$和（）之间的区别 [英] Difference between $ and ()

问题描述

我开始学习Haskell，遇到了一个我不能理解的问题。我有一个方法用于从键值列表中找到值（来自这个页面）： p>

 让findKey key xs = snd。头。过滤器（\（k，v） - > key == k）$ xs 
  

摆弄一下，决定用这种方式摆脱$ sign：

 让findKey key xs = snd。头。过滤器（\（k，v） - > key == k）（xs）
  

，它甚至不解析（过滤器应用于太多的argumens错误）。我已经读过$ sign用来简单地替换括号，我无法弄清楚为什么简单的代码改变是不好的。有人可以解释给我吗？

解决方案

中缀运算符（$）就是功能应用程序。换句话说，

  fx  - 和
f $ x 
  

是一样的。由于Haskell括号仅用于区分优先级（以及元组符号和其他一些小地方，我们也可以用其他几种方式写上面的内容：

pre $ $ $ $ $ $ $ （$）
f（x）
（f）（x） - 甚至
（f）$（x）

在任何情况下，上述表达式都表示相同的事情：将函数 f 应用于参数 x 。

那么为什么有这样的语法呢？ （$）有两个原因是有用的。

1. 它的优先级非常低，可以支持很多括号有时候


2. 很高兴为函数应用程序的操作提供明确的名称

$ b

在第一种情况下，请考虑以下深度右嵌套的函数应用程序

  f（g（h（i（jx））））
  

有点难以阅读，有点难以知道你有正确的括号。然而，它只是一堆应用程序，所以应该使用（$）来表示这个短语。确实有

  f $ g $ h $ i $ j $ x 
  

有些人觉得这样比较容易阅读。更现代的风格也包含了（。），以强调这个短语的整个左侧只是一个组合的管道函数

  f。 G 。 H 。一世 。 j $ x 
  

正如我们上面所见，这句话与

$ b相同
$ b

 （f。g。i。j）x 
  

有时更好阅读。

---

有时候我们想要能够传递函数应用的想法。例如，如果我们有一个函数列表

  lof :: [Int  - > Int] 
 lof = [（+1），（减1），（* 2）] 
  

我们可能希望通过一个值来映射应用程序，例如将数字 4 应用于每个函数

 > map（\fun-> fun 4）lof 
 [5，3，8] 
  

<但是由于这只是函数应用程序，所以我们也可以使用（$）部分的语法来更明确一些。

 > map $ 4 $ 
 [5，3，8] 
  

I started learning Haskell and I encountered a problem I can't just understand. I've got a method used to find value from a list of key-value list (from this page):

let findKey key xs = snd . head . filter (\(k,v) -> key == k) $ xs  

I tried fiddling with a bit and decided to get rid of $ sign in this way:

let findKey key xs = snd . head . filter (\(k,v) -> key == k) ( xs )

However, it doesn't even parse (filter applied to too many argumens error). I've read that $ sign is used to simply replace parenthesis and I can't figure out why this simple change of code is bad. Could someone explain it to me?

解决方案

The infix operator ($) is just "function application". In other words

 f   x     -- and
 f $ x

are the same. Since in Haskell parentheses are only used to disambiguate precedence (and for tuple notation and a few other minor places, see comments) we can also write the above in a few other ways

 f     x
 f  $  x
(f)    x
 f    (x)
(f)   (x)    -- and even
(f) $ (x)

In every case, the above expressions denote the same thing: "apply the function f to the argument x".

So why have all this syntax? ($) is useful for two reasons

1. It has really low precedence so it can stand in for a lot of parentheses sometimes
2. It's nice to have an explicit name for the action of function application

---

In the first case, consider the following deeply right-nested function application

f (g (h (i (j x))))

It can be a little difficult to read this and a little difficult to know you have the right number of parentheses. However, it's "just" a bunch of applications so there ought to be a representation of this phrase using ($). Indeed there is

 f $ g $ h $ i $ j $ x

Some people find this easier to read. More modern style also incorporates (.) in order to emphasize that the whole left side of this phrase is just a composed pipeline of functions

 f . g . h . i . j $ x

And this phrase is, as we saw above, identical to

(f . g . h . i . j)  x

which is sometimes nicer to read.

---

There are also times when we want to be able to pass around the idea of function application. For instance, if we have a list of functions

lof :: [Int -> Int]
lof = [ (+1), (subtract 1), (*2) ]

we might want to map application by a value over them, for instance apply the number 4 to each function

> map (\fun -> fun 4) lof
[ 5, 3, 8 ]

But since this is just function application, we can also use section syntax over ($) to be a bit more explicit

> map ($ 4) lof
[ 5, 3, 8 ]

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