即使缺少参数，此功能为何仍起作用? [英] Why does this function work even though the argument is missing?

问题描述

我正在尝试理解以下代码:

I'm trying to understand the following piece of code:

import Data.Char (ord)

encodeInteger :: String -> Integer
encodeInteger = read . concatMap ch
    where ch c = show (ord c)

但是当将 encodeInteger 定义为需要一个字符串的函数时，我看不到它如何工作，但是在第二行中，该函数是在没有该字符串参数的情况下实现的.

But I don't see how this can work when encodeInteger is defined as a function that takes a string, but in the second line, the function is implemented without that string argument.

此外， concatMap (根据hoogle)具有一个功能和一个列表，但仅提供了功能 ch .

Also, concatMap (according to hoogle), takes a function and a list, but only the function ch is provided.

为什么此代码仍然有效?该论点以某种方式神奇地通过了吗?与欺骗有关吗?

Why does this code still work? Is the argument somehow magically passed? Has it something to do with currying?

为什么不能这样更改它?

edit: And why doesn't it work to change it like this:

encodeInteger :: String -> Integer
encodeInteger a = read . concatMap ch a
    where ch c = show (ord c)

推荐答案

基本定义一个函数

f = g

与定义函数相同

f x = g x

在特定情况下，您可以使用

In your specific case, you can use

encodeInteger a = (read . concatMap ch) a

定义您的功能.括号是必需的，否则解析为

to define your function. The parentheses are needed, otherwise it is parsed as

encodeInteger a = (read) . (concatMap ch a)

和 concatMap ch 不是函数，不能组成.最多你可以写

and concatMap ch a is not a function and can not be composed. At most you could write

encodeInteger a = read (concatMap ch a)
-- or
encodeInteger a = read $ concatMap ch a

关于为什么 concatMap ch 仅接受一个参数?".这是部分应用程序，在Haskell中非常常见.如果有

About "why concatMap ch takes only one argument?". This is a partial application, which is very common in Haskell. If you have

f x y z = x+y+z

您可以使用更少的参数调用 f ，并获得其余参数的函数结果.例如， f 1 2 是采用 z 并返回 1 + 2 + z 的函数.

you can call f with fewer arguments, and obtain as the result a function of the remaining arguments. E.g., f 1 2 is the function taking z and returning 1+2+z.

具体来说，感谢Currying，没有一个函数接受两个或更多参数.每个函数始终只接受一个参数.当您拥有类似

Concretely, thanks to Currying, there's no such a thing as a function taking two or more arguments. Every function always takes only one argument. When you have a function like

foo :: Int -> Bool -> String

然后 foo 接受一个参数，即 Int .它返回一个函数，该函数需要一个 Bool ，最后返回一个 String .您可以通过编写文字来可视化

then foo takes one argument, an Int. It returns a function, which takes a Bool and finally returns a String. You can visualize this by writing

foo :: Int -> (Bool -> String)

无论如何，如果您查找currying和部分应用程序，则会发现很多示例.

Anyway, if you look up currying and partial application, you will find plenty of examples.

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