scala标准库提供了一个Either类型，它可以说是Option的升级版。与Option相同，Either也有两种状态：Left和Right，分别对应Option的None和Some，不同的是Left可以返回一个值。我们通常用这个值来表述异常信息。scalaz也提供了自己版本的Either，并用/来分辨表示，以及两种状态-/和/-。我想scalaz特别提供/是有原因的：/不单是一种类型，它是一种type class。更重要的是/是一种Monad，具备了函数组合能力（composibility）。如此能够方便把Either功能整合到FP编程中去。我们先看看/的定义：scalaz/Either.scala
sealed abstract class /[+A, +B] extends Product with Serializable {
...
def isLeft: Boolean =
this match {
case -/(_) => true
case /-(_) => false
}
/** Return `true` if this disjunction is right. */
def isRight: Boolean =
this match {
case -/(_) => false
case /-(_) => true
}
...
/** Return the right value of this disjunction or the given default if left. Alias for `|` */
def getOrElse[BB >: B](x: => BB): BB =
this match {
case -/(_) => x
case /-(b) => b
}
/** Return the right value of this disjunction or the given default if left. Alias for `getOrElse` */
def |[BB >: B](x: => BB): BB =
getOrElse(x)
/** Return the right value of this disjunction or run the given function on the left. */
def valueOr[BB >: B](x: A => BB): BB =
this match {
case -/(a) => x(a)
case /-(b) => b
}
/** Return this if it is a right, otherwise, return the given value. Alias for `|||` */
def orElse[AA >: A, BB >: B](x: => AA / BB): AA / BB =
this match {
case -/(_) => x
case /-(_) => this
}
/** Return this if it is a right, otherwise, return the given value. Alias for `orElse` */
def |||[AA >: A, BB >: B](x: => AA / BB): AA / BB =
orElse(x)
...
与Option相同：/也提供了函数来获取运算值（Right[A]）,如getOrElse。那么如何获取异常信息呢？可以用swap后再用getOrElse:
/** Flip the left/right values in this disjunction. Alias for `unary_~` */
def swap: (B / A) =
this match {
case -/(a) => /-(a)
case /-(b) => -/(b)
}
/** Flip the left/right values in this disjunction. Alias for `swap` */
def unary_~ : (B / A) =
swap
"ah, error!".left[Int].getOrElse("no error") //> res2: Any = no error
"ah, error!".left[Int].swap.getOrElse("no error") //> res3: String = ah, error!
(~"ah, error!".left[Int]).getOrElse("no error") //> res4: String = ah, error!
与Option一样，/也有两种状态：
/** A left disjunction
*
* Often used to represent the failure case of a result
*/
final case class -/[+A](a: A) extends (A / Nothing)
/** A right disjunction
*
* Often used to represent the success case of a result
*/
final case class /-[+B](b: B) extends (Nothing / B)
/实现了map和flatMap：
/** Map on the right of this disjunction. */
def map[D](g: B => D): (A / D) =
this match {
case /-(a) => /-(g(a))
case b @ -/(_) => b
}
/** Bind through the right of this disjunction. */
def flatMap[AA >: A, D](g: B => (AA / D)): (AA / D) =
this match {
case a @ -/(_) => a
case /-(b) => g(b)
}
注意flatMap：如果状态为/- 则连续运算g(b)，如果状态为-/ 则立即停止运算返回-/状态。这与Option功能相当。我们用for-comprehension来证明：
1 val epok = for {
2 a <- /-(3)
3 b <- /-(2)
4 } yield a + b //> epok : scalaz./[Nothing,Int] = /-(5)
5 val epno = for {
6 a <- /-(3)
7 c <- -/("breaking out...")
8 b <- /-(2)
9 } yield a + b //> epno : scalaz./[String,Int] = -/(breaking out...)
10 if (epno.isLeft) (~epno).getOrElse("no error") //> res5: Any = breaking out...
/在for-comprehension里的运算行为与Option一致。不过这个/写法比较别扭。/type class为任何类型提供了注入方法left和right: scalaz.syntax/EitherOps.scala
final def left[B]: (A / B) =
-/(self)
final def right[B]: (B / A) =
/-(self)
}
trait ToEitherOps { //可以为任何类型A注入方法
implicit def ToEitherOps[A](a: A) = new EitherOps(a)
}
现在这个for-comprehension可以这样写：
1 val epok1 = for {
2 a <- 3.right
3 b <- 2.right
4 } yield a + b //> epok1 : scalaz./[Nothing,Int] = /-(5)
5 val epno1 = for {
6 a <- 3.right
7 c <- "breaking out...".left[Int]
8 b <- 2.right
9 } yield a + b //> epno1 : scalaz./[String,Int] = -/(breaking out...)
10 if (epno1.isLeft) (~epno1).getOrElse("no error") //> res6: Any = breaking out...
这样表述是不是清晰直白多了。