight
.Projects this
Either
as aRight
.Because
Either
is right-biased, this method is not normally needed. -
def
swap: Either[B, A]
If this is a
Left
, then return the left value inRight
or vice versa.If this is a
Left
, then return the left value inRight
or vice versa.val left: Either[String, Int] = Left("left") val right: Either[Int, String] = left.swap // Result: Right("left")
, val right = Right(2) val left = Left(3) for { r1 <- right r2 <- left.swap } yield r1 * r2 // Right(6)
Examples: -
final
def
synchronized[T0](arg0: ⇒ T0): T0
- Definition Classes
- AnyRef
-
def
toOption: Option[B]
Returns a
Some
containing theRight
value if it exists or aNone
if this is aLeft
.Returns a
Some
containing theRight
value if it exists or aNone
if this is aLeft
.Right(12).toOption // Some(12) Left(12).toOption // None
-
def
toSeq: collection.immutable.Seq[B]
Returns a
Seq
containing theRight
value if it exists or an emptySeq
if this is aLeft
.Returns a
Seq
containing theRight
value if it exists or an emptySeq
if this is aLeft
.Right(12).toSeq // Seq(12) Left(12).toSeq // Seq()
-
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
- def toTry(implicit ev: <:<[A, Throwable]): Try[B]
-
final
def
wait(): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
-
final
def
wait(arg0: Long, arg1: Int): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws( ... )
- final def wait(arg0: Long): Unit
-
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
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.collection.parallel.immutable
- Immutable, parallel data-structures such asParVector
,ParRange
,ParHashMap
orParHashSet
scala.collection.parallel.mutable
- Mutable, parallel data-structures such asParArray
,ParHashMap
,ParTrieMap
orParHashSet
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.sys
- Interaction with other processes and the operating systemscala.util.matching
- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect
- Scala's reflection API (scala-reflect.jar)scala.xml
- XML parsing, manipulation, and serialization (scala-xml.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.