什么时候应该使用$（并且总是可以用括号替换）？ [英] When should I use $ (and can it always be replaced with parentheses)?

问题描述

从我正在阅读的内容中， $ 被描述为对其参数应用函数。然而，在Lisp中它看起来并不像（apply ...），因为它是一个二元运算符，所以看起来它唯一的作用是有时避免使用括号，例如 foo $ bar quux 而不是 foo（bar quux）。我理解它吗？后面的形式是否被认为是坏风格？

解决方案

如果开启和关闭的距离之间的距离大于良好的可读性权限，或者您有多层嵌套括号，则优先选择括号。

例如

p>

  i（h（g（fx）））
  

可以被重写

  i $ h $ g $ fx 
  

换句话说，它表示右联合函数应用程序。这是很有用的，因为普通的函数应用程序与左边相关联，即以下

  ihgfx 
  
 
 
 
 ...可以改写如下 
 
 
 （（（ih）g）f）x 
  
其他方便的用途 $（$）函数包括用它压缩一个列表： 
 
 
  zipWith（$）fs xs 
  
这适用于函数列表中的每个函数 fs 添加到列表 xs 中的相应参数中，并将结果收集到列表中。与序列fs x 相比，它将函数列表 fs 应用于一个参数 x 并收集结果;和 fs< *> xs 将列表 fs 中的每个函数应用于列表 xs 的每个元素。  
From what I'm reading, $ is described as "applies a function to its arguments." However, it doesn't seem to work quite like (apply ...) in Lisp, because it's a binary operator, so really the only thing it looks like it does is help to avoid parentheses sometimes, like foo $ bar quux instead of foo (bar quux). Am I understanding it right? Is the latter form considered "bad style"?
解决方案
$ is preferred to parentheses when the distance between the opening and closing parens would otherwise be greater than good readability warrants, or if you have several layers of nested parentheses.

For example
i (h (g (f x)))
can be rewritten
i $ h $ g $ f x
In other words, it represents right-associative function application. This is useful because ordinary function application associates to the left, i.e. the following
i h g f x
...can be rewritten as follows
(((i h) g) f) x
Other handy uses of the ($) function include zipping a list with it:
zipWith ($) fs xs
This applies each function in a list of functions fs to a corresponding argument in the list xs, and collects the results in a list. Contrast with sequence fs x which applies a list of functions fs to a single argument x and collects the results; and fs <*> xs which applies each function in the list fs to every element of the list xs.

                        
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