斯卡拉.按名称调用的高阶函数.是否有意义 [英] scala. higher order function calling by name. does it make sense

问题描述

只是想澄清一下.如果我们使用 higher-order function (f. 接受另一个函数作为参数).指定 "=>" 符号来调用它 by-name 是否有意义.似乎 arg-function 正在调用 by-name 无论如何?

Just want to clarify. If we use higher-order function (f. that accepts another function as argument). Does it make any sense specify "=>" sign to call it by-name. It seems arg-function is calling by-name anyhow?

有一个例子:

// 1.
  // the function that accepts arg-function with: two int params and returning String
  // the function passing v1 & v2 as parameters to arg-function, invoking arg-function 2 times, connecting the result to one string
  def takeFunction1(f: (Int, Int) => String, v1:Int, v2:Int ): String = {
     f(v1, v2) + f(v1, v2)
  }

  // 2. same as #1 but calling arg-function by-name
  def takeFunction2(f: => ((Int, Int) => String), v1:Int, v2:Int ): String = {
    f(v1, v2) + f(v1, v2)
  }    

  def aFun(v1:Int, v2:Int) : String = {
    (v1 + v2).toString
  }

  // --

 println( takeFunction1( aFun, 2, 2) )

 println( takeFunction2( aFun, 2, 2) )

如果我想这样称呼它呢?:

And what if I want to call it like this ?:

println( takeFunction2( aFun(2,2)), ... ) // it tries to evaluate immediately when passing

推荐答案

不同之处在于，如果您将调用作为第一个参数传递给一个返回 (Int, Int) => 的函数.要使用的字符串值，对生成器函数的调用仅使用传值计算一次，而在传值的情况下每次使用参数时都要计算一次.

The difference is that if you pass as the first argument a call to a function that returns the (Int, Int) => String value to use, this call to the generator function is evaluated only once with pass-by-value, compared to being evaluated each time the argument is used in the case of pass-by-name.

相当人为的例子:

var bar = 0

def fnGen() = {
  bar += 1
  def myFun(v1:Int, v2:Int) = {
    (v1 + v2).toString
  }
  myFun _
}

现在使用 fnGen 运行一些上述方法的调用:

Now run some calls of your methods above using fnGen:

scala> println( takeFunction1( fnGen(), 2, 2) )
44

scala> bar
res1: Int = 1

scala> println( takeFunction2( fnGen(), 2, 2) )
44

scala> bar
res3: Int = 3

如您所见，调用 takeFunction1 只会增加 bar 一次，而调用 takeFunction2 会增加 bar 两次.

As you can see, calling takeFunction1 increments bar only once, while calling takeFunction2 increments bar twice.

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