Scala:如何创建一个允许我在调用时使用点表示法的函数? [英] Scala: How can I create a function that allows me to use dot notation when calling it?

问题描述

尽管阅读了Scala 风格，但我对此已经困惑了一段时间指南 - 多次调用方法.

我希望能够调用这个方法

I want to be able to call this method

def foldRVL[A,B](l: List[A], z: B)(f: (A, B) => B) = //"Right-Via-Left"
  l.reverse.foldLeft(z)((a, b) => f(b, a))

使用这样的点符号List(1,2,3).foldRVL(0)(_ + _).

而不是这样:foldRVL(List(1,2,3), 0)(_ + _).

此外，有时我看到的代码显示的方法实际上要么在其签名中采用零个参数，要么比我预期的参数少一个，并且仍然使用点符号正确地采用参数.这是如何运作的?我问这个是因为这些方法使用点表示法，所以也许如果我写了这样的东西，我可以解决我的问题.

Also, sometimes I've seen code that shows methods that actually either takes zero parameters in their signature, or one fewer parameters than I would expect them to take, and still properly take a parameter using dot-notation. How does this work? I ask this because those methods work with dot-notation, so maybe if I wrote something like that I could solve my problem.

推荐答案

对于问题的第一部分，您可能需要查看 隐式类:

For the first part of your question, you probably need to look at implicit classes:

  implicit class RichRVLList[A](l:List[A]) {
    def foldRVL[B](z: B)(f: (A, B) => B) = //"Right-Via-Left"
      l.reverse.foldLeft(z)((a, b) => f(b, a))
  }

  List(1,2,3).foldRVL(1)(_ + _)  // output: res0: Int = 7

您可以使用隐式包装器丰富"现有类来添加"新方法.

You can "enrich" existent class using implicit wrapper to "add" new methods.

至于第二部分，您可能想要隐式参数.隐式参数是按类型从当前作用域推导出来的.下面的示例中使用了一些预定义的隐式值，例如 Numerics:

As for the second part, probably you want implicit parameters. Implicit parameters are deduced from the current scope by type. There are some predefined implicit values, such as Numerics, that were used in the example below:

  def product[T](els:TraversableOnce[T])(implicit num:Numeric[T]) = {
    els.fold(num.one)((x1, x2) => num.times(x1, x2))
  }      

  product(List(1, 2, 3)) // res1: Int = 6
  product(List(1, 2.5, 3)) //res2: Double = 7.5

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