函数式编程中的返回值

Question

我开始使用 Scala 探索“函数式编程”。 我想知道我们如何在函数式编程中返回一个值。我写了一个递归函数

def calculateSum(mainlist: List[Int]): Int = {
      def Sum(curentElem:Int = 0,thislist:List[Int],): Int = {
       if (list.isEmpty) curentElem
       else loop(curentElem + thislist.head, thislist.tail)
      //curentElem
     }
      Sum((List(0,1,2,3,4)))
      println ("returned from Sum : " + curentElem)

  }

• 我是否应该在函数的最后一行添加“curentElem”（就像我在注释行中所做的那样）！

更新： 我刚刚解决了这个问题：

object HelloScala  {    
def main(args: Array[String]): Unit = {     
      val s = sum(0, List(0,1,2,3,4))
      println("returned from Sum : " + s )
    }  

def sum(currentElem: Int, thislist: List[Int]): Int = {
      thislist match {
        case Nil => currentElem
        case head :: tail => sum(currentElem + head, tail)
      }

    }
}

Answer 1

如果你真的想打印结果，那么你可以这样做

def calculateSum(mainlist: List[Int]): Int = {
   def sum(currentElem: Int, thislist: List[Int]): Int = {
      if (thislist.isEmpty) curentElem
      else sum(currentElem + thislist.head, thislist.tail)
      //curentElem
   }
   val s = sum(0, mainlist)
   println("returned from Sum : " + s)
   s
}

如果你不这样做：

def calculateSum(mainlist: List[Int]): Int = {
   def sum(currentElem: Int, thislist: List[Int]): Int = {
      if (thislist.isEmpty) curentElem
      else sum(currentElem + thislist.head, thislist.tail)
      //curentElem
   }
   sum(0, mainlist)
}

一种使用模式匹配的方法（您将在 scala 中经常使用）：

def calculateSum2(mainlist: List[Int]): Int = {
    def sum(currentElem: Int, thislist: List[Int]): Int = {
      thislist match {
        case Nil => currentElem
        case head :: tail => sum(currentElem + head, tail)
      }
    }
    sum(0, mainlist)
}

Nil 是一个空列表。