Haskell：如何使用let-in构造？

Question

我写了一个sort函数。这很奇怪。

sort :: (Eq a, Ord a) => [a] -> [a]
sort xs = repeat'' sort' (xs) (length xs) 

repeat'' f xs 0 = xs
repeat'' f xs n = repeat'' f (f xs) (n-1)

sort' (x:y:xs)
        | x < y     = y : sort' (x:xs)
        | otherwise = x : sort' (y:xs)
sort' x = x

我如何使用let-in构造来美化它？

我认为我的尝试看起来并不好：

sort :: (Eq a, Ord a) => [a] -> [a]
sort xs = let 
             r f xs 0 = xs
             r f xs n = r f (f xs) (n-1)
          in r f' xs $ length xs where
               f' (x:y:xs) | x < y     = y : f' (x:xs) 
                           | otherwise = x : f' (y:xs)
               f' a = a