短路的函数列表返回\/

Question

所以，我有一个昂贵的方法与此签名

def func(param: Int): \/[String, Int] 

我试图环比则params的列表并返回\/[String, List[Int]]但停止循环，只要该方法返回-\/ 。

我想出了这一点：

scala> def func(i: Int) = { 
    | if(i > 1) { println{"!!"} ;"error".left[Int]} 
    | else i.right[String] 
    | } 
func: (i: Int)scalaz.\/[String,Int] 

scala> val toList = (dis: scalaz.\/[String,Int]) => {dis.map(i => List(i))} 
toList: scalaz.\/[String,Int] => scalaz.\/[String,List[Int]] = <function1> 

scala> val composed = (func _) andThen toList 
composed: Int => scalaz.\/[String,List[Int]] = <function1> 

scala> (1 to 3).toList.foldLeftM(List[Int]().right[String]){(dis, i) => 
    | for{ 
    |  a <- dis 
    |  b <- composed(i) 
    | } yield a |+| b 
    | } 
<console>:17: error: no type parameters for method foldLeftM: (f: (scalaz.\/[String,List[Int]], Int) => G[scalaz.\/[String,List[Int]]])(implicit M: scalaz.Monad[G])G[scalaz.\/[String,List[Int]]] exist so that it can be applied to arguments ((scalaz.\/[String,List[Int]], Int) => scalaz.\/[String,List[Int]]) 
--- because --- 
argument expression's type is not compatible with formal parameter type; 
found : (scalaz.\/[String,List[Int]], Int) => scalaz.\/[String,List[Int]] 
required: (scalaz.\/[String,List[Int]], Int) => ?G[scalaz.\/[String,List[Int]]] 

       (1 to 2).toList.foldLeftM(List[Int]().right[String]){(dis, i) => 
            ^
<console>:17: error: type mismatch; 
found : (scalaz.\/[String,List[Int]], Int) => scalaz.\/[String,List[Int]] 
required: (scalaz.\/[String,List[Int]], Int) => G[scalaz.\/[String,List[Int]]] 
       (1 to 2).toList.foldLeftM(List[Int]().right[String]){(dis, i) => 
                      ^

什么这里的G[_]，什么是在foldLeftM正确的结果类型？

Answer 1

通过注意到有两个错误，我提出解决：

1. 误解约类型的初始/结转值，它应该是List[Int]代替\/

2. 人们必须的foldLeftM

   明确声明类型参数

   scala> def func(i: Int) = { 
   | if(i > 1) { println{"!!"} ;"error".left[Int]} 
   | else i.right[String] 
   | } 
   func: (i: Int)scalaz.\/[String,Int] 
   
   scala> val toList = (dis: scalaz.\/[String,Int]) => {dis.map(i => List(i))} 
   toList: scalaz.\/[String,Int] => scalaz.\/[String,List[Int]] = <function1> 
   
   scala> val composed = (func _) andThen toList 
   composed: Int => scalaz.\/[String,List[Int]] = <function1> 
   
   scala> val f = (dis: List[Int], i: Int) => composed(i).map(r => dis |+| r) 
   f: (List[Int], Int) => scalaz.\/[String,List[Int]] = <function2> 
   
   scala> (1 to 4).toList.foldLeftM[({type l[a] = String \/ a})#l, List[Int]](List[Int]())(f) 
   !! 
   res16: scalaz.\/[String,List[Int]] = -\/(error)