函数式编程功能

Question

如何编写两个列表或集合的并集的函数程序？

Answer 1

这取决于语言，但通常会有一个递归解决方案，涉及遍历标识共享元素的集合。

例如，在 Haskell 中的原生 Data.Set 类型，

union :: Ord a => Set a -> Set a -> Set a
union Tip t2  = t2
union t1 Tip  = t1
union t1 t2 = hedgeUnion (const LT) (const GT) t1 t2

hedgeUnion _     _     t1 Tip
  = t1
hedgeUnion cmplo cmphi Tip (Bin _ x l r)
  = join x (filterGt cmplo l) (filterLt cmphi r)
hedgeUnion cmplo cmphi (Bin _ x l r) t2
  = join x (hedgeUnion cmplo cmpx l (trim cmplo cmpx t2))
           (hedgeUnion cmpx cmphi r (trim cmpx cmphi t2))
  where
    cmpx y  = compare x y

或者更简单地说，对于列表：

unionBy                 :: (a -> a -> Bool) -> [a] -> [a] -> [a]
unionBy eq xs ys        =  xs ++ foldl (flip (deleteBy eq)) (nubBy eq ys) xs

Answer 2

这听起来像是一个家庭作业问题，但我会咬牙切齿。在Python中：

lambda x, y: x + filter(lambda z: z not in x, y)