Scalaz(19)- Monad: \/

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  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。

 

 

 

 

 

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