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问题描述

我一直在试图定义一个函数，给出一个整数列表一个整数n，返回一个布尔值指示n是否在列表中恰好发生一次。

我有这一点，但它不工作，我无法弄清楚

 一旦:: [A]  - ＆GT; （一 - ＆GT;布尔） -  GT;布尔
过滤器_ [] = []
过滤器P（X：XS）
    | P X [= X：过滤器p XS
    |否则=过滤器p XS什么，我想会是一个例子：
主营＆GT;一旦[2,3,2,4] 2
假
主营＆GT;一旦[1..100] 2
真正
 

解决方案

  ::一次（公式一）=＆GT; [一个]  -  GT;一个 - ＆GT;布尔
一旦XS X =（== 1）$ $长度过滤器（== X）XS
 

I've been trying to define a function which, given a list of Integers and an Integer n, returns a Boolean indicating whether n occurs exactly once in the list.

I have this, but it is not working and I cannot figure it out

once :: [a] -> (a -> Bool) -> Bool 
filter _ [] = []  
filter p (x:xs)   
    | p x       = x : filter p xs  
    | otherwise = filter p xs 

An example of what I want would be:
Main>  once [2,3,2,4] 2
False
Main> once [1..100] 2
True

解决方案

once :: (Eq a) => [a] -> a -> Bool
once xs x = (== 1) $ length $ filter (== x) xs

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