Represents a value of one of two possible types (a disjoint union). An instance ofEither is an instance of eitherscala.util.Left orscala.util.Right.
A common use ofEither is as an alternative toscala.Option for dealing with possibly missing values. In this usage,scala.None is replaced with ascala.util.Left which can contain useful information.scala.util.Right takes the place ofscala.Some. Convention dictates thatLeft is used for failure andRight is used for success.
For example, you could useEither[String, Int] to indicate whether a received input is aString or anInt.
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 thatRight is assumed to be the default case to operate on. If it isLeft, operations likemap andflatMap return theLeft value unchanged:
def doubled(i: Int) = i * 2Right(42).map(doubled) // Right(84)Left(42).map(doubled) // Left(42)SinceEither defines the methodsmap andflatMap, 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 iSincefor comprehensions usemap andflatMap, the types of function parameters used in the expression must be inferred. These types are constrained by theEither values. In particular, because of right-biasing,Left values may require an explicit type argument for type parameterB, the right value. Otherwise, it might be inferred asNothing.
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 // errorfor { 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]`Attributes
- Companion
- object
- Source
- Either.scala
- Graph
- Supertypes
- Known subtypes
Members list
Value members
Abstract methods
Returnstrue if this is aLeft,false otherwise.
Returnstrue if this is aLeft,false otherwise.
Left("tulip").isLeft // trueRight("venus fly-trap").isLeft // falseAttributes
- Source
- Either.scala
Returnstrue if this is aRight,false otherwise.
Returnstrue if this is aRight,false otherwise.
Left("tulip").isRight // falseRight("venus fly-trap").isRight // trueAttributes
- Source
- Either.scala
Concrete methods
Returnstrue if this is aRight and its value is equal toelem (as determined by==), returnsfalse otherwise.
Returnstrue 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"Value parameters
- elem
the element to test.
Attributes
- Returns
trueif this is aRightvalue equal toelem.- Source
- Either.scala
Returnsfalse ifLeft or returns the result of the application of the given predicate to theRight value.
Returnsfalse ifLeft or returns the result of the application of the given predicate to theRight value.
Right(12).exists(_ > 10) // trueRight(7).exists(_ > 10) // falseLeft(12).exists(_ => true) // falseAttributes
- Source
- Either.scala
ReturnsRight 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.
ReturnsRight 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)Attributes
- Source
- Either.scala
Binds the given function acrossRight.
Binds the given function acrossRight.
Value parameters
- f
The function to bind across
Right.
Attributes
- Source
- Either.scala
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
Attributes
- Example
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)- Source
- Either.scala
Appliesfa if this is aLeft orfb if this is aRight.
Appliesfa if this is aLeft orfb if this is aRight.
Value parameters
- fa
the function to apply if this is a
Left- fb
the function to apply if this is a
Right
Attributes
- Returns
the results of applying the function
- Example
val result = util.Try("42".toInt).toEitherresult.fold( e => s"Operation failed with $e", v => s"Operation produced value: $v")- Source
- Either.scala
Returnstrue ifLeft or returns the result of the application of the given predicate to theRight value.
Returnstrue ifLeft or returns the result of the application of the given predicate to theRight value.
Right(12).forall(_ > 10) // trueRight(7).forall(_ > 10) // falseLeft(12).forall(_ => false) // trueAttributes
- Source
- Either.scala
Executes the given side-effecting function if this is aRight.
Executes the given side-effecting function if this is aRight.
Right(12).foreach(println) // prints "12"Left(12).foreach(println) // doesn't printValue parameters
- f
The side-effecting function to execute.
Attributes
- Source
- Either.scala
Returns the value from thisRight or the given argument if this is aLeft.
Returns the value from thisRight or the given argument if this is aLeft.
Right(12).getOrElse(17) // 12Left(12).getOrElse(17) // 17Attributes
- Source
- Either.scala
Joins anEither throughLeft.
Joins anEither throughLeft.
This method requires that the left side of thisEither 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 aLeft[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, andjoinRight, are analogous toOption#flatten.
Attributes
- Source
- Either.scala
Joins anEither throughRight.
Joins anEither throughRight.
