Haskell foldr and foldl

Question

The code below is used to generate the intersection of two lists:

unionSet :: Eq a => [a] -> [a] -> [a]
unionSet a b = foldl (\acc x -> if elem x acc then acc else acc ++ [x]) a b

Why does the foldl function work but when I use foldr it generates errors?

Answer 1

foldr has the type

(a -> b -> b) -> [a] -> b -> b

while foldl has the type

(b -> a -> b) -> [a] -> b -> b 

Note the order of the two parameters.

(\x acc -> ...

would fix the error.