haskell - 使用 let 在线解析错误(可能是不正确的缩进或不匹配的括号)

Question

我明白了

parse error (possibly incorrect indentation or mismatched brackets)

用|就行了i > j = 交换轴 p j

apartition :: Ord a => STArray s Int a -> Int -> Int -> ST s Int
apartition axs p q = do 
  x <- readArray axs p
  let loop i j
    | i > j = do swap axs p j
                 return j
    | otherwise = do u <- readArray axs i
                     if u < x 
                       then do loop (i + 1) j 
                       else do swap axs i j
                            loop i (j − 1)
  loop (p + 1) q

我真的很困惑造成这种情况的原因，有人可以帮我解决问题吗？

Answer 1

这是一个缩进错误，您需要在循环定义开头的右侧放置至少一个字符，因此:

apartition :: Ord a => STArray s Int a -> Int -> Int -> ST s Int
apartition axs p q = do 
  x <- readArray axs p
  let loop i j
       | i > j = do swap axs p j
                    return j
       | otherwise = do u <- readArray axs i
                        if u < x 
                        then do loop (i + 1) j 
                        else do swap axs i j
                                loop i (j - 1)
  loop (p + 1) q