我需要编写一个函数，该函数返回相对于U（用户也输入的通用集）的A（提供）的称赞

Question

我认为我不允许使用任何非标准库。

我曾经尝试过像制表符和空格这样的缩进，但是我是Haskell的新手，所以很可能我错过了一些东西。

subset :: (Eq a) => [a] -> [a] -> Bool
subset [] ys = True
subset (x:xs) ys
 elem x ys = subset xs ys
 let otherwise = False

complement :: (Eq a) => [a] -> [a] -> Maybe [a]
 complement [] ys = Just ys
 complement xs ys
  subset xs ys = Just (filter (not.(`elem`xs))ys)
      otherwise = Nothing

输入数据（[1,2,3] [1..5]）=刚[4,5]的预期结果

Answer 1

似乎您正在尝试编写模式保护程序，但忽略了|。试试：

complement :: (Eq a) => [a] -> [a] -> Maybe [a]
complement [] ys = Just ys
complement xs ys
  | subset xs ys = Just (filter (not.(`elem`xs))ys)
  | otherwise = Nothing

subset中存在类似的问题。还有其他一些缩进问题，尽管可能是粘贴到SO中引入的。