union package:base-prelude

union :: Eq a => [a] -> [a] -> [a]
base-prelude BasePrelude
The union function returns the list union of the two lists. For example, 
>>> "dog" `union` "cow"
"dogcw"
Duplicates, and elements of the first list, are removed from the the second list, but if the first list contains duplicates, so will the result. It is a special case of unionBy, which allows the programmer to supply their own equality test.
unionBy :: (a -> a -> Bool) -> [a] -> [a] -> [a]
base-prelude BasePrelude
The unionBy function is the non-overloaded version of union.