let行出现解析错误(可能是缩进不正确或括号不匹配)

我明白了

解析错误(可能是缩进不正确或括号不匹配)

在线与 | i > j = do swap axs 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

我真的很困惑这件事的原因,有人能帮我澄清一下吗?