scala – 如何使用Kleisli组合与返回验证的功能？
发布时间：2020-12-16 09:19:34 所属栏目：安全 来源：网络整理
导读：如何撰写返回验证的两个函数？以下是我未尝试的尝试： scala def f: Int = Validation[String,Int] = i = if(i % 2 == 0) Success(i * 2) else Failure("Odd!")f: Int = scalaz.Validation[String,Int]scala def g: Int = Validation[String,Int] = i = if(i
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如何撰写返回验证的两个函数？以下是我未尝试的尝试：
scala> def f: Int => Validation[String,Int] = i => if(i % 2 == 0) Success(i * 2) else Failure("Odd!")
f: Int => scalaz.Validation[String,Int]
scala> def g: Int => Validation[String,Int] = i => if(i > 0) Success(i + 1) else Failure("Not positive!")
g: Int => scalaz.Validation[String,Int]
scala> kleisli(f) >=> kleisli(g)
<console>:16: error: no type parameters for method kleisli: (f: A => M[B])scalaz.Kleisli[M,A,B] exist so that it can be applied to arguments (Int => scalaz.Validation[String,Int])
--- because ---
argument expression's type is not compatible with formal parameter type;
found : Int => scalaz.Validation[String,Int]
required: ?A => ?M[?B]
kleisli(f) >=> kleisli(g)
^
scala> type Va[+A] = Validation[String,A]
defined type alias Va
scala> kleisli[Va,Int,Int](f) >=> kleisli[Va,Int](g)
<console>:17: error: could not find implicit value for parameter b: scalaz.Bind[Va]
kleisli[Va,Int](g)
^
这在Haskell工作,所以我期待着应该有一种这样做与Scalaz的方式. λ> let f i = if i `mod` 2 == 0 then Right $i * 2 else Left "Odd!" λ> let g i = if i > 0 then Right $i + 1 else Left "Not positive!" λ> let h = f >=> g λ> h 11 Left "Odd!" λ> h (-4) Left "Not positive!" λ> h 4 Right 9 解决方法
您需要导入Validation.Monad._,因为> =>需要绑定[M]
scala> import scalaz._,Scalaz._
import scalaz._
import Scalaz._
scala> def f: Int => Validation[String,Int] = i => if(i % 2 == 0) Success(i * 2) else Failure("Odd!")
f: Int => scalaz.Validation[String,Int]
scala> type Va[+A] = Validation[String,A]
defined type alias Va
scala> import Validation.Monad._
import Validation.Monad._
scala> kleisli[Va,Int](g)
res0: scalaz.Kleisli[Va,Int] = scalaz.Kleislis$$anon$1@4fae3fa6
scala> res0(11)
res1: Va[Int] = Failure(Odd!)
scala> res0(-4)
res2: Va[Int] = Failure(Not positive!)
scala> res0(4)
res3: Va[Int] = Success(9)
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