如何检查两组的显式并集是否为空？

Question

Below is my code for the explicit union of two lists. How would I get the length of the list rather than the list itself?

explicit_union :: (Eq a) => [a] -> [a] -> [a]
explicit_union as bs = foldl (\as b -> if elem b as then as else as ++ [b]) as bs

Answer 1

The Union is empty if and only if both sets are empty. So if you're computing the union just for that, you're wasting computational time.

If instead you're given the union or have to use it anyways, length is the most natural method.

However as you only want to know if it's empty, instead of length ls == 0 you can use take 1 ls == [] . This way Haskell -- as it's a lazy language -- only has to compute the first element, instead of the whole list as length would require

Edit

As @joseph pointed out, there's a Prelude function to check if a list is null: null :: [a] -> Bool