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
trueif this is aLeft,falseotherwise.Returns
trueif this is aLeft,falseotherwise.Left("tulip").isLeft // true Right("venus fly-trap").isLeft // false
-
abstract
def
isRight: Boolean
Returns
trueif this is aRight,falseotherwise.Returns
trueif this is aRight,falseotherwise.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
nelements 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
trueif !(this == that), false otherwise.
- Definition Classes
- AnyRef → Any
-
final
def
##(): Int
Equivalent to
x.hashCodeexcept for boxed numeric types andnull.Equivalent to
x.hashCodeexcept 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. Fornullreturns a hashcode wherenull.hashCodethrows 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 == thatis equivalent toif (x eq null) that eq null else x.equals(that).The expression
x == thatis equivalent toif (x eq null) that eq null else x.equals(that).- returns
trueif the receiver object is equivalent to the argument;falseotherwise.
- 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 aClassCastExceptionat 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
ClassCastExceptionif 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
trueif this is aRightand its value is equal toelem(as determined by==), returnsfalseotherwise.Returns
trueif this is aRightand its value is equal toelem(as determined by==), returnsfalseotherwise.// 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
trueif this is aRightvalue 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
eqmethod implements an equivalence relation on non-null instances ofAnyRef, and has three additional properties:- It is consistent: for any non-null instances
xandyof typeAnyRef, multiple invocations ofx.eq(y)consistently returnstrueor consistently returnsfalse. - For any non-null instance
xof typeAnyRef,x.eq(null)andnull.eq(x)returnsfalse. null.eq(null)returnstrue.
When overriding the
equalsorhashCodemethods, 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
trueif the argument is a reference to the receiver object;falseotherwise.
- 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
falseifLeftor returns the result of the application of the given predicate to theRightvalue.Returns
falseifLeftor returns the result of the application of the given predicate to theRightvalue.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
Rightwith the existing value ofRightif this is aRightand the given predicatepholds for the right value, orLeft(zero)if this is aRightand the given predicatepdoes not hold for the right value, orLeftwith the existing value ofLeftif this is aLeft.Returns
Rightwith the existing value ofRightif this is aRightand the given predicatepholds for the right value, orLeft(zero)if this is aRightand the given predicatepdoes not hold for the right value, orLeftwith the existing value ofLeftif 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
finalizemethod is invoked, as well as the interaction betweenfinalizeand non-local returns and exceptions, are all platform dependent. -
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
faif this is aLeftorfbif this is aRight.Applies
faif this is aLeftorfbif 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
trueifLeftor returns the result of the application of the given predicate to theRightvalue.Returns
trueifLeftor returns the result of the application of the given predicate to theRightvalue.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
formatstring.Returns string formatted according to given
formatstring. 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
Rightor the given argument if this is aLeft.Returns the value from this
Rightor 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
trueif the receiver object is an instance of erasure of typeT0;falseotherwise.
- Definition Classes
- Any
-
def
joinLeft[A1 >: A, B1 >: B, C](implicit ev: <:<[A1, Either[C, B1]]): Either[C, B1]
Joins an
EitherthroughLeft.Joins an
EitherthroughLeft.This method requires that the left side of this
Eitheris itself anEithertype. 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
EitherthroughRight.Joins an
EitherthroughRight.This method requires that the right side of this
Eitheris itself anEithertype. 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
Eitheras aLeft.Projects this
Eitheras aLeft.This allows for-comprehensions over the left side of
Eitherinstances, reversingEither's usual right-bias.For example
for (s <- Left("flower").left) yield s.length // Left(6)
Continuing the analogy with scala.Option, a
LeftProjectiondeclares thatLeftshould be analogous toSomein 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
trueif the argument is not a reference to the receiver object;falseotherwise.
- 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
toStringmethods of derived classes.A string used in the
toStringmethods of derived classes. Implementations may override this method to prepend a string prefix to the result oftoStringmethods.- returns
in the default implementation, the empty string
- Definition Classes
- Product
-
def
right: RightProjection[A, B]
Projects this
Eitheras aRight.Projects this
Eitheras aRight.Because
Eitheris right-biased, this method is not normally needed. -
def
swap: Either[B, A]
If this is a
Left, then return the left value inRightor vice versa.If this is a
Left, then return the left value inRightor 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
Somecontaining theRightvalue if it exists or aNoneif this is aLeft.Returns a
Somecontaining theRightvalue if it exists or aNoneif this is aLeft.Right(12).toOption // Some(12) Left(12).toOption // None
-
def
toSeq: collection.immutable.Seq[B]
Returns a
Seqcontaining theRightvalue if it exists or an emptySeqif this is aLeft.Returns a
Seqcontaining theRightvalue if it exists or an emptySeqif 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,ArrayorOptionwhich are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collectionand its sub-packages contain Scala's collections frameworkscala.collection.immutable- Immutable, sequential data-structures such asVector,List,Range,HashMaporHashSetscala.collection.mutable- Mutable, sequential data-structures such asArrayBuffer,StringBuilder,HashMaporHashSetscala.collection.concurrent- Mutable, concurrent data-structures such asTrieMapscala.collection.parallel.immutable- Immutable, parallel data-structures such asParVector,ParRange,ParHashMaporParHashSetscala.collection.parallel.mutable- Mutable, parallel data-structures such asParArray,ParHashMap,ParTrieMaporParHashSetscala.concurrent- Primitives for concurrent programming such asFuturesandPromisesscala.io- Input and output operationsscala.math- Basic math functions and additional numeric types likeBigIntandBigDecimalscala.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.Predefobject are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
Listis an alias forscala.collection.immutable.List.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
Stringis an alias forjava.lang.String.