haskell 求助

我刚刚开始学习haskell遇到个题目做不出来，要求写一个function.

它接受一个长度为偶数的 list 和一个条件返回新的list

findBonding :: Eq a => (a -> a -> Bool) -> [a] -> Maybe [(a,a)]

元组 list 满足所有元素都是从原list来的且出现一次 就是俩俩组合,且满足f

findBonding (\x -> \y -> x+y<6) [1, 2, 3, 4] 得到 [(1,2),(3,4)] , [(1,3),(2,4)] [(1,4),(2,3)]

findBonding (\x -> \y -> x*y<10) [1, 2, 3, 4] 得到 [(1,2),(3,4)] , [(1,3),(2,4)] , [(1,4),(2,3)]

这是我已经写的代码

findBonding :: Eq a => (a -> a -> Bool) -> [a] -> Maybe [(a,a)]

findBonding f xs

|length xs `mod`2/=0 =Nothing

| and [elem (y, x) rel | (x, y) <- rel]

= Just(rel)

| otherwise = Nothing

where

rel = [(a, b) | a <- xs, b <- xs, a /= b, f a b]

只能返回符合条件f的所有可能组合，但是弄不出俩俩组合的样子

  findBonding (\x -> \y -> x*y < 21) [2..7]
=> Just [(2,3),(2,4),(2,5),(2,6),(2,7),(3,2),(3,4),(3,5),(3,6),(4,2),(4,3),(4,5),(5,2),(5,3),(5,4),(6,2),(6,3),(7,2)]

这个条件只有一个可能的bond 就是[(2,7),(3,6),(4,5),(5,4),(6,3),(7,2)]

T T 我做了俩天了实在想不出该怎么做