scala.Either.scala Maven / Gradle / Ivy
/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2010, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
package scala
/**
* The Either type represents a value of one of two possible
* types (a disjoint union). The data constructors Left and
* Right represent the two possible values.
* The Either type is often used as an alternative to
* scala.Option where Left represents failure
* (by convention) and Right is akin to Some.
*
*
* @author Tony Morris, Workingmouse
* @version 1.0, 11/10/2008
* @since 2.7
*/
sealed abstract class Either[+A, +B] {
/**
* Projects this Either as a Left.
*/
def left = Either.LeftProjection(this)
/**
* Projects this Either as a Right.
*/
def right = Either.RightProjection(this)
/**
* Deconstruction of the Either type (in contrast to pattern matching).
*/
def fold[X](fa: A => X, fb: B => X) = this match {
case Left(a) => fa(a)
case Right(b) => fb(b)
}
/**
* If this is a Left, then return the left value in Right or vice versa.
*/
def swap = this match {
case Left(a) => Right(a)
case Right(b) => Left(b)
}
/**
* Joins an Either through Right.
*/
def joinRight[A1 >: A, B1 >: B, C](implicit ev: B1 <:< Either[A1, C]): Either[A1, C] = this match {
case Left(a) => Left(a)
case Right(b) => b
}
/**
* Joins an Either through Left.
*/
def joinLeft[A1 >: A, B1 >: B, C](implicit ev: A1 <:< Either[C, B1]): Either[C, B1] = this match {
case Left(a) => a
case Right(b) => Right(b)
}
/**
* Returns true if this is a Left, false otherwise.
*/
def isLeft: Boolean
/**
* Returns true if this is a Right, false otherwise.
*/
def isRight: Boolean
}
/**
* The left side of the disjoint union, as opposed to the Right side.
*
* @author Tony Morris, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class Left[+A, +B](a: A) extends Either[A, B] {
def isLeft = true
def isRight = false
}
/**
* The right side of the disjoint union, as opposed to the Left side.
*
* @author Tony Morris, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class Right[+A, +B](b: B) extends Either[A, B] {
def isLeft = false
def isRight = true
}
object Either {
class MergeableEither[A](x: Either[A, A]) {
def merge: A = x match {
case Left(a) => a
case Right(a) => a
}
}
implicit def either2mergeable[A](x: Either[A, A]): MergeableEither[A] = new MergeableEither(x)
/**
* Projects an Either into a Left.
*
* @author Tony Morris, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class LeftProjection[+A, +B](e: Either[A, B]) {
/**
* Returns the value from this Left or throws Predef.NoSuchElementException
* if this is a Right.
*
* @throws Predef.NoSuchElementException if the option is empty.
*/
def get = e match {
case Left(a) => a
case Right(_) => throw new NoSuchElementException("Either.left.value on Right")
}
/**
* Executes the given side-effect if this is a Left.
*
* @param e The side-effect to execute.
*/
def foreach[U](f: A => U) = e match {
case Left(a) => f(a)
case Right(_) => {}
}
/**
* Returns the value from this Left or the given argument if this is a
* Right.
*/
def getOrElse[AA >: A](or: => AA) = e match {
case Left(a) => a
case Right(_) => or
}
/**
* Returns true if Right or returns the result of the application of
* the given function to the Left value.
*/
def forall(f: A => Boolean) = e match {
case Left(a) => f(a)
case Right(_) => true
}
/**
* Returns false if Right or returns the result of the application of
* the given function to the Left value.
*/
def exists(f: A => Boolean) = e match {
case Left(a) => f(a)
case Right(_) => false
}
/**
* Binds the given function across Left.
*
* @param The function to bind across Left.
*/
def flatMap[BB >: B, X](f: A => Either[X, BB]) = e match {
case Left(a) => f(a)
case Right(b) => Right(b)
}
/**
* Maps the function argument through Left.
*/
def map[X](f: A => X) = e match {
case Left(a) => Left(f(a))
case Right(b) => Right(b)
}
/**
* Returns None if this is a Right or if the given predicate
* p does not hold for the left value, otherwise, returns a Left.
*/
def filter[Y](p: A => Boolean): Option[Either[A, Y]] = e match {
case Left(a) => if(p(a)) Some(Left(a)) else None
case Right(b) => None
}
/**
* Returns a Seq containing the Left value if it exists or an empty
* Seq if this is a Right.
*/
def toSeq = e match {
case Left(a) => Seq(a)
case Right(_) => Seq.empty
}
/**
* Returns a Some containing the Left value if it exists or a
* None if this is a Right.
*/
def toOption = e match {
case Left(a) => Some(a)
case Right(_) => None
}
}
/**
* Projects an Either into a Right.
*
* @author Tony Morris, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class RightProjection[+A, +B](e: Either[A, B]) {
/**
* Returns the value from this Right or throws
* Predef.NoSuchElementException if this is a Left.
*
* @throws Predef.NoSuchElementException if the projection is Left.
*/
def get = e match {
case Left(_) => throw new NoSuchElementException("Either.right.value on Left")
case Right(a) => a
}
/**
* Executes the given side-effect if this is a Right.
*
* @param e The side-effect to execute.
*/
def foreach[U](f: B => U) = e match {
case Left(_) => {}
case Right(b) => f(b)
}
/**
* Returns the value from this Right or the given argument if this is a
* Left.
*/
def getOrElse[BB >: B](or: => BB) = e match {
case Left(_) => or
case Right(b) => b
}
/**
* Returns true if Left or returns the result of the application of
* the given function to the Right value.
*/
def forall(f: B => Boolean) = e match {
case Left(_) => true
case Right(b) => f(b)
}
/**
* Returns false if Left or returns the result of the application of
* the given function to the Right value.
*/
def exists(f: B => Boolean) = e match {
case Left(_) => false
case Right(b) => f(b)
}
/**
* Binds the given function across Right.
*
* @param The function to bind across Right.
*/
def flatMap[AA >: A, Y](f: B => Either[AA, Y]) = e match {
case Left(a) => Left(a)
case Right(b) => f(b)
}
/**
* Maps the function argument through Right.
*/
def map[Y](f: B => Y) = e match {
case Left(a) => Left(a)
case Right(b) => Right(f(b))
}
/** Returns None if this is a Left or if the
* given predicate p does not hold for the right value,
* otherwise, returns a Right.
*/
def filter[X](p: B => Boolean): Option[Either[X, B]] = e match {
case Left(_) => None
case Right(b) => if(p(b)) Some(Right(b)) else None
}
/** Returns a Seq containing the Right value if
* it exists or an empty Seq if this is a Left.
*/
def toSeq = e match {
case Left(_) => Seq.empty
case Right(b) => Seq(b)
}
/** Returns a Some containing the Right value
* if it exists or a None if this is a Left.
*/
def toOption = e match {
case Left(_) => None
case Right(b) => Some(b)
}
}
@deprecated("use `x.joinLeft'")
def joinLeft[A, B](es: Either[Either[A, B], B]) =
es.left.flatMap(x => x)
@deprecated("use `x.joinRight'")
def joinRight[A, B](es: Either[A, Either[A, B]]) =
es.right.flatMap(x => x)
/**
* Takes an Either to its contained value within Left or
* Right.
*/
@deprecated("use `x.merge'")
def merge[T](e: Either[T, T]) = e match {
case Left(t) => t
case Right(t) => t
}
/** If the condition satisfies, return the given A in Left,
* otherwise, return the given B in Right.
*/
def cond[A, B](test: Boolean, right: => B, left: => A): Either[A, B] =
if (test) Right(right) else Left(left)
}
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