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.
the value being probed for possible equality
true if this instance can possibly equal that, otherwise false
Returns true if this is a Left, false otherwise.
Left("tulip").isLeft // true
Right("venus fly-trap").isLeft // false
Returns true if this is a Right, false otherwise.
Left("tulip").isRight // false
Right("venus fly-trap").isRight // true
The size of this product.
for a product A(x1, ..., xk), returns k
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.
the index of the element to return
the element n elements after the first element
IndexOutOfBoundsException if the n is out of range(n < 0 || n >= productArity).
Test two objects for inequality.
true if !(this == that), false otherwise.
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.
a hash value consistent with ==
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
true if the receiver object is equivalent to the argument; false otherwise.
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.
the receiver object.
ClassCastException if the receiver object is not an instance of the erasure of type T0.
Create a copy of the receiver object.
The default implementation of the clone method is platform dependent.
a copy of the receiver object.
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"
the element to test.
true if this is a Right value equal to elem.
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:
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).
true if the argument is a reference to the receiver object; false otherwise.
The equality method for reference types. Default implementation delegates to eq.
See also equals in scala.Any.
true if the receiver object is equivalent to the argument; false otherwise.
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
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)
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.
Binds the given function across Right.
The function to bind across Right.
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)
Applies fa if this is a Left or fb if this is a Right.
the function to apply if this is a Left
the function to apply if this is a Right
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"
)
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
Executes the given side-effecting function if this is a Right.
Right(12).foreach(println) // prints "12" Left(12).foreach(println) // doesn't print
The side-effecting function to execute.
Returns string formatted according to given format string. Format strings are as for String.format (@see java.lang.String.format).
Returns the runtime class representation of the object.
a class object corresponding to the runtime type of the receiver.
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
The hashCode method for reference types. See hashCode in scala.Any.
the hash code value for this object.
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.
true if the receiver object is an instance of erasure of type T0; false otherwise.
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.
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.
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
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")
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)
Equivalent to !(this eq that).
true if the argument is not a reference to the receiver object; false otherwise.
Wakes up a single thread that is waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
Wakes up all threads that are waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
Returns this Right or the given argument if this is a Left.
Right(1) orElse Left(2) // Right(1) Left(1) orElse Left(2) // Left(2) Left(1) orElse Left(2) orElse Right(3) // Right(3)
The name of the nth element of this product, 0-based. In the default implementation, an empty string.
the index of the element name to return
the name of the specified element
IndexOutOfBoundsException if the n is out of range(n < 0 || n >= productArity).
An iterator over the names of all the elements of this product.
An iterator over all the elements of this product.
in the default implementation, an Iterator[Any]
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.
in the default implementation, the empty string
If this is a Left, then return the left value in Right 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)
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
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()
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.
a String representation of the object.
© 2002-2019 EPFL, with contributions from Lightbend.
Licensed under the Apache License, Version 2.0.
https://www.scala-lang.org/api/2.13.0/scala/util/Either.html
Represents a value of one of two possible types (a disjoint union). An instance of
Eitheris an instance of either scala.util.Left or scala.util.Right.A common use of
Eitheris 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 thatLeftis used for failure andRightis used for success.For example, you could use
Either[String, Int]to indicate whether a received input is aStringor 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" }Eitheris right-biased, which means thatRightis assumed to be the default case to operate on. If it isLeft, operations likemapandflatMapreturn theLeftvalue unchanged:Since
Eitherdefines the methodsmapandflatMap, 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
forcomprehensions usemapandflatMap, the types of function parameters used in the expression must be inferred. These types are constrained by theEithervalues. In particular, because of right-biasing,Leftvalues 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 // 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]`2.7