Haskell operator vs function precedence

你是对的。此规则是Haskell语法定义的一部分。 Haskell Report . 特别地,请注意第3节中的表达式,即函数应用程序的参数(AN) fexp )必须是 aexp . AEXP允许运算符作为节的一部分,也允许在带圆括号的表达式中,但不允许使用裸运算符。

在 map foo $ xs ,haskell语法意味着它被解析为两个表达式,应用于二进制运算符。 $ . As sepp2k notes, the syntax (map foo $) is a left section and has a different meaning.

我不得不承认,我从来没有考虑过这个问题,事实上,我必须在报告中查一下,看看为什么操作员会有他们所做的行为。

Firstly, application (whitespace) is the highest precedence "operator".

其次,在haskell中,运算符和函数之间实际上没有区别,除了默认情况下运算符是中缀,而函数不是。您可以使用反勾号将函数转换为中缀

2 `f` x

(+) 2 3

So, your question is a bit confused.

Prelude> :info ($)
($) :: (a -> b) -> a -> b   -- Defined in GHC.Base

infixr 0 $

Prelude> :info (+)

class (Eq a, Show a) => Num a where
  (+) :: a -> a -> a

infixl 6 +

Showing both precedence and associativity.

In addition to the information provided by other answers already, note that different operators can have different precedences over other operators, as well as being left-/right- or non-associative. You can find these properties for the Prelude operators in the Haskell 98 Report fixity section .

+--------+----------------------+-----------------------+-------------------+
| Prec-  |   Left associative   |    Non-associative    | Right associative |
| edence |      operators       |       operators       |    operators      |
+--------+----------------------+-----------------------+-------------------+
| 9      | !!                   |                       | .                 |
| 8      |                      |                       | ^, ^^, **         |
| 7      | *, /, `div`,         |                       |                   |
|        | `mod`, `rem`, `quot` |                       |                   |
| 6      | +, -                 |                       |                   |
| 5      |                      |                       | :, ++             |
| 4      |                      | ==, /=, <, <=, >, >=, |                   |
|        |                      | `elem`, `notElem`     |                   |
| 3      |                      |                       | &&                |
| 2      |                      |                       | ||                |
| 1      | >>, >>=              |                       |                   |
| 0      |                      |                       | $, $!, `seq`      |
+--------+----------------------+-----------------------+-------------------+

Any operator lacking a fixity declaration is assumed to be left associative with precedence 9 .

记住,函数应用具有最高优先级(考虑优先级) 10 与表中的其他优先级相比) [1] .

The difference is that infix operators get placed between their arguments, so this

list = map foo $ xs

can be rewritten in prefix form as

list = ($) (map foo) xs

which, by the definition of the $ operator, is simply

list = (map foo) xs

Operators can be passed as function arguments if you surround them with parenthesis (i.e. map foo ($) xs , which would indeed be passed as (map foo ($)) xs )但是,如果您没有用括号括起来,那么您就可以正确地认为它们不能作为参数(或分配给变量)传递。

Also note that the syntax (someValue $) (何处) $ could be any operator) actually means something different: it is equivalent to \x -> someValue $ x , i.e. it partially applies the operator to its left operand (which in case of $ is a noop of course). 同样地 ($ x) partially applies the operator to the right operand. 所以 map ($ x) [f, g, h] would evaluate to [f x, g x, h x] .