确定Haskell列表中是否存在重复的元素

Question

我正在尝试测试重复项列表，但是当我编译并输入时

repeated [1,2,3,4] 

它输出True。 怎么了？

belongs :: Eq a => a -> [a] -> Bool
belongs n [] = False
belongs n (x:xs) | n == x = True
                 | otherwise = belongs n xs

repeated :: [Integer] -> Bool
repeated [] = False
repeated (x:xs) | belongs x xs = True
                | otherwise = belongs (head xs) xs

Answer 1

你要

repeated :: [Integer] -> Bool
repeated [] = False
repeated (x:xs) | belongs x xs = True
                | otherwise = repeated xs

Answer 2

“属于（头部xs）xs”检查xs的头部是否在xs内，这将始终为真。

（除非xs为空，否则您的程序将崩溃！“ head”是部分函数，​​使用空列表将崩溃）

这将解决它（也由@talex指出，但我也建议使其更通用，无需将其专门用于Integer）：

repeated :: Eq a => [a] -> Bool
repeated [] = False
repeated (x:xs) | belongs x xs = True
                | otherwise = repeated xs