“代码:="是什么意思?单位"在 Scala 中是什么意思? [英] What does "code: => Unit" mean in scala?

问题描述

有谁知道 Scala 中 => 单位的类型?我不知道 => Unit 的含义以及如何使用它.我定义了一个如下所示的函数:

Does anyone know the type of => Unit in scala? I don't know the meaning of => Unit and how to use it. I defined a function like below:

def test(code: => Unit){
   print("start ...")
   code
   print("end ....")
}

test(print(1))

这是否意味着带有返回 Unit 的任何参数的函数?

Does it means a function with any arguments returning Unit?

谢谢

推荐答案

这种参数被称为by-name参数

<代码>=>B 表示返回一个 B 值的代码块 a，它们的目的是仅在您调用参数时才对它们进行评估.

=> B represents a block a of code which return a B value, their purpose is that they are evaluated only when you call the parameter.

def foo(code: => Int) {
    println("Not yet evaluated")
    val result = code
    println("parameter evaluated %s, is it an int ? %s " format (
           result, result.isInstanceOf[Int]) )
}

您可以通过以下方式调用 foo :

And you can call foo in the following way :

foo(1) 

或

val a = 3
val b = 5
foo {
  val c = a * a
  c * b
}

另一种传递参数的方式是按值:参数在被发送到方法之前被评估

The other style of passing parameter is by-value : parameters are evaluated before they are sent to the method

def foo(code : Int) {
    println("Parameter already evaluated")
    val result = code
    println("parameter evaluated : " + result)
}

参考资料

摘自Scala 函数式编程

更多按名称参数和按值参数说明

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