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] // To use a filtered value, convert to an Option first, // which drops the Left case, as None contains no value: for { i <- right1.toOption if i > 0 } yield i
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
- Alphabetic
- By Inheritance
- Either
- Serializable
- Product
- Equals
- AnyRef
- Any
- by MergeableEither
- by any2stringadd
- by StringFormat
- by Ensuring
- by ArrowAssoc
- Hide All
- Show All
- Public
- Protected
Abstract Value Members
- abstract def canEqual(that: Any): Boolean
Checks whether this instance can possibly equal
that
.Checks whether this instance can possibly equal
that
.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
IndexOutOfBoundsException
if then
is out of range(n < 0 || n >= productArity).
Concrete Value Members
- 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 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 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 flatten[A1 >: A, B1](implicit ev: <:<[B, Either[A1, B1]]): Either[A1, B1]
Returns the right value if this is right or this value if this is left
Returns the right value if this is right or this value if this is left
val l: Either[String, Either[String, Int]] = Left("pancake") val rl: Either[String, Either[String, Int]] = Right(Left("flounder")) val rr: Either[String, Either[String, Int]] = Right(Right(7)) l.flatten //Either[String, Int]: Left("pancake") rl.flatten //Either[String, Int]: Left("flounder") rr.flatten //Either[String, Int]: Right(7)
Equivalent to
flatMap(id => id)
Example: - 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 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 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] toMergeableEither[B] performed by method MergeableEither in scala.util.Either.This conversion will take place only if B is a superclass of A (B >: A).
- Definition Classes
- MergeableEither
- def orElse[A1 >: A, B1 >: B](or: => Either[A1, B1]): Either[A1, B1]
Returns this
Right
or the given argument if this is aLeft
.Returns this
Right
or the given argument if this is aLeft
.Right(1) orElse Left(2) // Right(1) Left(1) orElse Left(2) // Left(2) Left(1) orElse Left(2) orElse Right(3) // Right(3)
- def productElementName(n: Int): String
The name of the nth element of this product, 0-based.
The name of the nth element of this product, 0-based. In the default implementation, an empty string.
- n
the index of the element name to return
- returns
the name of the specified element
- Definition Classes
- Product
- Exceptions thrown
IndexOutOfBoundsException
if then
is out of range(n < 0 || n >= productArity).
- def productElementNames: Iterator[String]
An iterator over the names of all the elements of this product.
An iterator over the names of all the elements of this product.
- Definition Classes
- Product
- 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 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: - 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 toTry(implicit ev: <:<[A, Throwable]): Try[B]
Deprecated Value Members
- 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.- Annotations
- @deprecated
- Deprecated
(Since version 2.13.0) Either is now right-biased, use methods directly on Either
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.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.collection.parallel
- Parallel collections (scala-parallel-collections.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.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
.