如何使用$运算符对Haskell进行函数应用？ [英] How does function application with the $ operator curry in Haskell?

问题描述

根据GHC，$的类型是

  * Main>：t（$）
（$）::（a-> b） - > a  - > b 
  

但是我可以输入以下代码

  * Main> map（$ 2）[（* 2），（+2），（/ 2）] 
 [4.0,4.0,1.0] 
  

根据$的签名，尽管我会假设我需要使用翻转函数，因为第一个参数是$ （a-> b）。

例如，我不能执行以下操作：

  curry_test :: Integer  - >字符串 - > String 
 curry_test x y =（show x）++++ y 
 * Main> let x = curry_test123
无法与实际类型为'[Char]'的$ Integer相匹配'
'在`curry_test'的第一个参数中，即`123'
在表达式中：curry_test123
在'x'的等式中：x = curry_test123
  

但我可以做
$ b $ pre $ let x = curry_test 2


解决方案

Infix操作符有特殊的规则。见本页： http://www.haskell.org/haskellwiki/Section_of_an_infix_operator

基本上，因为 $ 是一个中缀运算符，所以（$ 2）实际上修正了 2 作为 $ 的第二个参数，所以它相当于 flip（$）2 。

这个想法是让运算符的部分应用程序更加直观，例如，如果 map（/ 2）在列表中，您可以想象将列表中的每个元素放置在划分符号左侧的缺失位置。

如果要使用 curry_test 函数用这种方法，你可以做

$ p code> let x =（`curry_test `123）

I am learning haskell and am a little confused how the function application operator $ curry's.

According to GHC the type of $ is

*Main>:t ($)
($) :: (a->b) -> a -> b

But I can type the following code

*Main>map ($ 2) [(*2), (+2), (/2)]
[4.0,4.0,1.0]

According to the signature of $ though I would assume I would need to use the flip function because the first parameter to $ is (a->b).

For example, I can't do the following

curry_test :: Integer -> String -> String
curry_test x y = (show x) ++ " " ++ y
*Main> let x = curry_test "123"
    Couldn't match expected type `Integer' with actual type `[Char]'
In the first argument of `curry_test', namely `"123"'
In the expression: curry_test "123"
In an equation for `x': x = curry_test "123"

But I can do

let x = curry_test 2

解决方案

Infix operators have special rules. See this page: http://www.haskell.org/haskellwiki/Section_of_an_infix_operator

Basically, since $ is an infix operator, ($ 2) actually fixes 2 as the second argument of $, so it is equivalent to flip ($) 2.

The idea is to make partial application with operators more intuitive, so for example if you map (/ 2) over a list, you can imagine putting each element of the list in the "missing" spot on the left side of the division sign.

If you want to use your curry_test function this way, you could do

let x = (`curry_test` "123")

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