是美元运算符（$）被认为是坏形式吗？为什么？ [英] Is dollar operator ($) considered bad form? Why?

问题描述

在Haskell中，我经常用$'来写表达式。我发现它很自然和可读，但我有时读它是不好的形式，不明白为什么它应该是。

解决方案

以下都是良好的形式：

  foo = bar。巴兹。 quux 
 foo x = bar。巴兹。 quux $ x 
 foo x =（bar。baz。quux）x 
 foo x = bar（baz（quux x））
  

我把它们按我的喜好粗略的顺序，虽然一如既往的味道不同，上下文可能需要不同的选择。我也偶尔看到

  foo = bar 
。 baz 
。当每个 bar ，     code> baz 和 quux 子表达式很长。以下是不好的形式：
  foo x = bar $ baz $ quux $ x 
  
有两个原因，这不太可取。首先，在重构期间可以将较少的子表达式复制和粘贴到辅助定义中;与所有（$）运算符，只有包含 x 参数的子表达式是有效的重构，而（。）和（$）运算符甚至子表达式 baz 或 baz。 
 
 
 
第二个更喜欢（。）的原因是预期可能更改（$）的固定性;目前，（$）是 infixr ，意味着它与右边相关联，如下：
  foo x = bar $（baz $（quux $ x））
  
但是，（$）在更多的表达式中是有用的，如果 infixl ;例如像
  foo h = f（gx）（hy）
 foo h = f $ gx $ hy 
 foo h =（f $ gx）$ hy 
  
 ...目前无法无括号表示。使用 infixl 应用程序解析时，坏形式示例为
  foo x =（（bar $ baz）$ quux）$ x 
  
不同。所以，通过避免这种形式，未来的代码。
 
In Haskell, I often write expressions with $'s. I find it quite natural and readable, but I sometimes read it is bad form and do not understand why it should be.
解决方案
The following are all good form:
foo = bar . baz . quux
foo x = bar . baz . quux $ x
foo x = (bar . baz . quux) x
foo x = bar (baz (quux x))
I've put them in rough order of my preference, though as always taste varies, and context may demand a different choice. I've also occasionally seen
foo = bar
    . baz
    . quux
when each of the bar, baz, and quux subexpressions are long. The following is bad form:
foo x = bar $ baz $ quux $ x
There are two reasons this is less preferable. First, fewer subexpressions can be copied and pasted into an auxiliary definition during refactoring; with all ($) operators, only subexpressions that include the x argument are valid refactorings, whereas with (.) and ($) operators even subexpressions like bar . baz or baz . quux can be pulled out into a separate definition.

The second reason to prefer (.) is in anticipation of a possible change to the fixity of ($); currently, ($) is infixr, meaning it associates to the right, like so:
foo x = bar $ (baz $ (quux $ x))
However, ($) would be useful in more expressions if it were infixl; for example, something like
foo h = f (g x) (h y)
foo h = f $ g x $ h y
foo h = (f $ g x) $ h y
...which currently cannot be expressed without parentheses. The "bad form" example, when parsed with an infixl application, would be
foo x = ((bar $ baz) $ quux) $ x
which means something significantly different. So, future-proof your code by avoiding this form.

                        
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