函數式編程函數

Question

如何爲兩個列表或集合的聯合編寫函數式程序？

Answer 1

它依賴於語言，但通常會有一個遞歸解決方案，涉及遍歷集合標識共享元素。

例如在Haskell在本機Data.Set類型，

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 

或者更簡單地說，對於清單：

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

Answer 2

這聽起來像一個家庭作業的問題，但我會咬。在Python中：

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