/* __ *\
** ________ ___ / / ___ Scala API **
** / __/ __// _ | / / / _ | (c) 2003-2009, LAMP/EPFL **
** __\ \/ /__/ __ |/ /__/ __ | http://scala-lang.org/ **
** /____/\___/_/ |_/____/_/ | | **
** |/ **
\* */
// $Id$
package scala
import collection.{Iterable, Seq}
import collection.immutable.{List, Nil, ::}
/** <p>
* The <code>Either</code> type represents a value of one of two possible
* types (a disjoint union). The data constructors <code>Left</code> and
* <code>Right</code> represent the two possible values.
* The <code>Either</code> type is often used as an alternative to
* <code>scala.Option</code> where <code>Left</code> represents failure
* (by convention) and <code>Right</code> is akin to <code>Some</code>.
* </p>
*
* @author <a href="mailto:research@workingmouse.com">Tony Morris</a>, Workingmouse
* @version 1.0, 11/10/2008
* @since 2.7
*/
sealed abstract class Either[+A, +B] {
/**
* Projects this <code>Either</code> as a <code>Left</code>.
*/
def left = Either.LeftProjection(this)
/**
* Projects this <code>Either</code> as a <code>Right</code>.
*/
def right = Either.RightProjection(this)
/**
* Deconstruction of the <code>Either</code> 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 <code>Left</code>, then return the left value in <code>Right</code> or vice versa.
*/
def swap = this match {
case Left(a) => Right(a)
case Right(b) => Left(b)
}
/**
* Returns <code>true</code> if this is a <code>Left</code>, <code>false</code> otherwise.
*/
def isLeft = false // Default here, overriden in Left
/**
* Returns <code>true</code> if this is a <code>Right</code>, <code>false</code> otherwise.
*/
def isRight = false // Default here, overriden in Right.
}
/**
* The left side of the disjoint union, as opposed to the <code>Right</code> side.
*
* @author <a href="mailto:research@workingmouse.com">Tony Morris</a>, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class Left[+A, +B](a: A) extends Either[A, B] { override def isLeft = true }
/**
* The right side of the disjoint union, as opposed to the <code>Left</code> side.
*
* @author <a href="mailto:research@workingmouse.com">Tony Morris</a>, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class Right[+A, +B](b: B) extends Either[A, B] { override def isRight = true }
object Either {
/**
* Returns the <code>Left</code> values in the given <code>Iterable</code> of <code>Either</code>s.
*/
@deprecated("use `for (Left(a) <- es) yield a'")
def lefts[A, B](es: Iterable[Either[A, B]]) =
es.foldRight[List[A]](Nil)((e, as) => e match {
case Left(a) => a :: as
case Right(_) => as
})
/**
* Returns the <code>Right</code> values in the given<code>Iterable</code> of <code>Either</code>s.
*/
@deprecated("use `for (Right(a) <- es) yield a'")
def rights[A, B](es: Iterable[Either[A, B]]) =
es.foldRight[List[B]](Nil)((e, bs) => e match {
case Left(_) => bs
case Right(b) => b :: bs
})
/** Transforms an Iterable of Eithers into a pair of lists.
*
* @param xs the iterable of Eithers to separate
* @return a pair of lists.
*/
@deprecated("use `for ((Left(l), Right(r)) <- es partition isLeft) yield (l, r)'")
def separate[A,B](es: Iterable[Either[A,B]]): (List[A], List[B]) =
es.foldRight[(List[A], List[B])]((Nil, Nil)) {
case (Left(a), (lefts, rights)) => (a :: lefts, rights)
case (Right(b), (lefts, rights)) => (lefts, b :: rights)
}
/**
* Projects an <code>Either</code> into a <code>Left</code>.
*
* @author <a href="mailto:research@workingmouse.com">Tony Morris</a>, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class LeftProjection[+A, +B](e: Either[A, B]) {
/**
* Returns the value from this <code>Left</code> or throws <code>Predef.NoSuchElementException</code>
* if this is a <code>Right</code>.
*
* @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 <code>Left</code>.
*
* @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 <code>Left</code> or the given argument if this is a
* <code>Right</code>.
