如何读取 Scala 函数参数链 [英] How to read a chain of Scala function parameters

问题描述

我正在尝试读取以下 2 个函数的参数列表:

I am trying to read the parameter-list of the following 2 functions:

1. def foo(action: => String => String) = "bar"
2. def foo(action: => () => String => String) = "bar"

1. 名为foo"的函数接收名为action"的函数，该函数接收/返回???
2. 一个名为foo"的函数接收一个名为action"的函数，该函数返回一个返回 ???

推荐答案

1. action 是一个按名称传递的函数，它接受一个 String 并返回一个 String.
2. action 是一个按名称传递的函数，它不需要返回一个函数，该函数接受一个 String 并返回一个 String

1. action is a passed-by-name function that takes a String and returns a String.
2. action is a passed-by-name function that takes nothing to return a function that takes a String and returns a String

现在您可能会问，好吧，按名称传递参数是什么意思?"好吧……那是完全不同的蠕虫罐头.基本上，按名称传递的参数仅在函数中使用时进行评估，并且每次在函数中使用时都会进行评估.这允许的是类似短路的东西，如下

Now you might ask, "Well, what does it mean for a parameter to be passed-by-name?" Alright... that's a whole different can of worms. Basically, a passed by name parameter is only evaluated when it's used in the function, and every time that it's used in the function. What this allows for is something like short-circuiting, as follows

def orOperator(left: Boolean, right: => Boolean) : Boolean = if (left) true else right

在这种情况下，如果发现 left 为真，运算符将短路(并在不计算/评估 right 的情况下终止).

In this case, the operator will short-circuit (and terminate without computing/evaluating right) if it finds left to be true.

所以......你对这些参数的看法是相似的.它们是不求值的函数——出于某种原因——除非/直到它们在函数体中被命名.我不明白这样做的动机，但是......事情就是这样.我希望这会有所帮助.

So... what you have with these parameters is something similar. They are functions that do not evaluate—for some reason—unless/until they are named in the function body. I don't understand the motivation for that, but... that's how it is. I hope that helps.

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