为什么 scala 在打印它被调用的行之前调用传递给 println 的方法? [英] Why does scala call the method passed to println before printing the line it is called in?

问题描述

程序看起来像这样......

The program looks like this ...

object Delay{

    def main(args: Array[String]){
        delayed(time())
    }

    def time()={
        println("Getting time in nanoseconds : ")
        System.nanoTime
    }

    def delayed(t: => Long)={
        println("In delayed Method")
        println("Param : "+t)
    }
}

输出是...

    In delayed Method
    Getting time in nanoseconds :
    Param : 139735036142049

我的问题是为什么Param :"这个词打印在Getting time ..."之后而不是像，

My question is why does the word "Param :" print after the "Getting time ..." and not like,

    In delayed Method
    Param : 
    Getting time in nanoseconds : 139735036142049

推荐答案

之所以看到这个执行顺序，是因为 t 是一个 by-name 参数您的代码:

The reason why you see this execution order, is because t is a by-name parameter in your code:

def delayed(t: => Long)

如果您使用 by-value 参数定义了 delayed 方法，如下所示:

If you defined your delayed method with a by-value parameter like so:

def delayed(t: Long)

time() 函数会在调用延迟之前被评估，而你会得到以下输出:>

Getting time in nanoseconds : 
In delayed Method
Param : 139735036142049

诀窍是按名称参数仅在使用时被调用，并且每次被使用.来自 Scala 文档:

The trick is that by-name parameters are called only when they're used, and every time they're used. From the Scala docs:

By-name 参数仅在使用时进行评估.它们与按值参数形成对比.

By-name parameters are only evaluated when used. They are in contrast to by-value parameters.

按名称参数的优点是，如果它们不在函数体中使用，则不会对其进行评估.另一方面，按值参数的优点是它们只计算一次.

By-name parameters have the advantage that they are not evaluated if they aren’t used in the function body. On the other hand, by-value parameters have the advantage that they are evaluated only once.

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