写作重叠功能

Question

我想编写一个输入两个列表的多态函数，并告诉我这两个列表是否包含一个公共元素。我写这个函数的尝试如下：

overlaps :: (Eq a) => [a] -> [a] -> Bool
overlaps x:xs y:ys  
   | x `is_element_of` ys = True
   | otherwise = False

，其中

is_element_of :: (Eq a) => a -> [a] -> Bool
is_element_of e list = case list of
[] -> False
x: xs -> e == x || e `is_element_of` xs

然而这不起作用......是否可以对两个列表进行模式匹配？这是写这个函数的可能方法吗？

Answer 1

您的is_elem_of个功能Data.List已作为elem存在。使用它，overlaps很容易编写。

overlaps []     _  = False
overlaps (x:xs) ys = x `elem` ys || overlaps xs ys

正如您所知，可以在列表上进行模式匹配。如果要在列表上编写函数而不使用模式匹配，可以使用foldr函数。

overlaps xs ys = foldr (\x acc -> x `elem` ys || acc) False xs

Answer 2

我想你会想要这样的东西（未经测试）：

overlaps :: (Eq a) => [a] -> [a] -> Bool
overlaps [] _        = False
overlaps _ []        = False
overlaps (x:xs) list = x `myelem` list || overlaps xs list

myelem :: (Eq a) => a -> [a] -> Bool
myelem _ []     = False
myelem x (y:ys) = x == y || myelem x ys

使用模式匹配方法的AFAIK更具惯用性。