我一直在尝试定义一个函数,给定一个整数列表和一个整数n,返回一个布尔值,指示n是否在列表中恰好出现一次.
我有这个,但它不起作用,我无法弄明白
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