*/
def getOrElse[AA >: A](or: => AA) = e match {
case Left(a) => a
case Right(_) => or
}
/**
* Returns <code>true</code> if <code>Right</code> or returns the result of the application of
* the given function to the <code>Left</code> value.
*/
def forall(f: A => Boolean) = e match {
case Left(a) => f(a)
case Right(_) => true
}
/**
* Returns <code>false</code> if <code>Right</code> or returns the result of the application of
* the given function to the <code>Left</code> value.
*/
def exists(f: A => Boolean) = e match {
case Left(a) => f(a)
case Right(_) => false
}
/**
* Binds the given function across <code>Left</code>.
*
* @param The function to bind across <code>Left</code>.
*/
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 <code>Left</code>.
*/
def map[X](f: A => X) = e match {
case Left(a) => Left(f(a))
case Right(b) => Right(b)
}
/**
* Returns <code>None</code> if this is a <code>Right</code> or if the given predicate
* <code>p</code> does not hold for the left value, otherwise, returns a <code>Left</code>.
*/
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 <code>Seq</code> containing the <code>Left</code> value if it exists or an empty
* <code>Seq</code> if this is a <code>Right</code>.
*/
def toSeq = e match {
case Left(a) => Seq(a)
case Right(_) => Seq.empty
}
/**
* Returns a <code>Some</code> containing the <code>Left</code> value if it exists or a
* <code>None</code> if this is a <code>Right</code>.
*/
def toOption = e match {
case Left(a) => Some(a)
case Right(_) => None
}
}
/**
* Projects an <code>Either</code> into a <code>Right</code>.
*
* @author <a href="mailto:research@workingmouse.com">Tony Morris</a>, Workingmouse
* @version 1.0, 11/10/2008
*/
final case class RightProjection[+A, +B](e: Either[A, B]) {
/**
* Returns the value from this <code>Right</code> or throws
* <code>Predef.NoSuchElementException</code> if this is a <code>Left</code>.
*
* @throws Predef.NoSuchElementException if the projection is <code>Left</code>.
*/
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 <code>Right</code>.
*
* @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 <code>Right</code> or the given argument if this is a
* <code>Left</code>.
*/
def getOrElse[BB >: B](or: => BB) = e match {
case Left(_) => or
case Right(b) => b
}
/**
* Returns <code>true</code> if <code>Left</code> or returns the result of the application of
* the given function to the <code>Right</code> value.
*/
def forall(f: B => Boolean) = e match {
case Left(_) => true
case Right(b) => f(b)
}
/**
* Returns <code>false</code> if <code>Left</code> or returns the result of the application of
* the given function to the <code>Right</code> value.
*/
def exists(f: B => Boolean) = e match {
case Left(_) => false
case Right(b) => f(b)
}
/**
* Binds the given function across <code>Right</code>.
*
* @param The function to bind across <code>Right</code>.
*/
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 <code>Right</code>.
*/
def map[Y](f: B => Y) = e match {
case Left(a) => Left(a)
case Right(b) => Right(f(b))
}
/** Returns <code>None</code> if this is a <code>Left</code> or if the
* given predicate <code>p</code> does not hold for the right value,
* otherwise, returns a <code>Right</code>.
*/
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 <code>Seq</code> containing the <code>Right</code> value if
* it exists or an empty <code>Seq</code> if this is a <code>Left</code>.
*/
def toSeq = e match {
case Left(_) => Seq.empty
case Right(b) => Seq(b)
}
/** Returns a <code>Some</code> containing the <code>Right</code> value
* if it exists or a <code>None</code> if this is a <code>Left</code>.
*/
def toOption = e match {
case Left(_) => None
case Right(b) => Some(b)
}
}
/**
* Joins an <code>Either</code> through <code>Left</code>.
*/
def joinLeft[A, B](es: Either[Either[A, B], B]) =
es.left.flatMap(x => x)
/**
* Joins an <code>Either</code> through <code>Right</code>.
*/
def joinRight[A, B](es: Either[A, Either[A, B]]) =
es.right.flatMap(x => x)
/**
* Takes an <code>Either</code> to its contained value within <code>Left</code> or
* <code>Right</code>.
*/
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 <code>Left</code>,
* otherwise, return the given B in <code>Right</code>.
*/
def cond[A, B](test: Boolean, right: => B, left: => A) =
if(test) Right(right) else Left(left)
}