有多少参数采用 Haskell 的 foldr 函数? [英] How many arguments takes the foldr function of Haskell?

问题描述

我是 Haskell 的新手，我正在阅读这本书 "Real世界 Haskell".在本书的第 4 章中，作者要求作为练习使用 fold 重写 groupBy 函数.该书的一位读者(Octavian Voicu)给出了以下解决方案:

I am new to Haskell and I am reading the book "Real World Haskell". In the Chapter 4 of the book the author asks as an exercise to rewrite the groupBy function using fold. One of the readers of the book (Octavian Voicu ) gave the following solution:

theCoolGroupBy :: (a -> a -> Bool) -> [a] -> [[a]]
theCoolGroupBy eq xs = tail $ foldr step (\_ -> [[]]) xs $ (\_ -> False)
                       where step x acc = \p -> if p x then rest p else []:rest (eq x)
                                          where rest q = let y:ys = acc q in (x:y):ys

我的问题很简单:我知道 foldr 有 3 个参数:一个函数、一个初始值和一个列表.但是在代码的第二行 foldr 需要 4 个参数.为什么会这样?谢谢.

My question is simple: I know that foldr takes 3 arguments: a function, an initial value and a list. But in the second line of the code foldr takes 4 arguments. Why this happens? Thank you.

推荐答案

Scott 的回答是正确的，foldr 的结果是一个函数，所以这就是为什么看起来 foldr 需要 4 个参数.foldr 函数接受 3 个参数(函数、基数、列表):

Scott's answer is correct, the result of the foldr is a function, so this is why it seems that foldr takes 4 arguments. The foldr functions does take 3 arguments (function, base, list):

*Main> :type foldr
foldr :: (a -> b -> b) -> b -> [a] -> b

这里我只举一个不那么复杂的例子:

I'll just give here an example that is less complex:

inc :: Int -> (Int -> Int)
inc v = \x -> x + v

test = inc 2 40  -- output of test is 42

在上面的代码中，inc 接受一个参数，v，并返回一个将其参数增加 v 的函数.

In the above code, inc takes one argument, v, and returns a function that increments its argument by v.

如下所示，inc 2的返回类型是一个函数，所以它的参数可以简单地加在最后:

As we can see below, the return type of inc 2 is a function, so its argument can simply be added at the end:

*Main> :type inc
inc :: Int -> Int -> Int
*Main> :type inc 2
inc 2 :: Int -> Int
*Main> :type inc 2 40                                                        
inc 2 40 :: Int

括号可以用来强调返回值是一个函数，但在功能上和上面的代码是一样的:

Parentheses could be used to emphasize that the return value is a function, but functionally it is identical to the above code:

*Main> (inc 2) 40
42

PS:我是原评论的作者 :)

PS: I'm the author of the original comment :)

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