This method requires that the right side of thisEither 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 aRight[Either[A, C]] then the containedEither[A, C] will be returned, otherwise this value will be returned unmodified.
Attributes
- Example
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- Source
- Either.scala
Projects thisEither as aLeft.
Projects thisEither as aLeft.
This allows for-comprehensions over the left side ofEither instances, reversingEither's usual right-bias.
For example
for (s <- Left("flower").left) yield s.length // Left(6)Continuing the analogy withscala.Option, aLeftProjection declares thatLeft should be analogous toSome in some code.
// using Optiondef interactWithDB(x: Query): Option[Result] = try Some(getResultFromDatabase(x)) catch { case _: SQLException => None }// this will only be executed if interactWithDB returns a Someval 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 Eitherdef 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")Attributes
- Source
- Either.scala
The given function is applied if this is aRight.
The given function is applied if this is aRight.
Right(12).map(x => "flower") // Result: Right("flower")Left(12).map(x => "flower") // Result: Left(12)Attributes
- Source
- Either.scala
Attributes
- Source
- Either.scala
Returns thisRight or the given argument if this is aLeft.
Returns thisRight 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)Attributes
- Source
- Either.scala
If this is aLeft, then return the left value inRight or vice versa.
If this is aLeft, then return the left value inRight or vice versa.
Attributes
- Example
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)- Source
- Either.scala
Returns aSome containing theRight value if it exists or aNone if this is aLeft.
Returns aSome containing theRight value if it exists or aNone if this is aLeft.
Right(12).toOption // Some(12)Left(12).toOption // NoneAttributes
- Source
- Either.scala
Returns aSeq containing theRight value if it exists or an emptySeq if this is aLeft.
Returns aSeq containing theRight value if it exists or an emptySeq if this is aLeft.
Right(12).toSeq // Seq(12)Left(12).toSeq // Seq()Attributes
- Source
- Either.scala
Attributes
- Source
- Either.scala
Deprecated methods
Projects thisEither as aRight.
Projects thisEither as aRight.
BecauseEither is right-biased, this method is not normally needed.
Attributes
- Deprecated
[Since version 2.13.0]Either is now right-biased, use methods directly on Either- Source
- Either.scala
Inherited methods
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.
Value parameters
- n
the index of the element name to return
Attributes
- Returns
the name of the specified element
- Throws
IndexOutOfBoundsExceptionif the
nis out of range(n < 0 || n >= productArity).- Inherited from:
- Product
- Source
- Product.scala
An iterator over the names of all the elements of this product.
An iterator over the names of all the elements of this product.
Attributes
- Inherited from:
- Product
- Source
- Product.scala
An iterator over all the elements of this product.
An iterator over all the elements of this product.
Attributes
- Returns
in the default implementation, an
Iterator[Any]- Inherited from:
- Product
- Source
- Product.scala
A string used in thetoString methods of derived classes.
A string used in thetoString methods of derived classes. Implementations may override this method to prepend a string prefix to the result oftoString methods.
Attributes
- Returns
in the default implementation, the empty string
- Inherited from:
- Product
- Source
- Product.scala
Inherited and Abstract methods
Checks whether this instance can possibly equalthat.
Checks whether this instance can possibly equalthat.
A method that should be called from every well-designed equals method that is open to be overridden in a subclass. SeeProgramming in Scala, Chapter 28 for discussion and design.
Value parameters
- that
the value being probed for possible equality
Attributes
- Returns
true if this instance can possibly equal
that, otherwise false- Inherited from:
- Equals
- Source
- Equals.scala
The size of this product.
The size of this product.
Attributes
- Returns
for a product
A(x1, ..., xk), returnsk- Inherited from:
- Product
- Source
- Product.scala
The nth element of this product, 0-based.
The nth element of this product, 0-based. In other words, for a productA(x1, ..., xk), returnsx(n+1) where0 <= n < k.
Value parameters
- n
the index of the element to return
Attributes
- Returns
the element
nelements after the first element- Throws
IndexOutOfBoundsExceptionif the
nis out of range(n < 0 || n >= productArity).- Inherited from:
- Product
- Source
- Product.scala