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Scalaz（19）－ Monad： // - Monad 版本的 Either

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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