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
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 thatLeft
is used for failure andRight
is used for success.For example, you could use
Either[String, Int]
to indicate whether a received input is aString
or anInt
.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:Since
Either
defines the methodsmap
andflatMap
, it can also be used in for comprehensions:Since
for
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
.2.7